INTRINSICALLY SAFE (IS) ACTIVE POWER SUPPLIES

Transcription

1 INTRINSICALLY SAFE (IS) ACTIVE POWER SUPPLIES by Mark Edward Walpole Assoc. Dip. Elec. Eng., B Eng. (Hons.) Submitted for the Degree of Master of Engineering (research) Queensland University of Technology Faculty of Built Environment and Engineering School of Electrical and Electronic Systems Engineering Brisbane March, 2003

2 Keywords (ii) intrinsic safety, intrinsically safe, active power supply, modelling, equivalent circuit, intrinsic safety Standards, intrinsic safety assessment method

3 Abstract (iii) Intrinsically safe (IS) active power supplies subjected to certain transient load conditions can deliver power to a circuit at significantly higher levels than indicated on their nameplate ratings. During a transient load such as an intermittent shortcircuit, energy is transferred from the power supply to the short-circuit and an electrical arc may form when the short-circuit is applied or removed. This poses a spark ignition risk as energy is transferred from the arc to the surrounding atmosphere. Currently various International and Australian Standards define the performance requirements for IS electrical apparatus. A duly accredited laboratory is required to establish the intrinsic safety compliance of an apparatus with the Standards. It involves an assessment of the apparatus and may include testing. The assessment of the apparatus determines adequate segregation, separation, construction, and selection of components. The tests performed on the apparatus include a temperature rise test and in some cases, the sparking potential of the circuit is tested using the spark test apparatus (STA). Testing the sparking potential of active power supplies to establish compliance adds significantly to the time and costs involved in establishing compliance. A new alternative assessment method is proposed in this report to augment or replace the testing phase of the compliance certification process for active power supplies. The proposed alternative assessment method (PAAM) is derived from a determination of the steady-state and transient output characteristics of the active power supply under consideration. Parameters such as peak output current, time constant of peak current decay, and the output voltages at these times are measured from the circuit's output characteristics. These measurements can subsequently be used to derive the topology and component values of an equivalent circuit. The resulting equivalent circuit is then considered like a linear power supply and the sparking potential can be determined using existing assessment methods. This thesis investigates in detail the equivalent circuit of a number of direct current (DC) active power supplies whose transient output characteristics exhibit predominantly capacitive behaviour. The results of the PAAM using the equivalent circuit are then compared with results achieved using the current testing procedure with a STA.

4 Abstract (iv) A small sample of active power supplies is used to generate data from which a relationship between the current testing procedure and the PAAM can be established. The PAAM developed in this research project can be used as a pre-compliance check by designers, manufacturers, or IS testing stations. A failure of this test would indicate that the active power supply s sparking energy is not low enough to be regarded as intrinsically safe. The PAAM requires fewer resources to establish a result than the STA. The benefits of a simplified spark ignition test would flow on from designers and manufacturers to end users.

5 Table of Contents (v) Chapter 1 Introduction Background Research Objectives Research Program Scope of Thesis Publications...5 Chapter 2 Review of IS Power Supplies and Intrinsic Safety Evolution of Intrinsic Safety Mechanism of Electrical Arcs Mechanisms of Ignition Energy Transferred from the Electric Arc Development of the Principles of Intrinsic Safety IS Power Supplies Evolution of IS Power Supplies Modern IS Power Supplies Design Methodologies of IS Power Supplies Types and Terminology of IS Power Supplies Three Types of IS Power Supplies Definition of IS Power Supplies Terminology IS Active Power Supplies Intrinsic Safety Standards Current Australian and International Standards Comparison of AS and AS/NZS Participants in Ensuring Intrinsic Safety Accredited Intrinsic Safety Testing and Certification Bodies Certification, Assessment and Testing of IS Power Supplies Certification Determining Conformance to a Standard Assessment of IS Active Power Supplies Testing IS Active Power Supplies using the STA Summary...36

6 Table of Contents (continued) (vi) Chapter 3 Electrical Investigation of the STA Introduction to the STA Low Voltage Electric Arcs and the STA Periodic and Randomness of the STA Electrical Circuit of the STA Sensitivity of the STA Summary Chapter 4 Characteristics of IS Active Power Supplies Sample IS Active Power Supplies Steady-state Output Characteristics Transient Output Characteristics Measuring Transient Characteristics using the STA Measuring Transient Output Characteristics using a Relay Limitations in Measuring Transient Output Characteristics Transient Characteristics of Sample IS Active Power Supplies Summary Chapter 5 Development of the PAAM Assessment Methods for IS Active Power Supplies The RLC Equivalent Circuit Model Experimental Verification of the RLC Equivalent Circuit Model The RC Equivalent Circuit Model Experimental Verification of the RC Equivalent Circuit Model The Proposed Alternative Assessment Method (PAAM) Limitations of the PAAM Summary Chapter 6 Experimental Evaluation of the PAAM Sample IS Active Power Supplies Sample Active Power Supply Parameter Measurements Example Application of PAAM Comparison with Spark Testing Results Summary

7 Table of Contents (continued) (vii) Chapter 7 Conclusions and Further Research Conclusions Further Research References 105 Appendices A 1. Generic Block Diagram of IS Active Power Supply A 2. Measured Output Characteristic using STA A 3. Measured Output Characteristic using Relay A 4. No-load to Short-circuit Output Characteristic A 5. Ignition Curves for well defined Circuits Resistive circuits Group I capacitive circuits Group I inductive circuits...115

8 List of Tables (viii) Tables Table 2-1: UK. National Coal Board DC IS power supplies [3]...13 Table 2-2: Summary of SIMTARS recommended design methodology [9]...16 Table 2-3: Definition of active and passive power supplies...19 Table 2-4: Definition of linear and non-linear power supplies...19 Table 2-5: Defining the types of IS power supplies...20 Table 2-6: Maximum values of V and I for Group I active power supplies [11]...22 Table 2-7: Relevant Acts and Regulations [17]...25 Table 2-8: Summary of SIMTARS intrinsic safety assessment procedure [9]...33 Table 2-9: Summary of SIMTARS intrinsic safety testing procedure [9]...34 Table 4-1: Measured steady-state parameters sample active power supplies...55 Table 4-2: Measured transient parameters test circuit with STA...59 Table 4-3: Measured transient parameters test circuit with a relay...61 Table 4-4: Instantaneous voltage and current for inductors and capacitors...63 Table 5-1: Component equations for the RLC equivalent circuit model...73 Table 5-2: Experimental RLC equivalent circuit model component values...74 Table 5-3: Component equations for the RC equivalent circuit model...80 Table 5-4: Experimental RC equivalent circuit model component values...81 Table 6-1: Measured transient parameters of sample active power supplies...93 Table 6-2: PAAM calculating component values (RC equiv. cct. model) - PS Table 6-3: PAAM component values (RC equiv. cct. model) for PS 1, 2 and Table 6-4: Comparison of results - PAAM vs. STA testing...100

9 List of Figures (ix) Figures Figure 2-1: Energy system of an electrical arc...8 Figure 2-2: Ignition kernel growth vs. ignition energy and quenching distance...10 Figure 2-3: Power supply circuit topologies and their V-I characteristic [12]...18 Figure 3-1: Plan and elevation views of STA wire holder and cadmium disk [24]...38 Figure 3-2: Oblique view of the STA wire holder and the cadmium disk...39 Figure 3-3: STA wire and cadmium disk making and breaking contact...40 Figure 3-4: STA making contact - discharging a capacitive circuit...42 Figure 3-5: Test circuit with STA and wire path for a single traverse...43 Figure 3-6: Measured output current (I O ) and voltage (U O ) for a single traverse...44 Figure 3-7: Periodic make and break of wires on the cadmium disk...45 Figure 3-8: Measured periodic make and break waveform...45 Figure 3-9: Geometry of arc scribed by the wire on cadmium disk...46 Figure 3-10: STA electrical circuit...48 Figure 3-11: STA calibration circuit with current measuring resistance...50 Figure 3-12: Measured V and I waveforms for the STA calibration circuit...51 Figure 4-1: Block diagram of sample IS active power supply DC stage...53 Figure 4-2: Steady-state test circuit...54 Figure 4-3: Steady-state output characteristics...55 Figure 4-4: Transient characteristics test circuit with STA...57 Figure 4-5: Measured transient output characteristics (STA)...58 Figure 4-6: Transient characteristics test circuit with a relay...60 Figure 4-7: Measured transient output characteristics (relay)...61 Figure 4-8: Power supply output capacitance external discharge path...63 Figure 4-9: Active power supply NL to FL transient characteristics...64 Figure 4-10: Active power supply FL to SC transient characteristics...65 Figure 4-11: Active power supply NL to SC transient characteristics...66 Figure 4-12: Active power supply NL to SC transient characteristics...68 Figure 5-1: PAAM - RLC equivalent circuit model topology...72 Figure 5-2: Experimental RLC equiv. cct. and steady-state characteristic...75 Figure 5-3: Experimental RLC equivalent circuit transient tests...75 Figure 5-4: Over damped RLC equiv. cct. NL to SC transient characteristics...76 Figure 5-5: Over damped RLC equiv. cct. SC to NL transient characteristics...77 Figure 5-6: Under damped RLC equiv. cct. NL to SC transient characteristics...78 Figure 5-7: Under damped RLC equiv. cct. SC to NL transient characteristics...79 Figure 5-8: PAAM - RC equivalent circuit model topology...80

10 List of Figures (x) Figure 5-9: Experimental RC equiv. cct. and steady-state characteristic...81 Figure 5-10: Experimental RC equivalent circuit - transient test circuit...82 Figure 5-11: Measured RC equiv. cct. NL to SC transient characteristics...82 Figure 5-12: Measured RC equiv. cct. SC to NL transient characteristics...83 Figure 5-13: Illustration of ignition curve safe and unsafe areas...86 Figure 6-1: Measured transient output current response for PS Figure 6-2: PAAM RC equivalent circuit model for PS Figure 6-3: PAAM ignition curve plots for PS 1, 2 and 3 [24]...97

11 Abbreviations (xi) AIT AS AS/NZS BS BSI BVS CENELEC ETCC FOS HSE (M) IEC IS JASANZ LEL MIC MIE MEIC NATA NSW NZ NZS PAAM PS QLD QUT RC RLC SIMTARS SMRE STA UEL UK UL Auto ignition temperature Australian Standard Australian and New Zealand Standard British Standard British Standards Institute Berggewerkschaftliche Versuchsstrecke (Approval organisation) European Committee for Electrotechnical Standardisation SIMTARS Engineering Testing and Certification Centre Factor of Safety Health and Safety Executive (Mining) International Electrotechnical Commission Intrinsically Safe Joint Accreditation System of Australia and New Zealand Lower Explosion Limit Minimum Ignition Current Minimum Ignition Energy Most Easily Ignited Concentration National Association of Testing Authorities, Australia New South Wales New Zealand New Zealand Standard Proposed alternative assessment method Power supply Queensland Queensland University of Technology Resistive and Capacitive Resistive, Inductive and Capacitive Safety In Mines Testing And Research Station (Approval organisation) Safety in Mines Research Establishment (Approval organisation) Spark Test Apparatus Upper Explosion Limit United Kingdom Underwriters Laboratories (Approval organisation)

12 Statement of original authorship (xii) The work contained in this thesis has not been previously submitted for a degree or diploma at any other higher education institution. To the best of my knowledge and belief the thesis contains no material previously published or written by another person except where due reference is made. Signed.. Mark Walpole Date: 27 / 3 / 2003

13 Acknowledgements (xiii) I would like to express my appreciation to my principal supervisor Dr. Tee Tang for his patience, guidance, and wisdom. I would like to thank the following for providing industry support for this research:- - Australian Coal Association Research Program (ACARP) for providing the scholarship and research grant, - Safety In Mines Testing And Research Station (SIMTARS) for providing facilities and technical support to perform this research and - Oakey Creek Coal Mining Company for providing active power supplies. Finally but by no means the least I wish thank my parents and brother who continually provide me with support and encouragement in all my endeavours. For my three children Alex, Ryan and Jessica.

14 - 1 - Chapter 1 Introduction 1.1 Background Intrinsically safe (IS) active power supplies have received some degree of notoriety in recent times. In December 1998 during the re-certification of an AUSTDAC Pty. Ltd. IS active power supply it was discovered that the device failed to meet requirements set out in the Australian Standard (AS) for intrinsic safety AS The IS active power supply involved was in then current use in hazardous areas of underground coal mines throughout Australia. The New South Wales (NSW) Department of Mineral Resources issued safety alerts (SA /12/98 and SA99/01 5/2/99) [1]. The cost to the underground coal mines is uncertain according to Bell and Hookham [2] who stated more than 50 mines in NSW and Queensland (QLD) were affected. The safety alerts were issued following the results of tests performed at TestSafe Australia (formerly known as, Londonderry Occupational Safety Centre) in NSW, an accredited testing laboratory, using the Spark Test Apparatus (STA) [1]. The tests revealed that the IS active power supply in question was capable of generating incendive electrical sparks under certain operating conditions. In the subsequent months, additional intrinsic safety certificates and mining approvals of other IS active power supplies were revoked. These events illustrated the onerous nature of the task that the intrinsic safety accreditation laboratories and certification bodies have in establishing that equipment submitted to them for certification complies with the relevant AS thus ensuring the safety of these devices in hazardous areas. Power supply manufacturers are continually pressured by the industry to provide IS power supplies that can deliver more power. Active power supplies can deliver more power and have been used extensively in non-intrinsically safe industries. Their deployment in underground coal mines poses a number of challenges to the issue of intrinsic safety. Current Australian and International Standards were written for passive power supplies and do not adequately cover aspects of the assessment and testing of active power supplies.

15 Chapter 1 Introduction In the remainder of this chapter, the objectives of this research project are presented followed by the details of research program, the scope of the research, and a list of associated publications. 1.2 Research Objectives This research aims at establishing the intrinsic safety requirements for IS active power supplies. The objectives of the research and investigation are summarised as follows:- identify the different forms of IS power supplies currently in operation and clearly define the properties of each type and the differences between active, passive, linear and non-linear power supplies investigate the industry standard intrinsic safety assessment and testing practices for IS active power supplies analyse IS active power supply circuitry to determine energy outputs likely to cause gas ignitions under dynamic conditions formulate and test a proposed alternative assessment method (PAAM) 1.3 Research Program The major milestones of this research project are presented under the following headings with a brief description of the research activities undertaken. Literature review - A literature search to establish the current body of knowledge and any other ongoing research activities related to IS power supplies Definition of IS power supplies An investigation of both the static and dynamic output characteristics of IS active power supplies as currently used by the coal mining industry A review of typical assessment and testing practices with reference to the Safety In Mines Testing And Research Station (SIMTARS) IS active power supplies assessment and testing procedures used during the compliance process.

16 Chapter 1 Introduction A discussion of technologies used to control the output of IS power supplies Including investigation of IS active power supply circuits in order to derive a functional block diagram and identify critical intrinsic safety parameters. Development of a PAAM for IS active power supplies - The STA was investigated to determine the methods by which IS active power supplies are tested. This leads on to development of an assessment method for IS active power supplies using the output steady-state and transient characteristics of the IS active power supply. Designing, building and testing - A small number of sample IS active power supplies are subjected to the PAAM and test outcomes are compared to results derived from spark testing using the STA. 1.4 Scope of Thesis Chapter 2 incorporates a literature review, which in addition to identifying the main issues related to IS active power supplies introduces the fundamental concepts of gas ignition and spark generation. The literature review covers the statutory requirements, National and International approval schemes, National and International Standards, intrinsic safety assessment and testing practices, and summarises the major works of researchers in these areas. National and International testing stations were contacted and requested to contribute to this research project by providing access to their policies, procedures and instructions for the assessment and testing of IS active power supplies. Unfortunately, only one response was received stating that there were no procedures available. Based on the minimal response received, it is assumed that these documents either do not exist or are unavailable for review. Access to SIMTARS policies, procedures and instructions enabled a review of SIMTARS assessment and testing procedures, which is presented in Chapter 2. Recent events that impact on assessment and testing practices have also been reviewed and are included in Chapter 2.

17 Chapter 1 Introduction In Chapter 3, the STA is investigated to determine its electrical characteristics and the methods by which the sparking potential of active power supplies are tested. Parameters in the output characteristics of the active power supply are identified. These parameters determine whether the power supply is intrinsically safe. In chapter 4, a number of IS active power supply circuits are analysed to identify the methods used to control the output energy. IS power supply manufacturers circuit diagrams are not readily available and access to SIMTARS intrinsic safety certification documentation is restricted by client privacy agreements. However, a functional block diagram is developed for active IS power supplies. A concept for a PAAM is developed in Chapter 5. It makes use of the output steadystate and transient characteristics of the active power supply. The PAAM uses an equivalent circuit developed to represent the active power supply. Using the equivalent circuit, an IS active power supply can be assessed similarly to the existing current practices used for passive power supplies. In Chapter 6, the PAAM is tested and the results analysed in order to investigate the possibility of a relationship between the assessment of the equivalent circuit and the results of spark testing the IS active power supply. A number of commercially available IS active power supplies are subjected to the PAAM and they are spark tested using the STA. The results from the two methods are compared in order to develop a possible correlation. The PAAM developed in this research may reduce or eliminate the need for spark testing IS active power supplies. In Chapter 7, the implications of this research and possible further research direction are discussed.

18 Chapter 1 Introduction Publications During the period of research, the following papers were published: (1) Turner, D., Barnier, G. and Walpole, M., Assessment, Testing, and Certification of Intrinsically Safe Active Power Supplies, Proceedings of Mining Health and Safety Conference 2000, Townsville, August (2) Walpole, M. and Tang, T., Modelling Active Power Supplies for Intrinsic Safety Assessment, Proceedings of Australasian Universities Power Engineering Conference AUPEC 2002, CD-ROM, Melbourne, October 2002.

19 - 6 - Chapter 2 Review of IS Power Supplies and Intrinsic Safety The general topic of intrinsic safety and its application has been well documented [3-8]. By contrast only a limited amount of literature exists concerning the topics of design, characterisation, assessment and testing of intrinsically safe (IS) power supplies [9-16]. It should be noted that most of the available research literature relating to IS power supplies has been generated by testing laboratories [9-16]. Section 2.1 of this chapter gives a brief history of intrinsic safety and reviews the mechanism of electrical arcs, mechanism of ignition, energy transfer from the arc, and the principles of intrinsic safety. The development of IS power supplies is presented in Section 2.2 and the types and terminology defined in Section 2.3. IS active power supplies including their recent research activities are presented in Section 2.4. The application of the relevant Standards are summarised in Section 2.5. The certification process including the assessment and testing of IS power supplies is presented in Section 2.6. In Section 2.7 the main themes of the literature review are summarised. 2.1 Evolution of Intrinsic Safety The history of intrinsic safety dates back to the period between , during which the British Safety in Mines Research Establishment (SMRE) and other international research laboratories developed the explosion protection technique. This action was triggered by a series of colliery accidents in England, involving explosions of fire damp. Subsequent investigations identified that sparking contacts made by a signalling system in an atmosphere of coal gas were the most probable cause of explosion [3]. Fire damp, a gas consisting mainly of methane is generally associated with coal seams, being produced during the process of formation of coal [3]. Fire damp, also known as coal gas, can be released during mining activity or may occur due to the proximity of the coal seam itself.

20 Chapter 2 Review of IS Power Supplies and Intrinsic Safety Mechanism of Electrical Arcs When two electrodes are separated in air by a distance called the spark gap (d sg ) as illustrated in Figure 2-1 (a) a spark will occur if the applied voltage between the electrodes exceeds the breakdown voltage of the dielectric and there is sufficient supply of current [5]. When the applied voltage across the spark gap is reduced below the breakdown voltage of the dielectric, the arc will extinguish unless the arc itself has altered the dielectric strength. It is usual for the arc to alter the dielectric either by ionisation of the molecules or by contamination as a result of combustion. Combustion byproducts such as carbon in the spark gap may result in a reduction of dielectric strength. The energy available to the arc across the electrodes is a function of voltage, current and time. In the closed energy system of power supply, electrodes and arc as illustrated in Figure 2-1 (a), the available electrical energy at the electrodes is converted into heat, light, sound, and other forms of electromagnetic radiation as in Figure 2-1 (b). The heat generated due to the resistance of the arc path can be either conducted into the electrodes, or by convection/radiation/conduction into the surrounding matter. In the case where the surrounding matter is a flammable gaseous mixture, an explosive ignition occurs if there is sufficient energy associated with the arc Mechanisms of Ignition Ignition is defined as the initiation of combustion of a flammable material. An ignition occurs when there is sufficient energy in the electric arc to cause flammable gas molecules in close proximity to the arc to be heated to a point above their auto ignition temperature (AIT). If no further energy is supplied from the electric arc at this point, the ignition will be quenched. Should additional energy be supplied via the electric arc then the ignition kernel grows as more flammable gas molecules are heated above the AIT and ignite. If the energy supplied by the arc is sufficient for the ignition kernel diameter to exceed the quenching distance, the thermal energy generated by the ignition will become self-sustaining, and a process known as explosion results [5].

21 Chapter 2 Review of IS Power Supplies and Intrinsic Safety Series resistance R DC High Voltage Source Arc Electrode holder dsg Electrode Contact separation or Arc length or Spark gap Electrode Electrode holder (a) High voltage spark generator Electrode Arc path - ionisation Heat conducted into electrodes Arc resistance Arc path - ionisation Electrode Heat - conducted to gas near the arc so that the gas heats Light Sound other electromagnetic radiation Heat conducted into electrodes (b) Magnified view of arc Figure 2-1: Energy system of an electrical arc The smallest amount of energy required to ignite the most easily ignited concentration (MEIC) of the gas or vapour is called the minimum ignition energy (MIE). Scientific research institutions have established the values for the MEIC and MIE of the most commonly used flammable gasses and vapours [5]. When the concentration of the flammable gas is below the lower explosion limit (LEL), or above the upper explosion limit (UEL), an ignition cannot occur. Between the LEL and UEL, it is possible, depending on the amount of energy in the spark, to generate an ignition. A number of factors, such as the volume of gas, temperature, humidity, and atmospheric pressure, act to directly influence the MIE and thus the

22 Chapter 2 Review of IS Power Supplies and Intrinsic Safety LEL and UEL. The LEL and UEL are typically expressed as a percentage, given by the normalised ratio of the volume of the flammable gas or vapours to the volume of air. As an example, the explosive concentrations of Methane gas are between LEL of 5% and UEL of 15.9% [17] and the MEIC is in the range of 5.6% and 9% [5] (Note: values are dependent upon environmental conditions and test apparatus used). Ignition may also occur as a result of high surface temperatures. If the temperature of a surface in contact with an explosive concentration of a flammable gas exceeds the AIT an explosive ignition will occur. The AIT of Methane is 537 o C [17]. In electrical circuits, heat is dissipated through the components due to the finite resistance of the current path. The surface temperature of an electrical component is dependent upon the power dissipation and the power rating of the component Energy Transferred from the Electric Arc Among the optimal conditions required in order to produce an arc is the use of high voltage in conjunction with an adequate current supply. Under these conditions, it is easy to quickly establish and maintain the arc. In a spark generator, the electrodes are shaped to a point using low ohmic material that minimises the conduction of heat away from the arc. The electrodes are positioned so that the points face one another and they are separated by the spark gap (d sg ). When the electrode separation is less or greater than a critical distance called the quenching distance (d q ) a significant increase in the applied energy is required to cause an ignition of the MEIC of an explosive gas mixture. The quenching distance is related to the size of the gas molecules. The quenching distance of the MEIC of Methane-air mixture is between mm. According to Magison, the value of MIE for Methane gas (coal gas) is 0.28 mj [5]. This is determined by using a high voltage capacitive discharge test apparatus and conditions that optimised the transfer of energy from the arc to the explosive gas mixture. Under these conditions, there is minimal energy loss from the arc so that most of the electrical energy in the arc is transferred to the surrounding MEIC of explosive gas mixture. Once sufficient energy is transferred from the arc into the gas, an explosive ignition occurs. For the MEIC of Methane-air, the breakdown voltage required across this spark gap is in the range of 8-10 kv. In practice, MIE is only a concern in high voltage circuits.

23 Chapter 2 Review of IS Power Supplies and Intrinsic Safety The output voltages of IS power supplies are usually low, typically in the range of 5-48V. The mechanism of ignition for low voltage conditions according to Magison [5] is similar to that for high voltage. That is, for an ignition to occur, the energy transferred from the electric arc to the MEIC of the explosive gas will need to exceed the MIE of the particular explosive gas. At lower voltages, the available energy in the electric arc is less than that for a high voltage arc. Consequently, the transfer of energy is not at the same rate or efficiency as for a high voltage arc. Ignition kernel growth in the spark gap (contact closing) U sg (V), I sg (A) 50 d k > d q and E gas > MIE results in explosion E gas (mj) Spark gap Voltage U sg MIE d k > d q Energy transferred to gas E gas d k = d q Spark gap current I sg d k < d q Ignition kernel growth Assumed contact closing velocity 0.2 m/s Figure 2-2: Ignition kernel growth vs. ignition energy and quenching distance Time (µs) Spark gap d sg (µm) In the case presented in Figure 2-2 the spark gap (electrode separation) distance (d sg ) is decreasing at a constant rate assuming a constant velocity, approaching zero upon physical contact. The ignition kernel diameter (d k ) growth is dependent upon the energy transferred to the gas (E gas ) and the relative difference between the ignition kernel diameter and both the quenching and spark gap distances. This case is more complex than for a fixed spark gap distance and is explained by the three phases in the ignition kernel growth. In the first two phases the spark gap distance is assumed to be greater than both ignition kernel diameter and quenching distance. The initial phase of ignition kernel growth occurs once the arc is established evident by the increase in spark gap current (I sg ) and results in energy transfer from the arc to the gas. Ignition kernel growth is relatively slow as the energy transferred to the gas is less than optimal since the ignition kernel diameter is less than quenching distance.

24 Chapter 2 Review of IS Power Supplies and Intrinsic Safety During the second phase of ignition kernel growth sufficient energy has transferred to the gas so that the ignition kernel diameter is a similar size to the quenching distance. During this phase the ignition kernel growth is at a maximum as the energy transferred to the gas is optimal. In the third phase stage the ignition kernel growth slows as the energy transferred to the gas is again less than optimal since the ignition kernel diameter is greater than quenching distance. If during the second or the beginning of the third phases the ignition kernel has reached a size where it is self sustaining then an explosive ignition will occur. However, the further into the third phase where energy transfer is less than optimal the less likely an explosive ignition will occur. This is due to two factors. The first is that as the ignition kernel grows an increasing amount of energy is required to heat the increasing volume of gas molecules and to overcome the losses at the periphery of the ignition kernel. Secondly in the case of the closing contacts where the ignition kernel diameter equals or exceeds the spark gap distance then the electrode heating will further reduce the effective energy transferred to the gas Development of the Principles of Intrinsic Safety The basic concept behind intrinsic safety relies on incorporating energy limitation to ensure that an explosive ignition cannot occur through either spark or thermal ignition. Therefore there will be insufficient energy available to heat the components and, should a spark occur, there will be insufficient energy within the circuits to cause an explosive ignition. In an electrical circuit the possible sources of spark ignition include:- discharge of energy in a capacitive circuit when the circuit is closed discharge of energy in an inductive circuit when the circuit is opened intermittent making and breaking of a resistive circuit hot wire fusing Intrinsically safe equipment utilises energy limitation by limiting its current or voltage, and/or the duration of their occurrence. The spark test apparatus (STA), also known as the break flash apparatus, is used to determine the sparking potential of the

25 Chapter 2 Review of IS Power Supplies and Intrinsic Safety electrical circuit (refer to Chapter 3). Alternatively, should the electrical circuit be well defined, the sparking potential of the electrical circuit can be determined by assessment using ignition curves defined in the intrinsic safety Standards. The ignition curves are presented in AS and AS/NZS which is derived from the International Standard IEC79. These Standards include ignition curves (refer Appendix A 5) for the following well defined electrical circuits:- linear DC voltage source and series current limiting resistor linear DC voltage source and shunt capacitance with/without series current limiting resistor linear DC voltage source and series air-cored inductor and current limiting resistor These curves are used in Chapter 6 to assess the intrinsic safety compliance of active power supplies. In an electrical circuit the possible sources of thermal ignition include:- heating of a small gauge wire strand glowing of a filament or track on a printed circuit board high surface temperature of components [9] IS equipment utilise components whose power ratings have been de-rated such that the AIT of the hazardous atmosphere is not exceeded. Temperature rise tests are performed to identify potential thermal ignition sources.

26 Chapter 2 Review of IS Power Supplies and Intrinsic Safety IS Power Supplies IS power supplies are designed, manufactured and certified to meet specific criteria in accordance with Australian or International Standards. These Standards specify the amount of energy that an IS power supply is permitted to deliver to the IS circuit. In this section a brief history covers the period from the early IS power supplies through to the modern day. The modern IS power supply features, as well as their design methodologies are described Evolution of IS Power Supplies Early IS power supplies used by the British coal mining industry had considerably higher output current and voltages than those permissible today. The United Kingdom (UK) National Coal Board designed and certified a range of mains fed DC power supplies to be used by the underground coal mining industry [3]. These IS power supplies and their specifications are summarised in Table 2-1. Each of the mains fed IS power supplies had a standby battery as indicated by the left hand set of arrows in Table 2-1 in the case of electrical supply failure. The right hand set of arrows indicate the specified voltage of the IS circuit being driven by the respective mains fed IS power supplies. Table 2-1: UK. National Coal Board DC IS power supplies [3] The UK National Coal Board power supplies were used extensively throughout the industry until 1965, when the new and more sensitive German STA was introduced. A number of the existing IS power supplies had their certification revoked after it was found that they were capable of generating incendive sparks on the new STA.

27 Chapter 2 Review of IS Power Supplies and Intrinsic Safety During the period from 1965 to 1970 IS apparatus became more complex, especially with the introduction of semiconductors [3], and, as a result, the relevant Standards were tightened. The research performed during this period was on simple IS power supplies where the limits of intrinsic safety were derived from experiments using the new STA Modern IS Power Supplies One of the most significant events in the history of electronics was the introduction of semiconductors. This development had far reaching effects. The introduction of solid state electronic systems created a need for regulated and stabilised DC power supplies [13]. Intensive automation and remote control in modern coal mines has induced an exponential increase in the number of electrical apparatus with type protection i - intrinsic safety. This causes the need for high-power intrinsically safe power supplies being able to supply as many apparatus as possible [11]. The development of IS power supplies follows that of general-purpose power supplies. The technical advances in general-purpose power supplies include the use of more complex feedback techniques that enable output current fold-back protection and switch-mode techniques. Both of these techniques involve the use of non-linear devices. The features in the modern IS power supply include: - voltage regulation and stabilisation filters to remove electrical noise over-voltage protection using semiconductor crowbar protection overload and short-circuit current protection using current limitation sophisticated fault detection, shutdown, and reset circuitry Modern IS power supplies are significantly more complex than their predecessors and this adds significantly to the effort associated with designing and testing these products. The costs associated with accreditation for intrinsic safety have also increased accordingly [18], [19].

28 Chapter 2 Review of IS Power Supplies and Intrinsic Safety Design Methodologies of IS Power Supplies Designing IS power supplies according to Magison [6] is an iterative process and he defines the tasks as: 1. - Establish the intrinsic safety design objective. This will define the material and temperature classes and will determine the type of intrinsic safety Design the product Document the design for the certifying authority to save time and money by easing the evaluation and certification process Document the design in manufacturing drawings and specifications to make it easy to control the details relevant to intrinsic safety and its certification throughout the life of the design. Magison goes on to further clarify Task 2. In Magison s treatment of IS power supplies he states that the Standards for intrinsic safety contain graphs of characteristics for resistive circuits. This ignition characteristic is only valid for power supplies whose V-I characteristic is a straight line; that is; the Thevenin equivalent circuit is a voltage source in series with a resistor [6]. This is also confirmed by Dill and Kanty who state The intrinsic safety of a power supply with current limitation by resistors can be simply assessed using published reference curves [11]. Green and Thurlow [13] stated that the principal methods of The design of an intrinsically safe power supply can be based on one of two methods: resistive limitation or zener diode clipping. The authors then go on to present the utilisation of a third method which utilises semiconductors for current limitation and voltage regulation. By using electronic devices to limit the current or spark duration an attempt has been made to avoid the disadvantages of resistive limitation [11]. An alternate approach proposed by SIMTARS uses an eight-step design procedure presented in Table 2-2. This method is more detailed and is specifically aligned to the intrinsic safety Standards AS and AS/NZS

29 Chapter 2 Review of IS Power Supplies and Intrinsic Safety Table 2-2: Summary of SIMTARS recommended design methodology [9]

30 Chapter 2 Review of IS Power Supplies and Intrinsic Safety Types and Terminology of IS Power Supplies IS power supplies are categorised by the method used to limit the output voltage and current. It is applicable to both battery and mains fed power supplies. The intrinsic safety Standards define mains fed IS power supplies as associated apparatus. Associated apparatus include both non-is and IS circuits where the non- IS circuits cannot adversely affect the IS circuits. In the case of a mains fed IS power supply it is only the IS low voltage output stage of the power supply that is considered in this thesis. In this section three types of IS power supplies are discussed followed by the definitions of the terminology relating to IS power supplies Three Types of IS Power Supplies Johannsmeyer and Kraemer [12] refer to three basic topologies of IS power supplies: linear, trapezoidal, and rectangular. These are the descriptions of the geometric shape of the output voltage versus output current (V-I) characteristics as shown in Figure 2-3. The V-I characteristics illustrate the effect on output voltage as a slowly decreasing resistive load (R L ) is applied to the output terminals. As the resistive load decreases from infinity, the output current increases from zero. Linear power supplies are typified by straight line output V-I characteristics where the gradient is determined by the series current limiting resistor (R) in Figure 2-3 (a). The electronic components in the output stage of a linear power supply are passive and are well defined. The output stage of a linear power supply does not contain energy storage components such as capacitors or inductors. The compliance process is by assessment and the use of the ignition curves published in the Standards as discussed in Section Trapezoidal power supplies have two linear sections. The first of these is where the zener diode voltage regulator limits the output voltage for a range of load currents. The second is where the load current exceeds the range that the zener diode can regulate. The series current limiting resistor (R) determines the gradient of the second section. The electronic components in the output stage of a trapezoidal

31 Chapter 2 Review of IS Power Supplies and Intrinsic Safety power supply with the exception of the zener diode are passive and are well defined. Figure 2-3: Power supply circuit topologies and their V-I characteristic [12] The output stage of a trapezoidal power supply is similar to the linear power supply. Where it does not contain energy storage components, the compliance process is by assessment and the use of the ignition curves. In the case where the output stage contains energy storage components the sparking potential of the circuit will need to be tested using the STA. Rectangular power supplies also have two linear sections. The first section is where voltage regulation occurs for a range of load currents up to the maximum output current. The second section is a current limited section where the output voltage is reduced as the current exceeds the maximum output value and enters overload.

32 Chapter 2 Review of IS Power Supplies and Intrinsic Safety The two linear sections indicate two modes of operation of rectangular power supplies. The normal operation is a constant voltage mode and under fault conditions a current limiting mode with voltage reduction. The electronic components in the output stage of a rectangular power supply can be active or passive and generally include energy storage components. The combination of energy storage and active components makes the dynamic behaviour difficult to determine. Consequently, the compliance process requires both assessment and spark testing using the STA Definition of IS Power Supplies Terminology A number of terms associated with IS power supplies are in common use without any clarification in the published literature. The terminology applicable to power supplies is tabulated in Table 2-3 and Table 2-4 defined by the author. Table 2-3: Definition of active and passive power supplies Power Supply Passive Active Description A power supply that does not include internal components for either voltage or current regulation. A power supply that includes internal components used for voltage, current, or a combination of both voltage and current regulation. Table 2-4: Definition of linear and non-linear power supplies Power Supply Linear Non-linear Description Steady-state characteristics of output voltage vs. output current is a single straight line. Steady-state characteristics of output voltage vs. output current is not a straight line. May include multiple straight line segments. In Table 2-5 the types of IS power supplies have been described in both the terminology used by Johannsmeyer and Kraemer [12] and the more common terminology of Table 2-3 and Table 2-4.

33 Chapter 2 Review of IS Power Supplies and Intrinsic Safety Table 2-5: Defining the types of IS power supplies IS Power Supply Linear Trapezoidal Rectangular Description A linear passive power supply. Steady-state characteristics of output voltage vs. output current has a single straight line. A non-linear active power supply. Steady-state characteristics of output voltage vs. output current has two straight line segments: 1. Voltage regulation segment (Normal operation) 2. Non-regulated segment (Overload operation) A non-linear active power supply. Steady-state characteristics of output voltage vs. output current has two straight line segments: 1. Voltage regulation segment (Normal operation) 2. Constant current segment (Overload operation) It is the non-linear active power supply that produces a rectangular output characteristic that is the focus of the remainder of this thesis.

34 Chapter 2 Review of IS Power Supplies and Intrinsic Safety IS Active Power Supplies IS active power supplies characterise themselves by their ability to regulate the output voltage whilst the current demanded is within a specified range. If an excessive amount of current is demanded, or a short-circuit occurs, the voltage will drop rapidly to ensure that energy delivered to the circuit is below the minimum energy required to ignite the specified explosive atmosphere. As IS active power supplies commonly have a rectangular output characteristics they cannot be assessed using the ignition curves published in the Standard and their intrinsic safety must be determined by performing spark ignition testing using the STA. The V-I characteristics of the three types of IS power supplies discussed in Section are steady-state characteristics. Steady-state characteristics are determined from the static behaviour or the behaviour due to slow variations. The difference between static and dynamic characteristics in power supply units usually results from the presence of capacitors and from the finite bandwidth of semiconductor elements [14]. The transient characteristics of IS power supplies are important as a significant amount of energy in an IS active power supply can be delivered from the energy storage components to the output terminals under fault conditions. Tomlinson and Widginton [14] investigated the dynamic behaviour of power supplies by examining the slew rate of the power supply. This is the rate at which the output voltage recovers after a transient short-circuit load is removed. It was discovered that power supplies with output voltage slew rates below 200 V/µs appeared to be safer as higher values of MIC were required to cause an ignition. The authors highlighted the need for great care when testing the intrinsic safety of constant-current power supplies exhibiting limited slewing rates as the slewing rate could be effectively increased by the addition of common circuit loads. The authors experimental circuit was a power supply with limited slewing rate. The addition of an external shunt capacitance reduces the voltage slewing rate providing

35 Chapter 2 Review of IS Power Supplies and Intrinsic Safety a spark quenching effect. This spark quenching effect seems to make the circuit safer. By reducing the value of the external shunt capacitance the slewing rate increases and it is possible that the circuit will become unsafe. There is a certain range of external shunt capacitance values where the spark quenching effect provides additional safety. Intuitively the addition of series resistance to a circuit should make the circuit less incendive. However, in this case, the addition of series resistance between the power supply and the external shunt capacitor made the circuit incendive again as the slew rate is increased. Dill and Kanty [11] researched the dynamic behaviour of IS power supplies with the aim of reducing the time taken to design or modify and test power supplies. Dill and Kanty suggested that one possibility for solving this problem is to analyse the dynamic behaviour of the electronically regulated (active) power supplies. The results are then used to decide whether or not a new spark-ignition (STA) test for intrinsic safety has to be made. The method presented by Dill and Kanty [11] is a comparative method, which relies on having previously obtained data from the power supply in question. If the static values are higher than before, all previous tests have to be repeated. If the static values are unchanged or lower, the dynamic behaviour has to be checked. The analysis is made with a substitute load, which simulates dynamic events. Most suitable for this purpose are electronic load modules, which can be regarded as resistors whose values can be controlled with frequencies up to some 100 khz [11]. Dill and Kanty went on to explain in detail the points on the steady-state output V-I characteristics to perform the dynamic tests. Dill listed the maximum output voltage and current for power supplies with active current limitation as shown in Table 2-6. The designer or manufacturer can expect difficulty in obtaining intrinsic safety compliance for power supplies with ratings exceeding these limits. Table 2-6: Maximum values of V and I for Group I active power supplies [11]

36 Chapter 2 Review of IS Power Supplies and Intrinsic Safety The concept of performing dynamic tests on active power supplies to determine their intrinsic safety such as those performed by Tomlinson and Widginton [14], and Dill and Kanty [11] are further explored as part of this research in Chapter 4. Dill presented a paper [10] with the theme of regulated IS power supplies and then applied five examples to highlight some of the deficiencies in the Standards. A summary of the examples and their findings are given below:- Example 1 Wrong use of curves - Dill highlights that the application of the ignition curves is only to very simple circuits and that they do not apply to regulated power supplies. Example 2 Inductances with shunt diodes - Dill argued the case of not using a zener diode as a shunt across an inductive coil such as a solenoid. Shunt diodes are used to provide a discharge path for the stored energy in the inductor. Zener diode shunts in comparison to normal diode shunts will make the solenoid act faster. The case presented is of an electronically regulated power supply supplying an inductor with a back to back zener diode shunt in series with the STA. He concluded that, In the open loop, measured across the terminals of the STA, the voltage will be the addition of the supply voltage and the zener voltage. The arcs will receive more energy [10]. Example 3 Regulated power supplies - By using electronic limiting devices for current or spark duration it has been tried to avoid the disadvantages of the resistive current limitation [10]. Dill explains the necessity of using the STA and in addition a current regulating device. This device simulates a load, which reduces the slewing rate of the voltage in the circuit just like a three-pin regulator in a subsequent electronic device could do, and which is similar to the effect of an inductance. Finally, it is necessary to say, that also the maximum values for external inductance and capacitance C for regulated current and voltage limitation cannot be taken from the curves in the standard [10]. Example 4 Influence of capacitors in parallel - Dill explained that shunt capacitance across a regulated power supplies only has a spark quenching effect only in a certain range of values, where the effect prohibits

37 Chapter 2 Review of IS Power Supplies and Intrinsic Safety ignition [10]. The addition of resistance between the power supply and capacitor increases the slew rate and the risk of ignition. Example 5 Combination of L and C - Dill highlighted a pitfall for the unwary in assumptions made from reading the certificate. But no standard and nearly no certificate tells you, that the maximum permissible external inductance (L EXT ) is determined for external capacitance (C EXT ) = 0 and the maximum permissible C EXT is determined for L EXT = 0 [10]. Dill states that this is the reason the German test houses decided to always certify with values that can be combined. This is applicable to all IS apparatus with entity parameters including IS power supplies. A number of IS active power supplies circuit diagrams were analysed but due to SIMTARS privacy agreements and proprietary information no documentation is available for inclusion in this thesis. These IS active power supply circuits and the following two IS active power supplies were used to derive a functional block diagram presented in Appendix A 1:- J.J Sammarco [18] developed a regulated IS, rechargeable power supply for portable electronic equipment for underground use. The power supply uses a number of semiconductor devices including a series current regulator and silicon controlled rectifier (SCR) crowbar protection. The regulated output is DC 5 V at 4 A. United States Patent # is an IS active power supply, employing a binary current interrupter connected between the power source and the electrical load [19]. The circuit employs a semiconductor switch to isolate the output and utilises a flip-flop to reset the switch, and uses semiconductor current and voltage regulation.

38 Chapter 2 Review of IS Power Supplies and Intrinsic Safety Intrinsic Safety Standards Legislation and subsequent Standards on intrinsic safety have been used in England since 1911 [3]. In Australia, the British Standards Institute (BSI) Standard for IS apparatus (BS 1259) was used until 1968, when AS 1829 was introduced [7]. The current Australian and International Standards are discussed in Sections In Section the two current Australian Standards AS and AS/NZS are compared. This is followed by an explanation of the roles and responsibilities of the parties involved in ensuring intrinsic safety. The final section introduces the Australian third party testing and accreditation bodies Current Australian and International Standards In the Australian legislation, the Statutory Acts presented in Table 2-7 are used to define the legal responsibilities related to underground mining and the use of electricity. These Acts refer to Australian Standards publications and make these Standards legal documents. A national scheme is used to manage and monitor compliance to the relevant legislation. Table 2-7: Relevant Acts and Regulations [17] The standards for intrinsic safety on principal are the result of research work. Most of this work was done in the years from [10]. There are currently two Australian Standards applicable to the design and construction of intrinsic safety apparatus and they are the recently introduced AS/NZS series which will eventually replace the AS 2380 series as it is phased out. The AS/NZS series is a direct adoption of the International Electrotechnical Commission (IEC) IEC 79 series. The AS 2381 series covers the selection, installation and maintenance of intrinsic safety equipment. A Handbook covering

39 Chapter 2 Review of IS Power Supplies and Intrinsic Safety electrical equipment for hazardous areas has also been published by Standards Australia. The current Australian Standards pertaining to intrinsic safety are as follows [17]:- AS Electrical equipment for explosive atmospheres - Explosion-protection techniques Part 1: General Requirements AS Electrical equipment for explosive atmospheres - Explosion-protection techniques Part 7: Intrinsic safety i AS/NZS Electrical equipment for explosive atmospheres - Selection, installation and maintenance Part 1: General requirements AS Electrical equipment for explosive atmospheres - Selection, installation and maintenance Part 7: Intrinsic safety i AS/NZS :2000 Electrical apparatus for explosive atmospheres Part 0: General Requirements AS/NZS :2000 Electrical apparatus for explosive atmospheres Part 11: Intrinsic safety i Standards Australia, HB Handbook Electrical equipment for hazardous areas The relationships between the various international bodies and committees that govern the International Standards are quite complex. A number of authors [7, 20] have questioned this complexity and referred to the many vested commercial and political interests involved. In brief the IEC Standards (IEC x series) are used as a basis for the European Committee for Electrotechnical Standardisation (CENELEC) Standards (EN 50 0xx series). Each CENELEC Standard is adopted and renumbered to a British Standard (BS 5501.x series). With the exception of the United States of America (USA) nearly all other nations are progressing towards the adoption of the International Standard [7]. The adoption by Australia and other nations of the IEC x series of Standards is a significant step toward the development of harmonised International Standards. Dill [10] highlighted a number of deficiencies in the intrinsic safety Standards and in the certification documents. In his preamble, Dill subtly criticised the committee s responsible for the Standards for their lack of contact with researchers in the field and failure to incorporate the latest knowledge in the Standards.

40 Chapter 2 Review of IS Power Supplies and Intrinsic Safety During the course of this research program, the new Queensland Coal Mining Safety and Health Act 1999 (see Table 2-7) and associated regulations were invoked. The main change relevant to IS active power supplies is that Mines Department approvals are no longer required in Queensland. Mine managers now have the responsibility of ensuring certified IS active power supplies are fit for their intended purpose Comparison of AS and AS/NZS The differences between the two Australian Standards for IS power supplies are:- minimum value of voltage for simple circuits has increased from 1.2 V (AS 2380) to 1.5 V (AS/NZS 60079) reduction in the number of assessment curves from ten curves catering for construction materials (AS 2380) to six curves (AS/NZS 60079) minor variations of the values in the ignition curves Both Standards still fail to sufficiently clarify the measurement of let through energy when testing crowbar (over-voltage) protection circuitry in IS power supplies. Both of the Standards prescribe an upper limit but do not define how the measurement is to be performed. The nameplate information for IS apparatus requires improvement. The method used by the German testing bodies (of quoting the limitations of the ranges for external inductance and capacitance together) would reduce the potential for the unwary to inadvertently connect an IS device to an unsafe cable or load. For IS power supplies additional parameters need to be included which define the V-I characteristics of the power supply as well as internal resistance, inductance and capacitance [12] Participants in Ensuring Intrinsic Safety The legal roles and responsibilities of the parties involved in ensuring intrinsic safety are defined within the Statutory Acts and associated Standards. In this section, these roles are discussed at length and the costs associated with the intrinsic safety process are highlighted in order to clarify the participation of the various stake holders.

41 Chapter 2 Review of IS Power Supplies and Intrinsic Safety Designers and/or suppliers of intrinsic safety equipment have a responsibility to ensure that their equipment is both functional and safe. The design strategy of a commercial product traditionally involves compromise between design and manufacture cost, device performance, and market pricing structures while still satisfying the requirements of the intrinsic safety Standards. Market demands, new technologies and competition all act to influence designers in their quest to produce a saleable item. The certification process also imposes a considerable cost burden, which must be considered. These costs are all ultimately passed on to the product purchaser. The factors that determine the cost and/or duration of the certification process falling within the responsibility of the party seeking certification are [15]:- type of certification requested quality of the design and manufacture of the equipment nature and complexity of the equipment level of pre-compliance review quality, completeness and accuracy of the associated documentation time taken to modify and resubmit the equipment if required quality and responsiveness of the communications between the party seeking certification and the accreditation body The role of the third party certification body is to assess and test where necessary to determine conformance to an Australian and/or International Standard. The services provided by certification bodies are utilised by designers, suppliers, and users of intrinsic safety equipment. The assessment, testing and certification process are themselves covered by relevant Standards to which the certifying body must conform to ensure that it retains its accreditation, i.e. its authority to certify equipment. In Australia, any testing of explosion protected equipment must be covered by the National Association of Testing Authorities, Australia (NATA) laboratory accreditation and the certification activities accredited by the Joint Accreditation System of Australia and New Zealand (JASANZ). The main factors that determine the cost and/or duration of the certification process, which are the responsibilities of the certification body are [15]:-

42 Chapter 2 Review of IS Power Supplies and Intrinsic Safety assessment, testing and certification processes, which are the methods used to determine conformance to the requested Standard quality and responsiveness of the communications between the certification body and the party seeking certification the current work volume of the certification body The users of the intrinsic safety equipment have a responsibility to ensure that their equipment is functional and safe throughout the serviceable life of the equipment. This responsibility includes the application and usage, maintenance and repair of the equipment, establishment and maintenance of documentation, and other statutory and inspectorate requirements. It also includes timely response to addressing any issues arising from publication of safety alerts, product recalls, and requests for re-certification. Australian and International Standards bodies set the requirements by which certification is determined. They have a responsibility to maintain these Standards, while responding to industry trends, and advances in technology. They also need to ensure that the Standards remain relevant with acceptable levels of risk associated with the use of the equipment in specified hazardous locations. Australian has a number of industry associations such as the Australian Coal Association (ACA) and the Association of Electrical and Electronic Manufacturers Australia (AEEMA), which promote, lobby and influence matters that impact upon industries using IS equipment Accredited Intrinsic Safety Testing and Certification Bodies Third party testing bodies are used to establish that a particular apparatus or system complies with the specified Standard. There have been examples of differences in the interpretation of the Standards between the third party testing bodies, both on an international and national level [21]. The impact on the NSW coal mining industry, because of the Safety Alerts issued in 1998, also raised questions by industry observers on the assessment, testing and certification process [21], [2]. In Australia the two main accredited laboratories capable of certifying IS equipment are Safety In Mines Testing And Research Station (SIMTARS) and TestSafe

43 Chapter 2 Review of IS Power Supplies and Intrinsic Safety Australia. Both SIMTARS and TestSafe Australia issue certificates and approvals for conformance with Australian Standards and legislative requirements for [22]:- Certificates of Conformity for Groups I and II explosion protected electrical equipment Certificates of Conformity for electrical equipment used in NSW or Queensland coal mines, and others Safety In Mines Testing And Research Station (SIMTARS) acts as a semiautonomous, professionally independent division of the Queensland Government's Department of Natural Resources and Mines. The testing, calibration, certification and other specialised services for electrical equipment used in hazardous locations is carried out by the Engineering Testing and Certification Centre (ETCC). Evidence of conformity issued by SIMTARS include [23]:- NATA reports for Australian and equivalent International Standards Certificates of conformity to intrinsic safety Standards to AS , AS/NZS , and others Some of the major International accredited testing bodies include:- Health and Safety Executive mining (HSE (M)) in Britain Berggewerkschaftliche Versuchsstrecke (BVS) in West Germany Underwriters Laboratories Inc (UL) in America These testing bodies are authorised to certify to the International intrinsic safety Standard and their own national Standard [7].

44 Chapter 2 Review of IS Power Supplies and Intrinsic Safety Certification, Assessment and Testing of IS Power Supplies The certification process in Australia confirms compliance to one of the two Australian Standards AS or AS/NZS A testing and certification body determines conformance to the Standards by circuit analysis, spark ignition testing, or a combination of both. Conformance to the thermal ignition requirements of the Standards can be determined by temperature rise tests. In practice, a significant part of the certification process involves assessment and testing. Generally, the various testing and certification bodies regard their procedures and assessment methods as proprietary information. These are therefore not generally available to the public. As part of this investigation, the author has summarised SIMTARS intrinsic safety assessment and testing procedures and they are presented in Sections and The certification process requires a number of reviews to be performed at critical points within the assessment and testing process. During the latter stages of the process, a final review takes place and if compliance is confirmed an appropriate certificate is issued Certification Determining Conformance to a Standard AS or AS/NZS categorises IS electrical apparatus initially by their location relative to the hazardous area. IS electrical apparatus are able to be located within a hazardous area. Associated equipment must be located in a safe area but the interconnecting wiring may enter the hazardous area. The Standards then further categorise IS electrical apparatus by whether the equipment is self-contained, part of a system, or entity concept equipment [24]. Generally IS power supplies are categorised for accreditation as associated electrical apparatus and are certified as entity concept equipment or as part of an integrated system. Associated electrical equipment require the following output parameters to be defined: Maximum output voltage (U O ), Maximum output current (I O ), Maximum external capacitance (C O ), Maximum external inductance (L O ), and Maximum external inductance to resistance ratio (L/R).

45 Chapter 2 Review of IS Power Supplies and Intrinsic Safety Assessment of IS Active Power Supplies As the complexity of the electrical/electronic circuitry increases it becomes more difficult and less reliable to determine the equipment s conformance to the relevant Standards by analysis alone. The interaction between the load and the active components within the active power supply cannot be easily analysed. It is for this reason that use of the ignition curves is not applicable for IS active power supplies [24]. In most situations IS active power supplies are subjected to a combination of both circuit analysis and spark ignition testing using the STA. A summary of SIMTARS general intrinsic safety assessment procedure is shown in Table 2-8 [9]. IS active power supplies are subjected to this assessment procedure with the exception of the initial part of Step 7. At this point IS active power supplies assessed by SIMTARS are subjected to spark testing using the STA as described in Section There is scope to make a number of improvements in the assessment process by increasing the pre-submission work and documentation. This would be performed by suitably qualified designers. During the assessment procedure outlined in Table 2-8 a number of steps repeat the work performed earlier by the designer. These steps are listed as follows:- 1. Identify all sources of energy 2. Identify components on which intrinsic safety depends 4. Segregation of components by creepage and clearance distances 5. Circuit Calculations - ratings for all components 6. Circuit Parameters - maximum voltages and currents determined 7. Identification of potential ignition sources - sparking and heating The additional work outlined above, if performed and documented by designers, would provide a self-review. A second benefit is a reduction in the assessment time and cost. Magison [6] identified this in his design methodology Task 3 as presented in Section

46 Chapter 2 Review of IS Power Supplies and Intrinsic Safety Table 2-8: Summary of SIMTARS intrinsic safety assessment procedure [9]

47 Chapter 2 Review of IS Power Supplies and Intrinsic Safety Testing IS Active Power Supplies using the STA It is essential to ensure the intrinsic safety of the active power supply is tested using the STA under numerous variations of the load parameters and over the full range of its output characteristics to ensure that incendive sparking is not possible. These are time consuming tests and are not required for linear power supplies. The sparking potential of linear power supplies with well defined circuits can be determined by assessment alone using ignition curves. A summary of the SIMTARS general intrinsic safety testing procedure is presented in Table 2-9 [9]. Table 2-9: Summary of SIMTARS intrinsic safety testing procedure [9] It is an exhaustive process to establish whether a circuit is IS at all possible circuit configurations and values under both normal and fault conditions. If the apparatus is being certified under the Entity Concept, then in addition to establishing the IS status, Entity Concept parameters (L O, C O and L/R ratio) must also be determined using a trial and error method.

48 Chapter 2 Review of IS Power Supplies and Intrinsic Safety Of the research performed on the STA the main areas of interest relate to the mechanics and materials used for the electrode wire and contact disk. In addition, some research has been done on the application of the STA to testing high current IS apparatus [25]. In Chapter 3 properties of the STA are investigated further. There is scope for possible improvement in the testing phase of the certification process. The testing procedure outlined in Table 2-9 is a lengthy and tedious exercise, contributing significantly to the costs involved. The STA uses up to four wires located in the wire holder. These tungsten wires are subjected to flexing and after a period they either bend or break off. The wire holder rotates at 80 rpm and, therefore, it is difficult to monitor the state of the wires during a test. Wires bend or break frequently and, if unobserved, this may require the test to be repeated. By sensing the current flow using additional circuitry, broken wires may be easily identified. This would allow a test to halt immediately and the wire to be replaced. The test could then be resumed with minimum lost time. There are a number of requirements stipulated in the Standards that must be maintained throughout the test for the results to be valid. These items include:- concentration of the explosive testing gas flow rate of explosive test gas through the testing chamber nominated voltage and current to the device under test number of revolutions of the wire holder occurrence of an explosive ignition results of the pre and post STA sensitivity check Partial automation, data acquisition, and recording would improve the operation and efficiency of the testing phase of the certification process.

49 Chapter 2 Review of IS Power Supplies and Intrinsic Safety Summary Hazardous areas with explosive atmospheres are common in industries such as processing, manufacturing, and in underground coal mines. Statutory requirements may stipulate that IS devices must be used. IS power supplies have been designed, manufactured and certified to meet specific criteria in accordance with intrinsic safety Standards. These Standards specify the amount of energy that the IS power supply is permitted to deliver to the IS circuit. The intrinsic safety accreditation process may involve both assessment and testing to determine conformance to the intrinsic safety Standards. Increase in the complexity of modern IS active power supplies has complicated the assessment and testing process and extended the time taken to determine conformance. The sparking potential of linear and trapezoidal type power supplies can be determined using ignition curves included in the intrinsic safety Standard. Researchers have observed significant levels of output energy when subjecting rectangular type (active) power supplies to dynamic load conditions. The Standard specifies that the sparking potential of active power supplies must be determined using the STA. The STA is used to determine whether this amount of energy can cause an explosive ignition. The dynamic behaviour of active power supplies has been investigated by a number of researchers and this literature was reviewed. The determination of the output energy when subjecting the active power supply to dynamic loads is further investigated in this thesis. In this chapter, two of the research goals of Section 1.2 have been fulfilled. -(a) the different types of IS power supplies and the relevant terminology have been clearly defined, and -(b) the existing practices for assessment and testing of active IS power supplies have been reviewed and opportunities for improvement identified. The remaining research goals require determining the limits of intrinsic safety for active power supplies and to this end the principal instrument for determining the sparking potential of IS circuits, the STA is investigated in the following chapter.

50 Chapter 3 Electrical Investigation of the STA The spark test apparatus (STA) is used to determine the sparking potential of the intrinsically safe (IS) power supply by simulating fault conditions likely to occur in the field. The main concern for spark ignition is the presence of exposed conductors that can touch (make) and then separate (break). The STA is connected to the circuit under test, produces a variety of makes, and breaks at different velocities and intervals. In this chapter, the STA is investigated to determine how the sparking potential of a device under test is established. Sections 3.1 and 3.2 give an introduction to the STA and identify how the sparking potential of an IS apparatus is determined. In Section 3.3 the periodic nature and randomness of the STA is discussed. This is followed by the measurements of the electrical circuit of the STA in Section 3.4. In Section 3.5 the sensitivity of the STA is discussed. 3.1 Introduction to the STA The STA consists of a small gas chamber to which a flammable test gas of known concentration is applied at a low flow rate. The chamber contains an insulated wire holder and an insulated cadmium disk as shown in Figure 3-1. The wires on the wire holder and cadmium disk simulate the electrical contacts in a switch that makes and breaks the circuit under test [24]. The wire holder is able to secure up to four wires and is positioned above the cadmium disk so that their circumferences overlap, as shown in Figure 3-1 and Figure 3-2. The wires are located equidistant around the edge of the wire holder and extend down so that they can make contact with the surface of the cadmium disk. Only one wire is able to be on the cadmium disk at any one time. The wire holder is driven at 80 rpm in a clockwise (CW) direction and the cadmium at 19.2 rpm in a counter-clockwise (CCW) direction. As the wire holder rotates, one of the four wires makes contact with the cadmium disk, traverses the surface of the cadmium disk, and then disconnects as illustrated in Figure 3-2. A short period lapses before the next wire makes contact with the cadmium disk [24].

51 Chapter 3 Electrical Investigation of the STA Figure 3-1: Plan and elevation views of STA wire holder and cadmium disk [24] The cadmium disk has two parallel grooves on its surface which cause the wire and cadmium disk surface to intermittently break contact as the wire end leaves the edge of the groove and then re-make contact when it reaches the other side of the groove. The angle at which the wire departs from the edge of a groove and the tension of the wire determines the speed of departure at the break, the speed of arrival of the make, and the time period the wire is located in the groove [24].

52 Chapter 3 Electrical Investigation of the STA rpm Wire holder Wire Wire path across cadmium disk 19.2 rpm Chordal grooves Cadmium disk Figure 3-2: Oblique view of the STA wire holder and the cadmium disk The ability to accurately replicate test results using the STA is affected by the statistical probability that an explosive ignition may occur. Ignition probability is based upon the occurrence of a potentially explosive ignition event every 1600 revolutions of the wire holder.

53 Chapter 3 Electrical Investigation of the STA Low Voltage Electric Arcs and the STA The STA facilitates the discharge of the energy storage components within the electrical circuit under test by repetitively making and breaking the electrical circuit. The sparks produced by the STA are located in the spark gap between the end of the wire and the cadmium disk. The dielectric in the spark gap is the prescribed flammable test gas at a known concentration. The conditions under which the end of the wire and the cadmium disk make and break the circuit during the STA operation are illustrated in Figure 3-3 (a) to (g). Wire holder Wire holder Wire holder Wire Wire Wire v1 v1 v1 Cadmium disk Cadmium disk Scoured disk surface Cadmium disk (a) Wire approaches side of disk (b) Wire makes with side of disk (c) Wire traverses surface of disk wire Wire holder Wire v2 Cadmium disk groove Wire holder Wire v2 disk surface Magnified view of wire across disk surface Cadmium disk Cadmium disk (d) Wire breaks with edge of groove (e) Wire makes with edge of groove Wire holder Wire holder Wire Wire v3 v1 Cadmium disk Cadmium disk (f) Wire approaches edge of disk KEY: (g) Wire breaks with edge of disk v = the relative velocity between the wire and the cadmium disk v 1 = 208 mm/s v 2 = mm/s dependent on angle to chordal groove v 3 = 250 mm/s Figure 3-3: STA wire and cadmium disk making and breaking contact

54 Chapter 3 Electrical Investigation of the STA When the voltage applied across the terminals of the STA is greater than the breakdown voltage of the dielectric an arc is formed. The arc will persist only while the applied voltage is greater than the breakdown voltage of the dielectric and sufficient current is available. It is assumed that the wire and cadmium disk have been separated for a period sufficiently long for the capacitor to be charged. The discharge of the capacitor will generally occur when the wire is making contact with the cadmium disk as in Figure 3-3 (a) and (b), and in greater detail in Figure 3-4. The discharge of an inductive energy storage component will generally occur when the wire breaks contact with the cadmium disk. This is after a period of time where the wire and cadmium disk have been in contact for a sufficient period to allow the inductor to be energised as in Figure 3-3 (d) and (f). Resistive circuits may also cause spark ignition where there is an intermittent making and breaking of a high current circuit as in Figure 3-3 (c). The energy available from the apparatus under test at the terminals of the STA can significantly exceed the minimum ignition energy (MIE) of the explosive test gas in the STA chamber without causing an explosive ignition. As discussed in Section the energy available at the spark gap typically exceeds the MIE as the amount of energy transferred from the arc to gas is highly dependent upon the physical shape, arrangement and materials of the electrodes. The exact amount of energy transferred from the arc to the surrounding flammable test gas in the STA is difficult to determine exactly and is dependent upon:- distance between the wire and cadmium disk geometry of the wire end, disk surface or edge at the spark gap duration of the time spent near the quenching distance relative velocity between the wire and cadmium disk the instantaneous values of arc voltage and current

55 Chapter 3 Electrical Investigation of the STA (a) (b) (c) v Wire v Arc v Edge of Cadmium disk viewed from above. d = d0 The wire is approaching the edge of the cadmium disk. t = 0 The voltage across the wire and cadmium disk is insufficient to breakdown the spark gap distance d0. d = d1 t = t1 d = d2 t = t2 The voltage across the wire and cadmium disk is sufficient to breakdown the spark gap distance d1 and an arc is formed. The voltage across the wire and cadmium disk is sufficient to breakdown the spark gap distance d2 and the arc continues. (d) v d = dq t = t3 The separation distance d = dq and the arc is quenched. (e) v d = 0 t = t4 The wire is now in contact with the disk and the separation distance d = 0. KEY: velocity v, separation distance d, and time t Rotational speed of wire holder = 80 rpm and wire path radius r = mm Angular velocity ω = (2 * π * 80 rpm) / 60 = 8.38 rad/s Linear velocity v = ω * r = 8.38 * = mm/s Figure 3-4: STA making contact - discharging a capacitive circuit The only indication that sufficient energy has been transferred from the arc to the surrounding explosive gas mixture is the occurrence of an explosive ignition. This confirms that an amount of energy equal to or greater than the MIE of the explosive gas mixture has been transferred from the electric arc to the test gas in the spark gap between the wire end and the cadmium disk.

56 Chapter 3 Electrical Investigation of the STA Periodic and Randomness of the STA When the STA is connected to the direct current (DC) resistive circuit as shown in Figure 3-5 (a) and the wire on the cadmium disk encounters both of the chordal grooves as shown in Figure 3-5 (b), the current (I O ) and voltage (U O ) waveforms are depicted in Figure 3-6. The spikes in the waveforms indicate when the wire breaks contact with the cadmium disk. 10 V R = 237 Ω Uo Io STA Uo Um Rm = 1Ω OSCILLOSCOPE A B Chnl. A - Uo Chnl. B - Io = Um / Rm Steady state measurements:- STA contacts open U O = 10V, I O = 0 ma STA contacts closed U O = 42 mv, I O = 42 ma (a) STA connected to resistive circuit. Wire positions on path:- a wire makes with disk Wire path b wire breaks with edge of groove c wire makes with edge of groove f e d c a b d wire breaks with edge of groove e wire makes with edge of groove f wire breaks with disk Cadmium disk rotation CCW (b) Wire path traversing cadmium disk Figure 3-5: Test circuit with STA and wire path for a single traverse The measured waveforms in Figure 3-6 for the output voltage and output current show small variations whilst the wire is over the surface of the disk. These variations are due to physical irregularities of both the wire and the disk. Irregularities may include: scratches on the disk surface, loose particles on the disk surface, worn edges on the side of the disk and grooves, wire bending and wire splitting.

57 Chapter 3 Electrical Investigation of the STA The period that the wire is over the surface of the disk is dependent on the rotational speed of the wire holder and the free length of the wire. The waveforms in Figure 3-6 were measured with the wire holder rotating at 80 rpm. The end of a single wire is on the disk for a period of ms followed by a 57.1 ms period with the wire off the disk before the next wire makes contact. The total period for the end of a wire is ms. As the four wires are located equidistant on the periphery of the wire holder then the ms period corresponds to 90 o CW rotation of the wire holder. A 90 o rotation of the wire holder through the 50:12 gearbox ratio results in a 21.6 o CCW rotation of the cadmium disk. ma Output current Io ms V Output voltage Uo ms a b c d e f wire positions on path Note: Wire positions a,b,c,d,e and f correspond to Figure 3-5. Figure 3-6: Measured output current (I O ) and voltage (U O ) for a single traverse The STA is periodic based on 12.5 revolutions of the wire holder, corresponding to 3 revolutions of the cadmium disk. The periodic nature of the STA is illustrated in Figure 3-7.

58 Chapter 3 Electrical Investigation of the STA Period of STA sec [wire holder at 80 rpm] Period of Cadmium disk sec [3 revs per STA period] Period of wire holder 0.75 sec [12.5 revs per STA period] Wire ON disk Period of wire sec [50 wires across disk to STA period] Wire OFF disk Time sec Figure 3-7: Periodic make and break of wires on the cadmium disk The measured current waveform in Figure 3-8 depicts two revolutions of the wire holder. The waveform shows eight periods corresponding to each of the four wires in the wire holder traversing the cadmium disk. The first period (0 to 188 ms) shows the wire intersecting the two chordal grooves on the cadmium disk. In the following period (188 to 375 ms), the cadmium disk has rotated and the wire intersects only one of the chordal grooves. During the fifth period at the 800 ms point, the cadmium disk has rotated to an angle where the groove is tangential to the wire path. The wire scrapes along one of the edges of the groove rapidly making and breaking the circuit under test. ma Output current Io Period ms Figure 3-8: Measured periodic make and break waveform

59 Chapter 3 Electrical Investigation of the STA The tungsten wire is harder than the cadmium disk and therefore, it scratches the surface of the cadmium disk. As the STA is periodic, the scouring forms a pattern as illustrated in Figure 3-9 (b). The wire path forms an arc on the cadmium disk and the geometry of the arc has been determined graphically in Figure 3-9 (a). The pattern in Figure 3-9 (a) matches that observed in the picture of the cadmium disk in Figure 3-9 (b). When a new cadmium disk with a smooth surface is used, the STA has poor sensitivity. A conditioning process is described in the intrinsic safety Standard. The purpose of the conditioning process is to roughen the surface of the cadmium disk. At the end of the conditioning process, a distinct pattern is observed on the surface of the cadmium disk as observed in Figure 3-9 (b). This process gradually improves the sensitivity of the STA to the point where it will successfully have an explosive ignition using the calibration circuit mm mm Cadmium disk Arc angle deg mm Arc radius Wire path on disk (arc) (a) Graphically determined using QuickCAD release 7 (b) photo of scoured cadmium disk Figure 3-9: Geometry of arc scribed by the wire on cadmium disk The sensitivity of the STA varies and is dependent upon both its physical properties and environmental conditions. The wire condition and humidity appear to be the main factors contributing to variations in STA sensitivity. These are minimised by air conditioning (i.e. controlling the humidity) of the atmosphere within the testing laboratory and ensuring that the test gas is at constant temperature with a low moisture content. Wires can be prepared to minimise splitting and require regular cleaning and straightening. Wire replacement is recommended if any deterioration of the wire is noticeable.

60 Chapter 3 Electrical Investigation of the STA Electrical Circuit of the STA The individual circuit loops within the STA were measured using an LRC meter to derive the electrical circuit presented in Figure 3-10 (a). The distributed components in Figure 3-10 (a) have been lumped together and are presented in Figure 3-10 (b) which illustrates the equivalent circuit of the STA. During the measurement, it was observed that the value of R WH and R CD varied as the wire holder was rotated. This indicates that the brush contact resistance with the rotating shaft varies. Average values for R WH and R CD are presented in Figure The Standard specifies maximum allowable values for the STA as self-capacitance 30 pf (contacts open), self-inductance 3 µh (contacts closed), and resistance of 0.15 Ω (contacts closed, measured at 1 A DC) [24]. The measured value for the resistance is considerably higher than the permissible values specified in the intrinsic safety Standard. The small values of series inductance, resistance, and shunt capacitance do not significantly load the circuit under test. The voltage available at the terminal of the STA is present across the wire holder and cadmium disk before the wire making contact with the side of the cadmium disk. The low internal resistance of the STA ensures the maximum short-circuit current is present during the time that the wire is in contact with the cadmium disk.

61 Chapter 3 Electrical Investigation of the STA WH External terminals CD R WH L WH R CONTACT WH Wire holder CD Cadmium disk av. average value C OPEN av. R WH = 1064 mω WH L WH = 0.5 µh R CD L CD R CONTACT = 34 mω CD C OPEN = 7 pf L CD = 1.1 µh av. R CD = 489 mω (a) Electrical circuit (distributed component) of the STA WH External terminals CD R CLOSED L CLOSED C OPEN R CLOSED R CONTACT L CLOSED WH = R WH + R CD = 1553 mω = L WH + L CD = 1.6 µh CD R CONTACT = 34 mω C OPEN = 7 pf (b) Electrical circuit (lumped component) of the STA Figure 3-10: STA electrical circuit

62 Chapter 3 Electrical Investigation of the STA Sensitivity of the STA The intrinsic safety Standard prescribes a test to determine the sensitivity of the STA. The calibration circuit is a DC inductive circuit as shown in Figure 3-11 and is intended to cause an explosive ignition in the STA test chamber. The sensitivity test is performed before and after all spark ignition tests. If the post sensitivity test fails then the spark ignition test is invalid. Widginton states ignitions can arise simply because of variations which are known to occur in the sensitivity of the spark test apparatus, or as a consequence of the probabilistic behaviour of the spark test apparatus [16]. The probabilistic behaviour of the STA is termed the probability of ignition which, for the ignition curves included in the intrinsic safety Standard, represents a probability of 1 ignition in approximately 400 revolutions of the wire holder (4 wires) resulting in approximately 1000 sparks [16]. This equates to a probability of < 1% [5]. The transient conditions that occur during the break of the inductive calibration circuit is the period when the stored energy in the inductor is delivered to the STA terminals and ultimately to the wire end and cadmium disk. It is at these times that peak energy and potential for ignition occur. Generally an explosive ignition occurs near an instance of a rapid rise in the available energy and not during steady-state periods. A rapid rise in available energy coincides with peak output current as the circuit is opened and has a short duration related to the time constant of the circuit. The steady-state circuit measurements are presented in Figure 3-11 and typical waveforms for voltage and current for the calibration circuit during a non-explosive ignition are presented in Figure 3-12 (a). As the STA calibration circuit is opened, an arc was formed. The energy transferred during this period was less than the minimum ignition energy (MIE) of the surrounding explosive test gas. In the case presented in Figure 3-12 (b), an explosive ignition did occur as the circuit was opened. The total amount of energy transferred to the test gas caused ignition kernel growth to exceed the quenching distance and hence become a selfpropagating flame front. When an explosive ignition occurs the energy value associated with the MIE of the test gas must have been transferred from the arc to the test gas. During the explosive ignition the measured peak values of the output

63 Chapter 3 Electrical Investigation of the STA voltage 197 V and output current exceeded 3.2 A resulting in the output power exceeding 630 W for a short duration. 24 V RS 214 Ω RL L Io 24 Ω 92.9 mh STA Inductor Uo Uo Um Rm = 1Ω OSCILLOSCOPE A B Chnl. A - Uo Chnl. B - Io = Um / Rm Steady state measurements:- STA contacts open U O = 24V, I O = 0 A STA contacts closed U O = 101 mv, I O = ma Figure 3-11: STA calibration circuit with current measuring resistance

64 Chapter 3 Electrical Investigation of the STA Non-explosive ignition wire and disk separating Upper trace: Output voltage U O Lower trace: Output current I O (a) Typical output voltage and current - Wire is traversing chordal groove and as the circuit opens a non-explosive ignition occurs Explosive ignition wire and disk separating Upper trace: Output voltage U O Lower trace: Output current I O (b) Output voltage and current during explosive ignition - As the circuit opens a explosive ignition occurs Figure 3-12: Measured V and I waveforms for the STA calibration circuit

65 Chapter 3 Electrical Investigation of the STA Summary In this chapter the research goal in Section 1.2 of investigating the intrinsic safety testing practices for IS active power supplies has been fulfilled. The principal instrument for testing and determining the sparking potential of active power supplies and other electrical circuits is the STA. The STA replicates conditions that are likely to occur in the field and hence is able to determine if the output energy from the electric circuit is low enough for it to be regarded as conforming to IS requirements. The investigations carried out as part of this thesis have determined that the STA has a periodic make and break sequence between the wires and cadmium disk. However, the roughened surface of the cadmium disk results in variable conditions throughout the duration of contact between the wire and the disk surface. The chordal grooves on the cadmium disk provide a range of separation and approach velocities between the wire and cadmium disk. The STA is sensitive to the physical condition of the wire and cadmium disk. For optimal performance straight wires where the wire end is without splits or deformity, and a conditioned cadmium disk with a roughened surface is required. The STA is also sensitive to environmental conditions, in particular to humidity. The STA wire and cadmium disk apply an intermittent transient short-circuit and open-circuit loads to the circuit under test. It is the transient response of the circuit under test that determines the output power during this transient period and thus the available output energy. In the case of active power supplies, the energy stored in components within the active power supply is transferred to the STA during these transient periods potentially creating a low energy electric arc. The amount of energy available at the output of an active power supply is dependent on the transient characteristics of the active power supply and this is investigated in Chapter 4.

66 Chapter 4 Characteristics of IS Active Power Supplies In order to carry out the investigations described within this chapter a number of direct current (DC) intrinsically safe (IS) active power supply samples were provided by a local underground coal mining company. The IS active power supplies had similar electrical circuit topology and varied only in their nominal DC output ratings. The three sample IS active power supplies are identified as PS 1, PS 2 and PS 3, and their nameplate ratings are presented in Table 4-1 In this chapter an analysis of the output steady-state and transient characteristics of this type of IS active power supply is undertaken. The sparking potential of these power supplies is identified and defined by parameters that are measured from their steady-state and transient output characteristics. 4.1 Sample IS Active Power Supplies Due to proprietary privilege, no documentation or circuit diagrams were available for the sample IS active power supplies. The circuits were traced and analysed and a generic block diagram is included in Appendix A 1. The functional block diagram of the output stage of a sample IS active power supply circuit is presented in Figure 4-1. The IS active power supply includes active components in the voltage regulator, current limiter, and over-voltage crowbar protection circuitry. In this particular IS active power supply the current limiter includes the intrinsic safety control circuitry. AC Line supply side of circuit not shown Bridge rectifier C Low pass filter V reg Voltage Regulator C Low pass filter IS Control circuit IS Cntrl I Limit Current Limiter Crowbar protection + IS DC Output - Output current sensing circuitry Over voltage sensing circuitry Figure 4-1: Block diagram of sample IS active power supply DC stage

67 Chapter 4 Characteristics of IS Active Power Supplies Steady-state Output Characteristics The steady-state output characteristics of three sample IS active power supplies were measured and are presented in Figure 4-3 with the measured values of all three sample power supplies presented in Table 4-1. The load resistance R LOAD in the test circuit of Figure 4-2 (a) is slowly reduced from infinity (open-circuit) to shortcircuit. At each measuring point, the output voltage and current are recorded once they have stabilised. The two linear sections of the steady-state output characteristics in Figure 4-3 are emphasised to illustrate the differences between no-load, full-load and short-circuit values. The two linear sections in Figure 4-3 correspond to the two operating modes of the sample IS active power supply. The normal mode of operation is from no-load to fullload where the voltage regulator in Figure 4-1 maintains a constant output voltage as the output current and load resistance varies. The fault mode of operation is where the current demand exceeds the rated full-load current. In this mode of operation the current limiter in Figure 4-1 limits the output current to approximately the full-load value and reduces the output voltage for further demands of output current as load resistance is reduced. The current limiter effectively reduces the output power available. DC Active Power Supply + Uo (a) Steady-state test circuit - Io Amps Short circuit (SC) Full load (FL) IO SC IO FL FAULT MODE [Current limiting] R LOAD NORMAL MODE [Voltage regulation] No load (NL) Volts UO SC UO FL UO NL (b) Steady-state measurement Figure 4-2: Steady-state test circuit

68 Chapter 4 Characteristics of IS Active Power Supplies PS 1, 2 and 3 Steady-state Output Characteristics PS 1 12V 1A PS 2 12V 2A PS 3 18V 1.25A 2000 Output current Io (ma) Output voltage Uo (V) Figure 4-3: Steady-state output characteristics Table 4-1: Measured steady-state parameters sample active power supplies Power supply identifier PS 1 PS 2 PS 3 Nameplate Rated U O (V) Rated I O (A) Steady-state U O NL (V) measurements U O FL (V) U O SC (V) (Refer to I O NL (A) Figure 4-2(b) I O FL (A) for description) I O SC (A) Note: Full-load measurements are with the output current set to the rated I O where U O = Output voltage I O = Output current NL = No-load (R LOAD = open circuit) FL = Full-load (R LOAD = U O FL / I O FL Ω) SC = Short-circuit (R LOAD = 0 Ω)

69 Chapter 4 Characteristics of IS Active Power Supplies Transient Output Characteristics While the steady-state output characteristics define the circuit behaviour to slow variation in load, the transient output characteristics define the circuit behaviour to rapid variation in load. Transient output characteristics are determined by rapidly switching between two load conditions and are used to analyse the dynamic behaviour of the IS active power supply. The transient output characteristics include two plots, one of output voltage versus time and the other output current versus time. Both output voltage and output current values must be considered as they determine the instantaneous output power and hence the output energy. IS active power supplies with energy storage components can output significantly higher amounts of instantaneous power than their steady-state output power when subjected to transient load conditions. During a transient, energy from the energy storage components is transferred to the output terminals and to the load and poses a spark ignition risk. IS active power supplies with predominantly capacitive energy storage components such as the three samples investigated here exhibit this behaviour on circuit closures. Of particular concern are intermittent short-circuits with low circuit resistance. Under these conditions it is possible that a low voltage arc can be formed at the site of the short-circuit as described in Section The instantaneous output power of a power supply is dependent on the values of output voltage and output current and can be determined by the following equation [26]: p o (t) = v o (t) x i o (t) (4.1) where p o (t) = instantaneous power p at time t v o (t) = instantaneous voltage v at time t i o (t) = instantaneous current i at time t

70 Chapter 4 Characteristics of IS Active Power Supplies The output energy of a power supply is dependent upon the values of output voltage and output current occurring during a defined period as determined by the following equation [26]: e o (t) = t 1 tp o (t) dt (4.2) where e o (t) = energy from time t 1 to time t As discussed in Section energy from an arc is transferred to the proximate atmosphere. All testing was carried out with a minimal air or test gas flow rate, after the output energy integration is reset to zero after a reasonably long period of no circuit current. An accumulative effect can occur if consecutive energy transfers occur within a very short time period. Typically, the second energy transfer from the energy storage components is smaller due to insufficient charging time to store any significant amounts of energy Measuring Transient Characteristics using the STA The transient output voltage and current of an IS active power supply were measured using a storage oscilloscope connected across the STA as shown in Figure 4-4. DC Active Power Supply + - STA IO RM Uo OSCILLOSCOPE A UM B Chnl. A - Uo Chnl. B - Io = UM / RM R M Current measuring resistor (1Ω) Uo Volts Uo NL Chnl. A Uo SC Io Peak Io SC Io NL t1 Io Amps t1 Chnl. B t2 t2 Uo(t) Io(t) time time Figure 4-4: Transient characteristics test circuit with STA When the STA is operated, it was observed that consecutive wire and cadmium disk closures did not always yield the same output voltages and output currents waveforms as shown in Figure 4-5. The measured results of Figure 4-5 shows a transition where the STA contacts closed providing a short-circuit. Although it is

71 Chapter 4 Characteristics of IS Active Power Supplies reasonable to expect the voltage and current to be the same it may not necessarily be so due to debris on the disk, wire fatigue, or splitting of the wire end. 7.61A I O 1.76A 0A 12.61V 9.44V U O 2.41V 0 V t 0 t Peak =160µs t SS =7.12ms Upper trace: Output current (I O ) Lower trace: Output voltage (U O ) Figure 4-5: Measured transient output characteristics (STA) IS active power supplies with predominantly capacitive energy storage components pose a spark ignition risk during the contact closure transient period. The parameters that define this transient period are; no-load output voltage, period from non-zero output current to peak output current, peak output current and corresponding output voltage values, duration of the output current decay from the peak value to steady-state value, and the steady-state output current and corresponding output voltage values. These parameters are measured from the transient no-load to short-circuit load applied by the STA when the maximum peak output current occurs. The maximum peak output current sample is identified after recording a number of sample waveforms. Transient waveforms are included in the sample if the energy storage components are fully charged by a suitable open circuit time, and there is no STA wire bounce on initial contact with the cadmium disk. This excludes contact closures that occurred as the wire traversed the chordal grooves on the cadmium

72 Chapter 4 Characteristics of IS Active Power Supplies disk. The transient with the maximum peak output current is selected from the recorded samples and the parameters measured. A sample size of 20 was found to be statistically significant and typically included a sample with the maximum peak output current. This sample size was determined by repetitive experiments and obtaining a population of measurements. Due to the inherent variations in the STA, numerous measurements were required to derive a population. It was found that within any 20 consecutive samples of the population, a sample occurred where the peak output current was equal to the maximum peak output current of the total population. The measured parameter values of maximum peak output current and corresponding voltage for the transient no-load to short-circuit load are presented in Table 4-2. Data values of the sample transient voltage and current waveform were tabulated in an Excel spreadsheet (see Appendix A 2) where the value for output power was determined using equation (4.1). The output energy was determined using equation (4.2) where the integration was approximated using the trapezoidal method. Plots of output current, voltage, power and energy are also included in Appendix A 2. Table 4-2: Measured transient parameters test circuit with STA Initial contact of wire and cadmium disk Time Time I O (t) (A) U O (t) (V) P O (t) (W) E O (t) (mj) t t Peak 160 µs t SS 7.12 ms Times of interest t 1 Contact makes and circuit closes at time t 1 t Peak Peak output current occurs at time t Peak t SS Steady-state conditions occur at time t SS The value of output energy when the peak output current occurs is approximately 5.53 mj. This is considerably higher than the MIE of Methane 0.28 mj [5]. This amount of energy is a spark ignition risk, though as discussed in Sections only some of this energy is transferred to the ignition process.

73 Chapter 4 Characteristics of IS Active Power Supplies Measuring Transient Output Characteristics using a Relay To overcome variations inherent in the STA a relay contact was substituted for the STA wire and cadmium disk as shown in Figure 4-6. A signal generator was used to provide an independent square wave voltage to drive the relay coil. By varying the signal generator frequency and voltage the relay closing speed could be adjusted to a value between 0 to 200 mm/s. However, although contact bounce was a problem at higher speeds. The relay contact applied a short-circuit load and a storage oscilloscope was used to measure the output voltage and current during the transition. DC Active Power Supply + - Contact closes at t1 opens at t2 IO RM Uo OSCILLOSCOPE A UM B Chnl. A - Uo Chnl. B - Io = UM / RM R M Current measuring resistor (1Ω) Uo Volts Uo NL Chnl. A Uo SC Io Peak Io SC Io NL t1 Io Amps t1 t2 Uo(t) time Chnl. B Io(t) time t2 Figure 4-6: Transient characteristics test circuit with a relay The measured results as shown in Figure 4-7 shows a transition where a shortcircuit was applied. During the experiment it was observed that the highest values of peak output current occurred when the relay was operated manually with the relay test button. This produced a slow closure of the relay contact with no contact bounce. This led us to believe that a wetted contact may provide an alternate solution to the problem of contact bounce.

74 Chapter 4 Characteristics of IS Active Power Supplies A I O 1.74A 0A 12.78V 9.09V U O 1.81V 0V t 0 t Peak =70µs t SS =6.02ms Upper trace: Output current (I O ) Lower trace: Output voltage (U O ) Figure 4-7: Measured transient output characteristics (relay) The same sampling strategy used in Section was applied to obtain a sample that included the maximum peak output current. The measured parameter values of the maximum peak output current and corresponding voltage for the transient noload to short-circuit load are presented in Table 4-3:. Data values of the sample transient voltage and current waveform were tabulated in an Excel spreadsheet (refer Appendix A 3) where the value for output power was determined using equation (4.1). The output energy was determined using equation (4.2). Plots of output current, voltage, power and energy are also included in Appendix A 3. Table 4-3: Measured transient parameters test circuit with a relay Relay contacts closing Time Time I O (t) (A) U O (t) (V) P O (t) (W) E O (t) (mj) t t Peak 70 µs t SS 6.02 ms where t 1,t Peak, and t SS are defined in Table 4-2.

75 Chapter 4 Characteristics of IS Active Power Supplies After comparing the sets of values in Table 4-2 and Table 4-3, it is evident that the relay contact does not produce the same results as those for the STA. The relay circuit has lower inductance and series resistance than the STA circuit. In Section 3.4 the STA was found to have series inductance and resistance. The STA s series impedance limits the current and the resistance damps the output characteristic behaviour. The series inductance increases the time it takes to reach peak output current and the resistance predominantly reduces the value of peak output current although it can also affect the rise time. In Section 3.2 the STA wire and cadmium disk closing speed for the initial contact was calculated as 208 mm/s. The relay contact was closed manually at a much slower closing speed resulting in higher values of peak output current. The contact closing speed affects the rate of change of voltage and current thus the instantaneous values of voltage and current. The relay method of measuring the transient characteristics produces results that deliver higher values of output energy in a shorter time period. This equates to a higher risk of sparking potential in terms of intrinsic safety. As described in Sections and it is the initial rapid increase in the available output energy that determines the amount of energy transferred to an explosive test gas in close proximity to the spark gap. To improve the correlation between the two sets of results, the relay contact circuit could include values of inductance and resistance so that the relay circuit impedance is the same as the STA. Alternatively a factor could be used to establish the relationship between the two sets of results and to cater for the variations inherent in the use of the STA. Statistical probability and analysis studies would be required to determine the appropriate factor and this would require a suitable sample size to attain acceptable values of confidence Limitations in Measuring Transient Output Characteristics The short rise time of the peak output current approached the limit of the measuring equipment used in this experiment. A digital storage oscilloscope with a 200 MHz bandwidth and a sample rate up to 2.5 GS/s was used to measure the peak output current. Variations in measured peak output current occurred as a result of coincidence of the oscilloscope sample period and circuit closure. An oscilloscope with a wider bandwidth, and higher sample rate would reduce variation in the measurement of peak output current.

76 Chapter 4 Characteristics of IS Active Power Supplies Measurement of output current is sensitive to changes in circuit resistance so it is recommended that short wire lengths with a cross-sectional area of greater than 0.75 mm 2, low resistance joints and clean relay contacts are utilised. The value of peak output current is dependent on the impedance of the discharge path. The discharge path is shown in Figure 4-8. DC Active Power Supply RC C + - Contact IO RM Uo OSCILLOSCOPE A UM B Chnl. A - Uo Chnl. B - Io = UM / RM R C = Effective series resistance (ESR) of C R M = Current measuring resistor Figure 4-8: Power supply output capacitance external discharge path The discharge path includes the effective series resistance (ESR) of the energy storage capacitors, wire resistances, wire contact resistances, relay contact resistance, and a current measuring resistor (1 Ω). This resistance value was selected as a conservative approximation to ensure that the proposed alternative assessment method (PAAM) was more sensitive to ensure pass margins with high levels of confidence. Parasitic inductance and capacitance are minimised by separated short wire lengths. When measuring the transient response of a circuit with either capacitance or inductance, the instantaneous values of current and voltage are dependent upon the rate at which the voltage or current is changing, as shown in Table 4-4. Table 4-4: Instantaneous voltage and current for inductors and capacitors Inductor L e L = L di / dt v R = i * R i i = 1/L e L dt i = v R / R v R R E = v R + e L = i * R + L di / dt E i Capacitor C i = C dv C / dt i = v R / R e L L vr v C = 1/C i dt v R = i * R E E = v R + v C = i * R + 1/C i dt = RC dv C / dt + v C vc where E applied source voltage i instantaneous circuit current v R instantaneous voltage across resistor R e L instantaneous voltage across inductor L v C instantaneous voltage across capacitor C R C

77 Chapter 4 Characteristics of IS Active Power Supplies Transient Characteristics of Sample IS Active Power Supplies Three transient output characteristics were obtained from the sample IS active power supplies. Transient output characteristics were obtained for both the normal and fault modes of operation to separately determine the transient behaviour of the relevant blocks in Figure 4-1. The third transient output characteristics encompassed both modes of operation and determined the transient behaviour during the transition between operational modes. For each of the three transient output characteristics a transient load is applied, the output voltage and current then stabilise. This is followed by the removal of transient load so that there are two transitions during a single test. The transient output characteristics for the normal mode of operation are determined by rapidly changing the load from no-load to full-load at time t 1 and, after a period to stabilise, rapidly removal of the load from full-load to no-load at time t 2. The test circuit and transient output characteristics for U O and I O are illustrated in Figure 4-9. During the transition from full-load to no-load at time t 2 in Figure 4-9 the output voltage over shoots and has a damped oscillation as it stabilises at the steady-state no-load output voltage as illustrated in the detail inset. Output current reduces rapidly from the steady-state rated full-load value to zero. The oscillation near time t 2 has a short period and decays quickly. During this transition, there is an increase in the output energy which is a potential source for spark ignition. The parameters defining the period near time t 2 are the values of the first two peaks of output voltage and the corresponding output currents. DC Active Power Supply + - R FL Contact closes at t1 opens at t2 Rm Uo OSCILLO- SCOPE A Um B Chnl. A - Uo Chnl. B - Io = Um / Rm Uo Volts Uo Peak Uo NL Uo FL Io FL Io NL t1 Io Amps Io(t) Uo(t) time time R FL full-load resistance U O output voltage R m current measuring resistor U m voltage across R m NL No-load FL Full-load I O - output current Figure 4-9: Active power supply NL to FL transient characteristics t1 t2 t2 Detail

78 Chapter 4 Characteristics of IS Active Power Supplies The transient output characteristics for the fault mode of operation are determined by the transition from full-load to a short-circuit at time t 1 and, after a period to stabilise, a rapid reduction in the load from a short-circuit to full-load at time t 2. The test circuit and transient output characteristics for U O versus time and I O versus time are illustrated in Figure During the transition from full-load to short-circuit at time t 1 in Figure 4-10 the output current rises rapidly to a peak followed by a non-linear decay to the steady-state fullload current. The output voltage reduces rapidly from the steady-state full-load value to the steady-state short-circuit value. During this transition, there is an increase in output energy, which is a potential source for spark ignition. The parameters that define the period near time t 1 are the value of peak output current and the output voltage. DC Active Power Supply + Contact closes at t1 opens at t2 R FL Uo A OSCILLO- SCOPE Uo Volts Uo FL Uo SC Figure 4-10: Active power supply FL to SC transient characteristics Uo(t) time Um B t1 t2 Chnl. A - Uo Io Amps Chnl. B - Io = Um / Rm Rm Io Peak - Io SC Io(t) Io FL time t1 t2 R FL full-load resistance U O output voltage R m current measuring resistor U m voltage across R m FL Full-load SC Short-circuit I O - output current The test circuit and transient output characteristics of a no-load to short-circuit transition are illustrated in Figure During the transition from no-load to shortcircuit at time t 1 in Figure 4-11 the output current rises rapidly to a peak followed by a non-linear decay to the steady-state full-load current. The output voltage reduces rapidly from the steady-state no-load value to the steady-state short-circuit value. During this transition, there is an increase in the output energy, which is a potential source for spark ignition. The parameters that define this period are the value of the peak output current, the time constant of the exponential decay, and the output voltages at these points.

79 Chapter 4 Characteristics of IS Active Power Supplies DC Active Power Supply + Contact closes at t1 opens at t2 Uo OSCILLO- SCOPE Uo NL Uo(t) A Uo SC time Um B t1 t2 Chnl. A - Uo Io Amps Chnl. B - Io = Um / Rm Rm Io Peak - Io SC Io(t) Io NL time t1 t2 R m current measuring resistor U O output voltage FL Full-load U m voltage across R m SC Short-circuit NL No-load I O - output current Figure 4-11: Active power supply NL to SC transient characteristics Considering the transient output current responses in Figure 4-10 and Figure 4-11 the initial current rise at time t 1 is attributed to the capacitive energy storage components in the circuit of Figure 4-1 discharging into the short-circuit load. As the contacts in the test circuit are closing at time t 1 an arc is formed and output energy is transferred to the arc. The peak output current and the initial part of the decay near time t 1 are caused by the rapid discharge of the energy storage capacitors. The peak of the initial output current rise is dependent upon the voltage across the energy storage capacitors and the circuit resistance between the energy storage capacitors and the short-circuit. The later part of the output current decay is due to the non-linear components in the intrinsic safety control and current limiter circuit of Figure 4-1. As the output current demand exceeds the rated value the IS control circuit drives the current limiter. This increases its resistance to limit the output current, reduces the output voltage, and limits the output power. The time between time t 1 and when the peak output current is reached is the response time of the current sensing and intrinsic safety control circuit of Figure 4-1. After the initial peak the output current returns to the steady-state short-circuit value. The period between the peak output current and when steady-state short-circuit values are reached is the response time of the intrinsic safety current limiter circuitry. On removal of the transient short-circuit at time t 2 there is no evidence of output voltage oscillation. The oscillation observed in the full-load to no-load transient

80 Chapter 4 Characteristics of IS Active Power Supplies characteristics of Figure 4-9 has been damped by intrinsic safety control and current limiting circuits which are active during the initial part of the short-circuit to no-load transition. The output current drops rapidly from the steady-state short-circuit value to zero. This indicates that there are minimal inductive energy storage components in the output stage of this type of IS active power supply. Values of the no-load to short-circuit transient characteristics voltage U O (t) and current I O (t) were tabulated in an Excel spreadsheet (refer Appendix A 4) where the value for output power as shown in Figure 4-12 (b) was determined using equation (4.1). The output energy as shown in Figure 4-12 (b) was determined using equation (4.2) where the integration was approximated using the trapezoidal method to calculate the area under the output power versus time curve. The peak output power occurs with the peak output current shortly after time t 1. The output energy rises rapidly from time t 1 to a knee and then continues to slowly increase due to the steady-state output power. The value of the output energy at the knee is the transient output energy rise and is the available energy in the arc that potentially can be transferred to the surrounding explosive test gas. The time between t 1 and the knee is the duration of the arc. On the removal of the transient short-circuit at time t 2 the output power drops rapidly to zero and the output energy ceases to rise.

81 Chapter 4 Characteristics of IS Active Power Supplies V 1A PS No-load to Short-circuit output characteristic Uo Io Output volts (Uo) time (ms) Output current (Io) Amps (a) Output current I O (t) and voltage U O (t) 12 V 1A PS No-load to Short-circuit output characteristic Po Eo Output power (Po) W time (ms) Output energy (Eo) uj (b) Output power P O (t) and energy E O (t) Figure 4-12: Active power supply NL to SC transient characteristics

82 Chapter 4 Characteristics of IS Active Power Supplies Summary The steady-state output characteristics of the sample IS active power supplies identified a normal mode where the output voltage is regulated and a fault mode where the output current is limited. Transient output characteristics can be determined by measuring instantaneous output voltages and currents using a storage oscilloscope and a relay contact to switch between two load conditions. Output voltage and output current values can be measured during the transient period. The transient output characteristics of the sample IS active power supplies identified a number of transient load conditions where the output power is significantly higher than the maximum steady-state output power. During these transient load conditions, there is a rise in the available output energy. This is a potential source of spark ignition. During the no-load to short-circuit load transient period there was a change over between the modes of operation of the sample IS active power supply, from normal mode to fault mode and during this time the highest simultaneous values of voltages and current were measured. The sample IS active power supplies analysed in this section have transient output characteristics consistent with circuits containing predominantly capacitive energy storage components. The parameters that define the transient output current and voltage in this chapter are used in Chapter 5 to develop a proposed alternative assessment method (PAAM).

83 Chapter 5 Development of the PAAM The research performed by Dill and Kanty [11] established a way of determining the sparking potential of a circuit utilising a comparative method. If the static and transient output characteristics of an intrinsically safe (IS) active power supply were recorded then any time later the sparking potential of that same power supply could be determined by comparing its present static and transient output characteristics with the recorded characteristics. The implications are that the steady-state and transient output characteristics contain sufficient information to determine the sparking potential of a circuit. Three alternative assessment methods are discussed in this chapter. In Section 5.1 the first two methods are briefly described followed by a third method based on the development of an equivalent circuit. The third method is the proposed alternative assessment method (PAAM). In Sections 5.2 to 5.4 the equivalent circuits models used in the PAAM are developed. The PAAM and its limitations are discussed in Sections 5.6 and 5.7 respectively followed by the conclusions in Section Assessment Methods for IS Active Power Supplies The first method is based on the determination of a finite value for the output energy derived from the transient output characteristics of an active power supply. This value of output energy could then be used to determine whether the active power supply s sparking potential is low enough to be regarded as intrinsically safe. Whilst this appears to be a simple technique, consideration should be given to the test conditions under which the transient output characteristics of the active power supply are produced. The test conditions should be such that there is an optimal transfer of energy from the electric arc to the surrounding test gas. The effective amount of energy transferred to the ignition process needs to be determined and a relationship established between the energy transferred to the test gas and the sparking potential in terms of intrinsic safety limitations. A determination of the effective amount of energy transferred to the ignition process and the development of a relationship between this energy and the sparking potential is beyond the scope of this thesis.

84 Chapter 5 Development of the PAAM The second method uses circuit analysis of the active power supply to determine the maximum power transfer under transient short-circuit conditions. This would require impedance matching between the internal impedance of the active power supply and the impedance of the transient short-circuit. Under transient short-circuit conditions the internal impedance of the active power supply can vary significantly. An assessment based on analysis of dynamic impedance was deemed too complex for consideration as a practical assessment method to determine sparking potential. The third method entitled proposed alternate assessment method (PAAM) is developed throughout the remainder of this thesis and features the modelling of an IS active power supply via the use of an equivalent circuit. Ideally the equivalent circuit would simplify the IS active power circuit, containing fewer components while still producing the same output characteristics as the IS active power supply. According to Dill and Kanty [11] the equivalent circuit can be used to establish the sparking potential if it has the same steady-state and transient output characteristics as the IS active power supply. The existing assessment method using the ignition curves included in the intrinsic safety Standard (refer Appendix A 5) is applicable to well defined circuits. A well defined circuit is a circuit such as a direct current (DC) voltage source and comprises of one of the following component combinations: a series resistor, or resistor and inductor, or resistor and capacitor. If the equivalent circuit is one of these well defined circuits then its sparking potential and that of the IS active power supply can be determined by using existing assessment techniques. Two equivalent circuit models are presented in this chapter. The first equivalent circuit discussed is entitled RLC equivalent circuit model where the circuit topology includes resistance, inductance and capacitance. The second equivalent circuit discussed is simplified RC equivalent circuit model as the circuit topology includes only a resistance and a capacitance. The RC equivalent circuit model is a well defined circuit.

85 Chapter 5 Development of the PAAM The RLC Equivalent Circuit Model The RLC equivalent circuit model attempts to model both conditions that occur where a transient output energy rise is observed during both the transient application of a short-circuit and the transient removal of a full-load. The circuit topology of the RLC equivalent circuit as presented in Figure 5-1 is determined by analysing the output characteristics of the sample IS active power supply. The steady-state output characteristics illustrated in Figure 4-2 show that the full-load voltage is slightly less than the no-load voltage, indicating the existence of a series resistance R S. The first transient considered is the no-load to short-circuit transition as described in Figure At time t 1, when the short-circuit is applied, the current rapidly increases from zero to a peak value, followed by a non-linear decay to the steadystate short-circuit value. The voltage during this period decays from the steady-state no-load voltage to the steady-state short-circuit voltage. This indicates a shunt capacitive energy storage component C with a corresponding series resistance R C which includes the effective series resistance (ESR) of the capacitor. The second transient considered is the full-load to no-load transition described in Figure 4-9. At time t 2, when the full-load resistance is removed, the current decays from steady-state full-load value to steady-state no-load value, indicating a series inductive energy storage component L with a corresponding series resistance R L. The output voltage exhibits an overshoot followed by an oscillation that decays to the steady-state no-load voltage, indicating a damped oscillatory circuit. US RS + RL L RC C Io + Uo U S - DC voltage source R S - Source resistance L - Inductor R L - Inductor resistance C - Capacitor R C - Capacitor ESR resistance - U O - Output voltage I O - Output current Figure 5-1: PAAM - RLC equivalent circuit model topology

86 Chapter 5 Development of the PAAM The damping factor (ξ) of a series RLC circuit exhibiting an under damped oscillation can be estimated from the ratio of the magnitude of the first two overshoots of the oscillation. The natural frequency of the oscillation (ω n ) can be estimated using the damped oscillation frequency (ω d ) and the damping factor [27]. The characteristic equation for the damped second-order response can be solved so that the damping factor and natural frequency are related to the series circuit component values. The component values for the RLC equivalent circuit model are determined using the equations presented in Table 5-1. The parameters measured in Table 5-1 are determined from the steady-state and transient characteristics of an IS active power supply. Table 5-1: Component equations for the RLC equivalent circuit model Component Equations U S = U O NL R S + R L = U O NL I O SC - R LOAD R C = U O NL I O Peak Note (i) Underdamped case ξ < 1, ω d < ω n Critically damped case ξ = 1 Over damped case ξ > 1 Description U O NL = SS no-load circuit voltage R LOAD known (external component) Note(i) I O SC = SS short-circuit current I O Peak = TS(i) peak output current Note(i) U O exhibits a damped oscillation U O exhibits an exponential like behaviour U O exhibits an exponential like behaviour L = (R S + R L + R C ) (2*ω n *ξ) C = 1 (L*ω n 2 ) Damping factor ξ = Natural frequency ω n = log e x 1 x 2 (π 2 - (log e x 1 x 2 ) 2 ) ω d (1 - ξ 2 ) where x 1 = amplitude of first overshoot of TS(v) x 2 = amplitude of first undershoot of TS(v) T = period of TS(v) oscillation ω d (damped frequency) = 1/T SS Steady-state characteristics in Figure 4-2 TS(i) NL SC - Current transient characteristics in Figure 4-11 TS(v) FL NL - Voltage transient characteristics in Figure 4-9 Note (i) - In some cases where R C or R L are calculated as low ohm values, special component types are selected such as a capacitor type with low ESR or manufactured such as an inductor with low internal resistance. [27] [27]

87 Chapter 5 Development of the PAAM Experimental Verification of the RLC Equivalent Circuit Model Once the RLC equivalent circuit model was defined, component values were determined for an over damped and under damped circuit. Experimental RLC equivalent circuits were constructed using the component values listed in Table 5-2, and tested to measure the steady-state and transient output characteristics. The value for R S is higher than typically found in power supplies. A high value for R S was used to ensure that the time constant involving the inductor was significantly different from the time constant related to the capacitor. This would allow identification of their respective affects on the circuit. Table 5-2: Experimental RLC equivalent circuit model component values Component Value Over damped Under damped U S DC voltage source 10 V 10 V R S Series resistance 216 Ω 216 Ω L Inductor (air cored) 92.8 mh 92.8 mh R L Inductor resistance 24 Ω 24 Ω C Capacitor µf 972 nf R C Capacitor (ESR) 0.91 Ω 5.2 Ω R M Current measuring resistor Ω Ω ξ - Damping factor The under and over damped experimental RLC test circuits presented in Figure 5-2 (a) produce the same steady-state characteristic for both the under and over damped cases as shown in Figure 5-2 (b). The DC voltage source U S has a current limit that is activated as the current demand exceeds full-load value. The steadystate characteristics for the under and over damped experimental RLC equivalent circuits formed a rectangular shape consistent with an active power supply.

88 Chapter 5 Development of the PAAM US RS + RL L RC C IO + UO RLOAD Output current (Io) ma Steady-state output characteristic RLC equivalent circuit model Output voltage (Uo) (Refer Table 5-2 for component values) (a) Experimental RLC equivalent circuit (Measured values) (b) Steady-state output characteristic Figure 5-2: Experimental RLC equiv. cct. and steady-state characteristic The under and over damped experimental RLC equivalent circuit output transient characteristics are measured using the circuit shown in Figure 5-3. The DC voltage source U S had its current limiter de-activated for the measurement of the transient output characteristics. In the case of the over damped experimental RLC equivalent circuit the no-load to short-circuit output transient is presented in Figure 5-4 and the short-circuit to no-load output transient presented in Figure 5-5. US RS + RL L RC C Io + Uo Contact closes at t1 opens at t2 RM UO UM OSCILLOSCOPE A B Chnl. A - Uo Chnl. B - Io = UM / RM (Refer Table 5-2 for component values) - Figure 5-3: Experimental RLC equivalent circuit transient tests

89 Chapter 5 Development of the PAAM Note(i) Io 41.59mA 23.15mA 0 A 9.99V U O 70mV 0 V t 0 t Minimum =0.23ms t SS =2.04ms t Peak =20µs Note(i) - Output current peak I O Peak (3.34 A at 20µs) not shown in this waveform Upper trace: Output current (I O ), Lower trace: Output voltage (U O ) Figure 5-4: Over damped RLC equiv. cct. NL to SC transient characteristics The peak output current can be estimated from an analysis of the capacitor discharge path by the following equation: I O Peak U O NL R C + R M...(5.1) The estimated value of peak output current for the over damped case is 4.1 A. It is expected that this estimated value will be higher than the measured value due to the exclusion of current path through the DC voltage source U S and circuit inductances. The measured value of 3.34 A is lower because of additional circuit loading by relay contact resistance, oscilloscope probes, current sensing resistance, and parasitic inductance.

90 Chapter 5 Development of the PAAM I O 41.65mA 0A 9.97V U O 70mV 0V t 0 t SS =14.76ms Upper trace: Output current (I O ) Lower trace: Output voltage (U O ) Figure 5-5: Over damped RLC equiv. cct. SC to NL transient characteristics When the experimental RLC equivalent circuit model (over damped) transient output characteristics in Figure 5-4 and Figure 5-5 are compared with those measured from sample IS active power supply. It is observed that the transient output current characteristics of the RLC equivalent circuit model in Figure 5-4 near time t Minimum falls below the steady-state value. The transient current characteristic of the sample IS active power supply as in Figure 4-12 (a), at no stage falls below the steady-state values. As a consequence, the output energy of the RLC equivalent circuit model is significantly lower during this period due to the current I O (t) undershoot directly after the peak output current at time t Peak. As the short-circuit load is removed during the short-circuit load to no-load transition at time t 2 the output voltage characteristics of the experimental RLC equivalent circuit model in Figure 5-5 replicates the behaviour of the sample IS active power supply illustrated in Figure The output power during this period rapidly drops to zero as the circuit is opened.

91 Chapter 5 Development of the PAAM In the case of the under damped experimental RLC equivalent circuit the no-load to short-circuit output transient is presented in Figure 5-6 and the short-circuit to noload output transient presented in Figure 5-7. These output transient characteristics attempt to replicate the behaviour of the sample IS active power supply at times t 1 and t mA I O 41.74mA 7.29mA 0A 9.95V U O 70mV 0V t 0 t Minimum =70µs t Peak =10µs t SS =1.81ms Upper trace: Output current (I O ), Lower trace: Output voltage (U O ) Figure 5-6: Under damped RLC equiv. cct. NL to SC transient characteristics Using equation 5.1, the estimated value of peak output current for the under damped case is 1.49A. As discussed previously it is expected that this estimated value will be higher than the measured value. The measured value of 89.5 ma is lower than expected because of additional circuit loading as previously discussed. The experimental RLC equivalent circuit model (under damped) transient output characteristics in Figure 5-6 and Figure 5-7 are compared to those measured from sample IS active power supply. The transient output current characteristics of the RLC equivalent circuit model in Figure 5-6 near time t Minimum falls below the steady state value. As with the over damped case the output energy of the RLC equivalent circuit model (under damped) is significantly lower during this period. Electronic

92 Chapter 5 Development of the PAAM circuit simulation of the RLC equivalent circuit model indicated that the inductance (L) is primarily responsible this behaviour. I O 41.63mA 0mA 15.27V 10.34V 10.01V 8.65V U O 0V t 0 t OS =630µs t OS2 =2.69ms t SS =4.28ms t US1 =1.66ms Upper trace: Output current (I O ), Lower trace: Output voltage (U O ) Figure 5-7: Under damped RLC equiv. cct. SC to NL transient characteristics In Section 4.4 it was established that the output energy of the sample IS active power supply during the transition from no-load to short-circuit load at time t 1 is significant and poses a spark ignition risk. This is a critical period and the RLC equivalent circuit model would be required to accurately predict the behaviour of the sample IS active power supply. A simplified model can be used to predict the behaviour during the no-load to short-circuit load transition at time t 1.

93 Chapter 5 Development of the PAAM The RC Equivalent Circuit Model The RC equivalent circuit model as shown in Figure 5-8 is the same as the RLC equivalent circuit model with the exception that the inductance component has been removed. The RC equivalent circuit component values can be determined from the steady-state and transient characteristics of an IS active power supply. US RS Io + RC C Uo U S - DC voltage source R S - Source resistance C - Capacitor R C - Capacitor ESR - U O - Output voltage I O - Output current Figure 5-8: PAAM - RC equivalent circuit model topology Only the steady-state and transient no-load to short-circuit characteristics of the sample IS active power supply are required to determine the component values. Table 5-3 shows the equations required. Table 5-3: Component equations for the RC equivalent circuit model U S = U O NL Component Equations R S = U O NL I - R L O SC R S R C = (R S + R L ) I O SC.R S I O Peak + I - R L Note(i) O SC τ.(r S + R L ) C = (R S. R C + R S. R L + R L. R C ) Description U O NL = SS output voltage R L known (external component) I SC = SS short-circuit current I O Peak = TS(i) peak output current τ = time constant of TS(i) peak current decay SS Steady-state characteristics in Figure 4-2 TS(i) Current transient characteristics in Figure 4-12 (a) Note (i) - In some cases where R C is calculated as low ohm values, special component types are selected such as a capacitor type with low ESR.

94 Chapter 5 Development of the PAAM Experimental Verification of the RC Equivalent Circuit Model A experimental RC equivalent circuit using component values as shown in Table 5-2. was constructed as shown in Figure 5-9 (a) and subsequently tested to measure the steady-state output characteristics presented in Figure 5-9 (b). Table 5-4: Experimental RC equivalent circuit model component values Component Value U S DC voltage source 10 V R S Series resistance 99.3 Ω C Capacitor µf R C Capacitor (ESR) 0.91 Ω R M Current measuring resistor Ω US RS RC C Io + Uo RLOAD Output current (Io) ma Steady-state output characteristic RC equivalent circuit model Output voltage (Uo) (Refer Table 5-4 for component values) (a) Experimental RC equivalent circuit (Measured values) (b) Steady-state characteristic Figure 5-9: Experimental RC equiv. cct. and steady-state characteristic The experimental RC equivalent circuit steady-state characteristics in Figure 5-9 (b) is a rectangular shape consistent with an active power supply. The experimental RC equivalent circuit output transient characteristics were measured using the circuit shown in Figure The no-load to short-circuit output transient is presented in Figure 5-11 and the short-circuit to no-load output transient presented in Figure 5-12.

95 Chapter 5 Development of the PAAM US RS RC C Io + Uo Contact closes at t1 opens at t2 RM UO UM OSCILLOSCOPE A B Chnl. A - Uo Chnl. B - Io = UM / RM - (Refer Table 5-3 for component values) Figure 5-10: Experimental RC equivalent circuit - transient test circuit 3.28A I O 105mA 0mA 10.04V U O 0.2V 0V t 0 t Peak =20µs t SS =220µs Upper trace: Output current (I O ), Lower trace: Output voltage (U O ) Figure 5-11: Measured RC equiv. cct. NL to SC transient characteristics

96 Chapter 5 Development of the PAAM Using equation 5.1, the estimated value of peak output current for the under damped case is 4.1 A. As discussed previously it is expected that this estimated value will be higher than the measured value. The measured value of 3.28 A is lower as discussed previously. I O 91.1mA 0mA 9.99V U O 0.33V 0V t 0 t SS =5.19ms Upper trace: Output current (I O ), Lower trace: Output voltage (U O ) Figure 5-12: Measured RC equiv. cct. SC to NL transient characteristics The experimental RC equivalent circuit no-load to short-circuit transient characteristics in Figure 5-11 are compared to those measured from sample IS active power supply in Figure 4-12 (a). The experimental RC equivalent circuit model is able to predict the behaviour of the sample IS active power supply during the no-load to short-circuit transient period near time t 1. When the short-circuit load is removed at time t 2 the RC equivalent circuit model has an exponential voltage rise whereas the sample active power supply has a exponential voltage with a faster rise time. This difference between the RC equivalent circuit model and the sample IS active power supply can be ignored as the output voltage near time t 2, has a minimal effect on the output energy, since the output current is zero.

97 Chapter 5 Development of the PAAM The value of the capacitance C in the RC equivalent circuit model is found to be significantly lower than the physical value of the capacitance in the output stage of the sample IS active power supply. This lower value of C is defined in this thesis as the effective capacitance of the active power supply. The current limiter of Figure 4-1 accounts for the difference between the effective capacitance and the physical value of the capacitance. The RC equivalent circuit is, in effect, modelling the nonlinear response of the current limiter. The response times of the current sensing circuit, IS control circuit and current limiter have a significant effect on the transient output energy.

98 Chapter 5 Development of the PAAM The Proposed Alternative Assessment Method (PAAM) Research performed by Dill and Kanty [11] was used as a basis for the research carried out in this project. The aim was the development of an alternative assessment method to determine the sparking potential of an active power supply. This method is based on the use of an equivalent circuit model conforming to the topology of one of the well defined circuits as defined in the intrinsic safety Standard. The output stage of a sample IS active power supply was modelled utilising an RC equivalent circuit comprised of a small number of passive components. The steady-state and transient output characteristics can be obtained from a simple test using a relay contact and a storage oscilloscope. Due to the transient nature of the signals being measured, a storage oscilloscope with a suitable bandwidth or sampling rate and input impedance is used in order to ensure the accuracy of the measurements. The response of the equivalent circuit throughout the period of application of a shortcircuit up to the occurrence of the peak output current is a function of the specific characteristics of the short-circuit. These include the rate at which the contacts are closing, the applied voltage, dielectric strength, and impedance of the discharge circuit. These specifics of the short-circuit determine the time period between the first conduction of current and the occurrence of peak output current, value of the peak output current and corresponding output voltages. The second stage of the transient response extends from the point where the peak output current occurs to the establishment of steady-state circuit conditions. This is due to the current sensing circuit, the IS control circuit and the current limiter within the IS active power supply. The current sensing circuit, the current limiter and IS control circuit have a finite response time. This information was used to develop the RC equivalent circuit model. The RC equivalent circuit model replicates the behaviour of the sample IS active power supply when subjected to a short-circuit. The RC equivalent circuit model is an equivalent linear power supply of the IS active power supply under investigation.

99 Chapter 5 Development of the PAAM If it is postulated that an active power supply can be adequately represented by a RC equivalent circuit model, the active power supply unit can then be assessed using the ignition curves in the intrinsic safety Standard. If the equivalent circuit model is assessed as intrinsically safe that is inside the safe area as illustrated in Figure 5-13 then the active power supply could also be considered intrinsically safe. Pass margins for the voltage and capacitance would be used to monitor how close an assessment is to the ignition curve thus providing an acceptable level of confidence. In cases where pass margins are small, the assessment should be confirmed using the STA. If the equivalent circuit model fails the assessment using the ignition curves then the active power supply would not be regarded as intrinsically safe. A series of appropriate pass margins need to be established via statistically means. Group I capacitive circuits Ignition curve for capacitive circuit Capacitance C (uf) SAFE AREA UNSAFE AREA Minimum igniting voltage U (V) Note: This is an illustration and is not to be used for assessment Figure 5-13: Illustration of ignition curve safe and unsafe areas

100 Chapter 5 Development of the PAAM Limitations of the PAAM The development of the PAAM to determine the sparking potential of active power supplies in this thesis was based on a small sample size. All of the sample active power supplies had similar circuit topologies, although they had different nominal voltages and currents, as shown in Table 4-1. Two of the sample active power supplies had output currents approaching the upper limit recommended by Dill in Table 2-6. These two power supplies are examples of active power supplies that approach the boundaries of intrinsic safety. A series of further investigations, utilising a larger sample size and including a more comprehensive variation in circuit topology and nominal output values are required in order to establish reliability of the proposed alternative assessment method. The speed of the relay contact operation has a direct impact on the test results and requires further consideration. It was determined experimentally that the maximum peak output current occurs when the relay contact is closed slowly with no contact bounce. It is anticipated that for a different circuit topology, the relay closing or opening speed may need to be altered to optimise the measurement of transient behaviour. The parameters measured from the transient output characteristics using the relay contact are higher than those measured using the STA. The higher measured values may cause PAAM result to fail the power supply or to indicate inadequate pass margins. Although this is undesired, it is erring on the side of safety. The existing assessment technique utilises a factor of safety (FOS) applied to both output voltage and current to provide a safety margin. The PAAM may not require the use of a FOS. A storage oscilloscope with high a input impedance and a suitably large bandwidth or high sampling rate is required to measure the transient behaviour. Repeatable results can be achieved using the relay contact and storage oscilloscope. The sampling strategy ensures the energy storage components have had enough time to fully charge before the relay contact closes and the transient behaviour is measured. Selection of the value of peak output current from a statistically significant sample

101 Chapter 5 Development of the PAAM size will ensure that the maximum peak output current is measured along with its associated time constant and output voltages. The measured peak output current is sensitive to circuit impedance. Estimation of the time constant of the non-linear peak output current decay affects the effective capacitance value used to determine the position on the ignition curve and hence the PAAM assessment result and pass margins. The non-linear behaviour of the output current decay closely approximates an exponential decay during its initial phase but varies from typical exponential behaviour as the output current stabilises at the steady-state short-circuit value. To ensure that the time constant of the output current decay is accurately estimated a trendline is selected so that it coincides with the initial values of output current decay and always exceeds the output current value. As the trend line values are either equal to or greater than the output current values the time constant is not under estimated. Over estimation of the time constant can be a problem and will lead to the PAAM result to be a fail or determine inadequate pass margin. The initial technique used in this research to estimate the time constant was to measure the period from the occurrence of the output current peak to the point where the output current had reduced by 63.8 % of the difference between the peak current value and the final steady-state short-circuit current value. This technique was replaced by transferring values of the transient output current to an Excel spreadsheet, plotting the characteristic and utilising the trend-line feature (refer to Figure 6-1). The application of additional external loads to the active power supply under test was not considered within the scope of this research because IS devices, including IS power supplies, can be assessed in isolation using the IS entity concept approach. Certification of active power supplies using the 'entity concept method' [24] requires the establishment of maximum values of external circuit inductances and capacitances are required to be determined. This is typically determined via the use of the STA and application of external capacitances and inductances to the active power supply, repetitive testing and alteration of the external component values until

102 Chapter 5 Development of the PAAM ignition occurs. The external components represent the combination of distribution cabling and IS devices (load) connected to the cable. The addition of external load resistance, which is equivalent to increasing the series shunt resistance reduces output current and hence improves the safety of the circuit. As worst conditions are attempting to be determined additional resistance should be minimised. In the case of additional external load inductance, capacitance or both inductance and capacitance, the distributed nature of the connected loads and the resistance between the distributed elements provides a degree of current limitation. The potential combination of energy from the output of the active power supply and the energy storage components in the load is a concern. It is anticipated in this case where the output current is predominantly a single order capacitive transient behaviour the RC equivalent circuit model can be applied. In other cases an alternate equivalent circuit model would need to be developed. The PAAM has not been validated in circumstances where external components are added to the active power supply under test. It is envisaged that further development of the PAAM would include the identification of a number of PAAM equivalent circuit models. These PAAM equivalent circuit models would cater for the varying types of output characteristic behaviour, including single order inductive, second order and higher order responses. Applying the PAAM to other types of power supplies has not been validated. Where the power supply to be tested exhibits similar output transient behaviour to the power supplies already examined it is anticipated that the PAAM RC equivalent circuit model can be applied. Where the power supply to be tested has different transient characteristics to the power supplies examined then a number of options are presented in the following paragraphs. The first option is to approximate the result of the PAAM by utilising the RC model with component values that result in transient characteristics which envelopes the transient characteristics of the power supply under test. This will ensure that the instantaneous values of current and voltage of the PAAM RC model always exceed those of the power supply under test. The duration of envelope would be critical and would have a direct effect on the pass margin confidence level. In the case where

103 Chapter 5 Development of the PAAM the fail margin confidence level is low it is possible that the PAAM may have unfairly produced a fail result so the next (second) option is recommended. In the second option where the transient characteristic exhibits a first order inductive, second and higher order behaviours a different PAAM equivalent circuit will need to be developed. A number of modelling techniques that synthesise an equivalent circuit from transient characteristics are well documented in control theory literature [27]. Alternately the transient behaviour of the power supply under test may be analysed piecewise by assessing adjacent periods of transient behaviour. In this case the PAAM RC model or another equivalent circuit model is utilised and each piecewise assessment result would need to have an adequate pass margin confidence level for the overall PAAM result to be defined as a pass. The duration of adjacent periods and the overall duration assessed would be critical and have a direct effect on the pass margin confidence interval.

104 Chapter 5 Development of the PAAM Summary The transient response of an active power supply results from rapid changes in load conditions. The amount of energy available at the output of an active power supply during transient conditions is dependent on the capacitance and/or inductance of the energy storage components, resistance between the energy storage components and the output, trigger level of the IS control circuit, response time of the IS control circuit, and characteristics of the current limiting device used in the IS control circuit. The circuit topologies used in the output stage of IS active power supplies can be modelled using an equivalent circuit. The circuit topology and component values of the equivalent circuit can be determined by measurement of parameters associated with the steady-state and transient output characteristics of the active power supply under assessment. The desired equivalent circuit topology is one of the well defined circuit topologies which has a corresponding ignition curve defined in the intrinsic safety Standard. The sparking potential of the active power supply under assessment can be determined by assessing the equivalent well defined circuit using existing methods and the appropriate ignition curve. If there is; (a) an adequate pass margin, the active power supply passes, (b) inadequate pass margin, the result is confirmed using the STA, and (c) failure, the active power supply fails. The acceptance of the PAAM requires further testing on a suitably sized sample of active power supplies with sufficient variation in circuit topology and nominal output ratings to establish a set of equivalent well defined circuit models. Subsequently suitable confidence intervals for the pass margins can be established by statistical analysis. In chapter 6 the PAAM is applied to the sample IS power supplies and the results compared to spark testing the sample IS power supplies using the STA.

105 Chapter 6 Experimental Evaluation of the PAAM In this section, the proposed alternative assessment method (PAAM) for determining the sparking potential of active power supplies is verified experimentally. A comparison is made between the results of the PAAM to the results obtained from testing using the spark test apparatus (STA). 6.1 Sample IS Active Power Supplies The three sample intrinsically safe (IS) active power supplies described in Section 4.1 were used to validate the new assessment method. All of the sample IS active power supplies have the same circuit topology as shown in Figure 4-1 with an energy storage capacitance of 4000 µf. The nominal output voltage and current ratings for each of these active power supplies are listed in Table 4-1. The sample IS active power supplies PS 1 and PS 2 both have a rated output voltage of 12 V. The rated output current for PS 1 is 1 A which is a mid-range value whereas PS 2 has a rated output current of 2 A. The rated output current for PS 2 nears the recommended maximum listed in Table 2-6. The sample 18 V IS active power supply PS 3 with a rated output current of 1.25 A exceeds the recommended maximum current limit for any power supply in the range of 12.5 V to 24 V. Both sample active power supplies PS 2 and PS 3 are examples of active power supplies that approach and test the boundaries of intrinsic safety. 6.2 Sample Active Power Supply Parameter Measurements The transient output characteristics of the sample IS active power supplies were determined using a relay and storage oscilloscope as discussed in Section The parameters outlined in Section 4.3 were determined from the transient waveforms. This experiment was repeated until a sufficient number of samples were obtained. The maximum peak output current was identified from the sample waveforms. The measured parameters for each of the sample active power supplies are tabulated in Table 6-1.

106 Chapter 6 Experimental evaluation of the model Table 6-1: Measured transient parameters of sample active power supplies Sample Identifier U O NL (V) U O SC * (V) PS PS PS * R M = 1 Ω U O Output volts I O Output current NL No-load SC Short-circuit Sample Identifier I O Peak (A) I O SC * (A) Period t 1 to I O Peak (µs) Period I O Peak to I O SC (µs) PS PS PS Note: Values are the worst case sample selected from a sample size of 30 The measured parameters in Table 6-1 were then used to derive component values of the RC equivalent circuit model presented in Figure 5-4, using the formulae in Table 5-3. The RC equivalent circuit component values for each of the sample IS power supplies is presented in Table 6-3. At this point in the PAAM each of the sample IS active power supplies has been simplified to a RC equivalent circuit model. The RC equivalent circuit model is an equivalent linear power supply.

107 Chapter 6 Experimental evaluation of the model Example Application of PAAM The RC equivalent circuit model components for the sample IS active power supply PS 1 are determined from the transient output current characteristics presented in Figure 6-1. The period of interest in the transient output current characteristics extends from the maximum peak output current to the steady-state short-circuit output current. The output current behaviour during this period is then matched to an exponential trend line, as shown in Figure 6-1 and defined in the following equation: I O Trend = (I O Peak - I O SS )e -t/τ + I O SS...(6.1) where I O Trend = Exponential model of active power supply output current I O I O Peak = Peak output current (worst case sample) I O SS = Steady state output current τ = time constant of exponential decay The criteria for the exponential trend line is that, the value of the trend line either equals or exceeds the value of the output current at all times. It is the value of peak output current and the initial period of the transient response that directly affects the value of output energy at the knee of the curve, refer to Figure As both output current and voltage reduce with time, the latter part of the transient is less significant.

108 Chapter 6 Experimental evaluation of the model Output current Io (amps) PSU 1 12V1A Ouput Current (Io) vs Time Measured transient response I O Peak Peak output current Measured transient response I O - Output current Trend line for I O I O Trend = ( ) e ( x10E(-6) x t) τ = 1/0.089x10-6 = 11.24µs Time (microseconds) Figure 6-1: Measured transient output current response for PS 1 Table 6-2 shows the calculations leading to the values of the RC equivalent circuit model components for the PS 1. The time constant τ of the transient output current decay is used to determine the RC equivalent circuit model shunt capacitance component value. Using the exponential trend line, the value of the time constant τ is determined by the inverse of the coefficient of time t in the exponential term. Table 6-2: PAAM calculating component values (RC equiv. cct. model) - PS 1 Component Calculation (Refer to Table 5-3 for component equations) U S U O NL = 12.7 V R S U O NL I - R L = 12.7 O SC 1-1 = 11.7 Ω R C R S (R S + R L ) I O SC.R S I 11.7 O Peak + I - R L = O SC ( ) x 1x = 0.5 Ω C τ.(r S + R L ) (R S. R C + R S. R L + R L. R C ) = 11.24x10-6 x ( ) (11.7x x1 + 1x0.5) = 7.9 µf The RC equivalent circuit model for the sample IS active power supply PS 1 is presented in Figure 6-2.

109 Chapter 6 Experimental evaluation of the model RS Io + U S - DC voltage source (12.71 V) US RC C Uo R S - Source resistance (11.7 Ω) C Capacitor (7.9 µf) R C - Capacitor ESR (0.5 Ω) - Figure 6-2: PAAM RC equivalent circuit model for PS 1 Applying this same technique to PS 2 and PS 3 results in the RC equivalent circuit component values as presented in Table 6-3. Table 6-3: PAAM component values (RC equiv. cct. model) for PS 1, 2 and 3 Sample identifier U S (V) R S (Ω) R C (Ω) C (µf) PS PS PS The RC equivalent circuit model can now be assessed using the existing assessment method. The RC equivalent circuit model circuit topology is a well defined circuit configuration included in the intrinsic safety Standard with its respective ignition curve (Group I capacitive circuits) as presented in Figure 6-3. Each sample IS active power supply was assessed using its values of U S, R C, and C as shown in Table 6-3 to determine a point of intersection on the appropriate ignition curve, refer to Figure 6-3. The appropriate ignition curve was determined by the value of R C. Where the value of R C occurs between the standard curves on the graph, the ignition curve is interpolated. If the point of intersection between U S and C lies on the left hand side of the ignition curve defined by R C, the pass margins are determined for both U S and C. The pass margin for U S is the horizontal distance between the point of intersection and the ignition curve. Similarly, the pass margin for C is the vertical distance between the point of intersection and the ignition curve.

110 Chapter 6 Experimental evaluation of the model Figure 6-3: PAAM ignition curve plots for PS 1, 2 and 3 [24]

111 Chapter 6 Experimental evaluation of the model Where the pass margins are acceptable, the active power supply has passed PAAM. Where the pass margins are not acceptable, the active power supply s sparking potential needs to be confirmed using the STA. If the point of intersection of C and Us falls to the right hand side of the appropriate ignition curve defined by R C, the active power supply has failed the PAAM. The results of the PAAM are presented in Table 6-4 as a pass or fail with the respective pass margins included.

112 Chapter 6 Experimental evaluation of the model Comparison with Spark Testing Results The three sample IS active power supplies were also subjected to spark testing using the STA. The STA was applied directly to the output terminals of the sample IS active power supplies. The test conditions for the STA complied with the requirements of Group I (underground coal mining) and using the FOS explosive test gas Hydrogen with a concentration ratio of 52:48 with air. Spark testing was carried out following the normal procedure, where the sensitivity of the STA was checked before and after each test. Only one wire was used in the STA wire holder to ensure that there was sufficient time available for the active power supply to recover from the previous short-circuit condition and that the output stage energy storage capacitors were fully recharged. Where there is one wire in the wire holder the Standard prescribes 1600 revolutions of the wire holder. The STA was connected to the terminals of the sample IS active power using one polarity for the first 800 revolutions and then for next 800 revolutions with the polarity reversed. The results of spark testing using the STA are presented in Table 6-4 as either a pass or fail outcome. For a pass to occur no explosive ignitions occurred during 1600 revolutions of the wire holder. For a fail to occur an explosive ignition occurred during the 1600 revolutions. PS 3 had an explosive ignition during STA testing. Two further tests were performed on this power supply but no explosive ignition resulted. This is an example of a situation where the ability of the STA to accurately replicate test results becomes questionable. The results presented in Figure 6-3 following the application of the PAAM of the three sample IS active power supplies indicate that they all fall on the left hand side of the appropriate ignition curve. Pass margins are then used to determine if the PAAM outcome is a pass or fail.

113 Chapter 6 Experimental evaluation of the model Table 6-4: Comparison of results - PAAM vs. STA testing PAAM STA Sample Pass margins Identifier Result U O (V) C (µf) Results PS 1 PASS PASS PS 2 PASS PASS PS 3 PASS FAIL The sample active power supply PS 3 provided the only example of a deviation between the results of the PAAM and STA testing. The capacitance pass margin for PS 3 is significantly smaller larger than that of PS 1 and PS 2, and the voltage pass margin for PS 3 is higher indicating that an a logical AND function i.e. both margins are required. The STA testing result obtained for PS 3 place this active power supply on the borderline of intrinsic safety. The spark testing results using the STA were obtained using Hydrogen as the test gas, which has a FOS of 1.5. As the pass margin has not been determined and the confidence interval is unknown, PS 3 may not be intrinsically safe. The results obtained using the PAAM do indicate a relationship with those obtained using the STA. The establishment of pass margins and confidence intervals for the PAAM would provide the data to help establish a mathematical definition of this relationship. It may also be sufficient to define this relationship by correlating the STA test results and the PAAM results using a larger sample of active power supplies. On initial inspection, it would be expected that PS 2 would have lower pass margins than PS 1. Meaning that it is closer to the intrinsic safety limit. PS 2 has the same rated output voltage as PS 1 but has twice the rated output current. However, further investigation reveals that the peak output current during the no-load to short-circuit transition is significantly higher for PS 1. As both PS 1 and PS 2 have the same period between the occurrence of the peak output current and the steady-state short-circuit value, significantly more energy is being transferred from PS 1 to the short-circuit and constitutes a potentially higher risk as an spark ignition source. This is reflected in the PS 1 being located closer to the ignition curve and hence the lower pass margins.

114 Chapter 6 Experimental evaluation of the model The sample power supply PS 3 has a higher rated output voltage than either PS 1 or PS 2. To be intrinsically safe it would be expected to have a lower effective capacitance than either PS 1 or PS 2. PS 3 does a lower effective capacitance. The PAAM capacitance pass margin for PS 3 is significantly lower than PS 1 or PS 2. The PAAM in this case recommends confirmation using spark ignition testing. The subsequent spark testing would confirm the PAAM result as an incendive ignition occurred and PS 3 failed. As a result of applying the PAAM, the sample power supplies PS 1 and PS 2 pass the spark ignition assessment phase of the intrinsic safety compliance process whereas PS 3 fails. 6.5 Summary The PAAM developed in this research project has been applied to a small number of sample active power supplies. There appears to be a correlation between the results produced by the PAAM and those obtained from spark testing using the STA. Further research is required to establish the confidence intervals for pass margins with consideration to FOS.

115 Chapter 7 Conclusions and Further Research 7.1 Conclusions The types of intrinsically safe power supplies have been defined and categorised in this research. The static and dynamic (transient) behaviour of active power supplies that exhibit a predominantly capacitive behaviour has been investigated. Parameters that define the amount of available output energy have been identified. The methods used in the assessment and testing of active power supplies as part of the intrinsic safety accreditation process have been reviewed, particularly in the determination of the sparking potential of active power supplies. A number of improvements to the current methods are proposed. Substantial energy can be available at the output of active power supplies under transient conditions. Transient conditions can occur during both normal operation and fault conditions. It is the fault conditions that give rise to concerns associated with the use of active power supplies due to the inherent energy stored within the output stage. The active power supplies investigated, when subjected to intermittent short-circuit fault conditions, capable of delivering output energy that pose a significant spark ignition risk. An alternate assessment method is proposed to determine the sparking potential of active power supplies. The proposed alternative assessment method (PAAM) determines the equivalent linear power supply for the active power supply under test. The equivalent linear power supply is then subjected to the existing assessment method using the ignition curves included in the Intrinsic Safety Standard. The results of applying the PAAM to the sample active power supplies were verified by performing traditional spark potential testing using the STA. The results of the PAAM show correlation with those derived using the STA, although the safety margins will still need to be established. The PAAM developed in this research project can be used as a pre-compliance check by designers, manufacturers, or IS testing stations. A failure of this test would indicate that the active power supply s sparking energy is not low enough to be

116 Chapter 7 Conclusions regarded as intrinsically safe. The PAAM requires fewer resources to establish a result than the STA. A simplified spark ignition test like PAAM would be beneficial to designers, manufacturers and end users. 7.2 Further Research Verification of the PAAM and submissions While this research project has endeavoured to establish a method of assessing the sparking potential of an active power supply, the PAAM still requires exhaustive testing and further validation by other concerned or specialist bodies. The PAAM would then need to be promoted amongst national and international testing stations in order to solicit further interest and promote acceptance. In addition, a submission would need to be prepared and delivered to the appropriate committees of the international, other national and the Australian Standards bodies. Development of IS power supply barrier In many intrinsic safety applications IS barriers are used to isolate intrinsic safety circuits from non-intrinsic safe circuits. These IS barriers are located in a safe area. Applying this concept to power supplies resulted in the concept of an IS power supply barrier. An IS power supply barrier would remove all of the intrinsic safety circuitry from the power supply and locate them in a separate device. This device would be situated between a conventional off-the-shelf power supply (non-is) and the hazardous area. The device would ensure that under all conditions there is insufficient energy in the circuit to cause an incendive spark. Development of an electronic IS testing device The output energy available at the output terminals of an active power supply can be determined from the relationship between voltage, current, and time. As these variables can be measured from the circuit, the amount of energy available at a potential spark can be determined. This is a measure of the potential spark energy that can be transferred to the surrounding gas. This information, combined with the

117 Chapter 7 Conclusions known MIE of the standard testing gases and physical properties of a making or breaking circuit, could be developed into a measuring device that indicates a measure of intrinsic safety.

118 References References [1] NSW Dept of Mineral Resources, "Power Supplies Warning", Mine Safety News, vol. June 1999 [2] S. Bell and M. Hookman, "Mines count cost of IS power supply bungle", Australia's Longwalls, vol. March 1999 [3] J. Hall, Intrinsic Safety, England: Marylebone Press Ltd, [4] Hillcon Consulting, "Ex - What is it?", What's New in Process Engineering, vol. June 2000, 2000 [5] E. C. Magison, Intrinsic Safety, USA: Instrument Society of America, [6] E. C. Magison, Electrical Instruments in Hazardous Locations, 4th ed, USA: Instrument Society of America, [7] Measurement Technology Limited (MTL), "Application Note A user's guide to intrinsic safety", MTL, Bedfordshire AN 9003, [8] Pepperl + Fuchs, "In view of Intrinsic Safety", Pepperl + Fuchs Pty Ltd, England [9] G. Barnier, "Intrinsic Safety - Assessment and Testing", [Offline], 2000, 03/05/2001, Available: SIMTARS Intranet. [10] W. G. Dill, "Intrinsic Safety: Design of complex systems, case studies", presented at EuropEx - The first World Seminar on the explosion phenomenon and on the application of explosion protection techniques in practice, Brussels, Belgium, [11] W. G. Dill and G. Kanty, "Analysis of the dynamic behaviour of electronically regulated power supplies as a substitute for testing intrinsic safety by sparkignition tests in explosive mixtures", presented at 23rd International Conference of Safety in Mines Research Institutes, Washington, DC, [12] U. Johannsmeyer and M. Kraemer, "Interconnection of Non-Linear and Linear Intrinsically Safe circuits", Physikalisch-Technische Bundesanstalt (PTB), Braunschweig Report No E, [13] R. Thurlow and P. J. Green, "Progress in the design of intrinsically safe power supplies", Mining Technology, vol. 59, [14] R. Tomlinson and D. W. Widginton, "Effects of slewing rate when testing intrinsically safe power supplies", Health and Safety Executive, Health and Safety Laboratories, London Report No. 2, 1978.

119 References [15] D. Turner, G. Barnier, and M. Walpole, "Assessment, Testing and Certification of Intrinsically Safe Active Power Supplies", presented at Queensland Mining Industry Health and Safety Conference 2000, Townsville, Australia, [16] D. W. Widginton, "Intrinsic safety reference curves: Some recent considerations.", presented at Fourth International Conference on Electrical Safety in Hazardous Areas, London, [17] Standards Australia, HB Handbook Electrical equipment for hazardous areas: Standards Australia International Ltd and Standards New Zealand, [18] J. J. Sammarco, "Intrinsically Safe 5-V, 4-A Rechargeable Power Supply", Bureau of Mines, Pittsburgh Research Center, Pittsburgh Report No. BUMINESIC9223, [19] J. C. Cawley, M. D. DiMartino, T. J. Fisher, R. L. King, and M. H. Uhler, "Power Supply for an Intrinsically Safe Circuit US Patent # ", Washington, DC: Department of the Interior, [20] P. S. Babiarz, "CEC, NEC, IEC, CENELEC: Harmony or discord?", INTECH., vol. V, 1998 [21] M. Hookman, "When IS is not safe", Australia's Longwalls, vol. March 1999 [22] TestSafe Australia, "TestSafe Australia - A Safety Testing and Research Centre", [Online], 2000,, Available: [23] SIMTARS, "SIMTARS - Safety in Mines Testing and Research Station", [Online], 2000, Last update 28 August 1998, Available: [24] Standards Australia and Standards New Zealand, AS/NZS :2000 Electrical apparatus for explosive gas atmospheres Part 11: Intrinsic safety i: Standards Australia International Ltd. and Standards New Zealand, [25] S. Halama, J. Cerri, and J. Bigourd, "Intrinsic safety of high intensity sources", presented at 22nd International conference of safety in mines research institutes, Beijing, China, [26] E. Hughes, Hughes Electrical Technology, Sixth ed, England: Longman Scientific & Technical, [27] J. Schwarzenbach and K. F. Gill, System Modelling and Control, 2nd ed, Britain: Edward Arnold Ltd, 1984.

120 Appendices A 1. Generic Block Diagram of IS Active Power Supply AC Input Voltage Source ON/OFF Isolation Protection Filter Transformer Bridge rectifier C Low pass filter R Voltage Sense V V reg Voltage Regulator IS Cntrl I reg Current Regulation & IS Control I R Current Sense Crowbar Protection OUTPUT STAGE OF IS ACTIVE DC POWER SUPPLY + IS DC Output -

121 Appendices A 2. Measured Output Characteristic using STA ORM(YOKOGAWA) Data for transient output characteristic measurement using STA Number of data 700 Trigger point Trigger time :04 Sample rate 50 khz Io =B17/$B$11 Tag name C01 C02 Uo =C17 Unit V V Po =G17*H17 No. Eo inst =(I17+I18)*0.5*$B$10* Eo accum =SUM($J$20:J20) sample interval 2.00E-05 sec resistance Rm ohms Lapsed Eo Eo Time Time Io Uo Po Inst Accum Sample No. Urm V Uo V msecs msecs Amps Volts Watts u J u J

122 Appendices Measured output characteristic using STA Uo Io 14 8 Output Volts (Uo) Output Current (Io) Amps time (ms) 0 Measured output characteristic using STA Po Eo Output power (Po) W Output energy (Eo) uj time (ms) 0

123 Appendices A 3. Measured Output Characteristic using Relay ORM(YOKOGAWA) Data for transient output characteristic measurement using a relay Number of data 2001 Trigger point 5760 Trigger time :22 Sample rate 100 khz Io =B17/$B$11 Tag name C01 C02 Uo =C17 Unit V V Po =G17*H17 No. Eo inst =(I17+I18)*0.5*$B$10* Eo accum =SUM($J$20:J20) sample interval 1.00E-05 sec resistance Rm ohms Lapsed Eo Eo Time Time Io Uo Po Inst Accum Sample No. Urm V Uo V msecs msecs Amps Volts Watts u J u J

124 Appendices Measured output characteristic using a relay Uo Io Output volts (Uo) time (ms) Output current (Io) Amps Measured output characteristics using a relay Po Eo Output power (Po) W Output energy (Eo) uj time (ms)

125 Appendices A 4. No-load to Short-circuit Output Characteristic ORM(YOKOGAWA) Number of data 100 Trigger point 1920 Trigger time :12 Sample rate 100 khz Io =B17/$B$11 Tag name C01 C02 Uo =C17 Unit V V Po =G17*H17 No. Eo inst =(I17+I18)*0.5*$B$10* Eo accum =SUM($J$17:J17) sample interval 1.00E-05 sec resistance Rm ohms Lapsed Eo Eo Time Time Io Uo Po Inst Accum Sample No. Urm V Uo V msecs msecs Amps Volts Watts u J u J

126 Appendices A 5. Ignition Curves for well defined Circuits Resistive circuits [24]

127 Appendices Group I capacitive circuits [24]

128 Appendices Group I inductive circuits [24]