LOW-VOLTAGE SWITCHGEAR AND CONTROLGEAR ASSEMBLIES

Transcription

1 EDITION 2006 LOW-VOLTAGE A GAMBICA TECHNICAL GUIDE SWITCHGEAR AND CONTROLGEAR ASSEMBLIES A guide to BS EN Incorporating Amendment No. 1 and Corrigenda Nos 1 + 2

2 Foreword Under current electrical and safety legislation it is a requirement that equipment purchased should be both fit for purpose and safe. This may be demonstrated through meeting the requirements of a European harmonised standard. In the field of switchgear and controlgear assemblies, this standard is BS EN This guide has been compiled by members of the GAMBICA Controlgear Systems Group to provide specifiers, designers and purchasers of switchgear and controlgear assemblies with a clearer understanding of BS EN to assist in the selection of fully compliant and safe products related to this standard. GAMBICA gratefully acknowledges the significant amount of time and effort put into the preparation of this guide by representatives of member companies. The greatest care has been taken to ensure the accuracy of the information contained in this guide, but no liability can be accepted by GAMBICA, or its members, for errors of any kind. Note: In this revision the UK National Annex has been removed, since it is no longer permitted by BSI to reproduce it on an open access website. Typical practice in the UK, however, is now taken into account in the types of construction. The guide seeks to highlight the most relevant areas of the specification and to give the purchaser confidence in the safety and reliability of the equipment. In addition, it explains commonly used terminology and areas of customer choice to assist in the decision making process. This Guide covers BS EN :1999 with the following changes: Amd. No dated 30 June 2004 Amd. No dated 31 January 2005 (Corrigendum No. 1) Amd. No dated 11 January 2006 (Corrigendum No. 2) BS EN :1999 incorporating Amendment No. 1 and Corrigenda Nos. 1 and 2 is equivalent to EN : A1:2004 and IEC : A1: The GAMBICA Association Ltd. This publication may be freely produced, in whole or part, provided its source is acknowledged. (i)

3 Contents 1. TYPE-TESTED AND PARTIALLY TYPE-TESTED ASSEMBLIES (TTA and PTTA) Type-tested assembly (TTA) Partially type-tested assembly (PTTA) Performance requirements 4 2. TYPE TESTING Enclosure and degree of protection Temperature rise Short-circuit withstand strength Effectiveness of the protective circuit Dielectric properties Clearances and creepage distances Mechanical operation ROUTINE TESTS THE SO-CALLED FAULT-FREE ZONE FORMS OF INTERNAL SEPARATION (FORMS 1-4) THE CE MARKING Low Voltage Directive EMC Directive Machinery Directive 31 ORDER CHECK LIST 32 (iii)

4 1. Type-tested and partially type-tested assemblies (TTA and PTTA) Introduction BS EN is Part 1 of the series of standards and is the main part covering the general requirements for type-tested and partially type-tested assemblies. The several subsidiary parts, which have to be read in conjunction with Part 1, deal with the particular requirements for certain specialised forms of assemblies (see Figure 1). BS EN Low-voltage switchgear and controlgear assemblies BS EN : Specification for type-tested and partially type-tested assemblies BS EN : Particular requirements for busbar trunking systems (busways) BS EN : Particular requirements for assemblies intended to be installed where unskilled persons have access for their use - Distribution boards BS EN : Particular requirements for assemblies for construction sites (ACS) BS EN : Particular requirements for assemblies intended to be installed outdoors in public places - Cable distribution cabinets (CDCs) for power distribution in networks Figure 1. The BS EN series In standards terminology, low-voltage control panels, motor control centres, distribution boards and the like are known collectively as "low-voltage switchgear and controlgear assemblies", or just "assemblies" for short. By "low-voltage" is meant voltages up to 1000 V a.c. or 1500 V d.c. The need to conform to standards Low-voltage switchgear and controlgear assemblies come under the EU Low Voltage Directive, which applies in all Member States of the EEA (European Economic Area - EU Member States, Norway, Iceland and Liechtenstein) and in Switzerland through a Mutual Recognition Agreement.The Directive is implemented in the UK by the Electrical Equipment (Safety) Regulations All assemblies placed on the EEA and Swiss market intended for use within it must bear the CE marking in an appropriate place, which indicates that the manufacturer is declaring compliance with the essential safety requirements of the Directive (see Chapter 6). The Directive requires electrical equipment to be safe and constructed in accordance with the principles generally accepted within the member states of the EU as constituting good engineering practice in relation to safety matters. It requires that the electrical equipment (e.g. an assembly), together with its component parts, is made in such a way as to ensure that it can be safely and properly assembled and connected. It also requires that measures are taken to ensure that 1

5 protection is assured against various hazards which might arise from the electrical equipment or by external influences on it. Just some of the hazards listed by the Directive include: direct and indirect contact with live parts - shock dangerous temperatures, arcs or radiation overloading insulation failures mechanical failures expected environmental conditions non-electrical dangers caused by the assembly. There is, of course, the proviso that an assembly is used in the application for which it was made, and that it is properly installed and maintained. Significantly, the Directive declares that its safety requirements are deemed to be met by electrical equipment which satisfies the safety provisions of harmonised standards. In the case of assemblies, the relevant harmonised standard is BS EN Therefore, quite apart from the assurance which conformity with the standard provides that an assembly will achieve acceptable levels of performance, safety and reliability, it is also the preferred and most straightforward means of demonstrating that the equipment meets UK legislative requirements. So, what is a TTA or PTTA? It is important to appreciate that, although BS EN is the basic standard for assemblies, it only considers those which can be classified either as "type-tested assemblies (TTA)" or "partially type-tested assemblies (PTTA)". There are no other classifications - the standard does not cater for assemblies built to less stringent design and test requirements, or which satisfy only some requirements of the standard. Such assemblies do not conform to BS EN Type tests are tests which are carried out on representative samples of assemblies as part of the process of verifying equipment designs and material selections. They are not to be confused with "routine tests" which are those carried out on actual production assemblies prior to dispatch/installation and which serve to check for manufacturing and material defects. See Chapters 2 and 3 for more details of type and routine tests. It is essential to recognise that the terms "typetested assembly (TTA)" and "partially typetested assembly (PTTA)" are closed and defined descriptors having very specific meanings within the context of the standard. They are not simply abbreviations - a "TTA", for example, is not just another way of saying "an assembly which has been subjected to some type tests"! 1.1 Type-tested assembly (TTA) A low-voltage switchgear and controlgear assembly conforming to an established type or system without deviations likely to significantly influence the performance, from the typical ASSEMBLY verified to be in accordance with this standard. [BS EN Clause ] 2

6 The term "type-tested assembly (TTA)" was coined because the difficulty was recognised in applying correctly the simple adjective "typetested" to the description of a production assembly. One reason for this is that assemblies, unlike many other products, are often individually customised. This means that a production assembly, even though clearly conforming to an established type or system, can rarely be identical in all respects to representative samples which have been type tested. There is always likely to be a requirement for some application-specific deviations, even if these deviations are quite minor ones. For example, there may well be a need for variations in the complement and arrangement of the switchgear and controlgear products within the assembly,or changes to individual circuit configurations. Therefore, the term "TTA" provides the necessarily qualified description of a production assembly. As can be seen from the definition of a TTA, the crucial factor is that the essential characteristics of the type-tested arrangements are retained and no deviation is such as to adversely affect the performance of the assembly in comparison to that of the type-tested samples. Within the definition of a TTA the phrase "without deviations likely to significantly influence the performance" does, of course, introduce a degree of subjectivity. However, in the case of certain deviations it should be selfevident that the performance is likely to be significantly (i.e. adversely) affected. Deviations which could significantly affect the performance include: Major structural changes to the assembly carcass. Reductions in busbar cross-sections, changes in busbar profiles and spacing. Changes to the type or quantity of busbar supports or the support structures. Since any of the above deviations could affect the mechanical strength of the busbar system, these would cause unpredictable, and possibly adverse, changes to the short-circuit withstand rating of the busbars. Exclusion of or changes to major shortcircuit protective devices taken account of in the programme of type tests. The effective short-circuit coordination between a major protective device, e.g. incoming fuse-switch or circuitbreaker, and an assembly or section thereof cannot be established other than by test. By removing or installing protective devices not taken account of in the programme of type tests there is the risk of major damage to the assembly under conditions of short-circuit. Reductions in compartment sizes. Components and wiring dissipate heat. If installed within compartments smaller than those established during the type test programme then higher temperature rises will be the result. These may exceed the permissible limits. The resultant overheating of components and wiring may cause subsequent component breakdowns, insulation failures and internal short-circuit faults. The effects of other deviations may not be so self-evident but here it is reasonable to expect that TTA manufacturers, on the basis of their 3

7 type-test data, will be able to define very closely in their associated engineering procedures the limits of their application design parameters. 1.2 Partially type-tested assembly (PTTA) A low-voltage switchgear and controlgear assembly, containing both type-tested and nontype-tested arrangements provided that the latter are derived (e.g. by calculation) from type-tested arrangements which have complied with the relevant tests. [BS EN Clause ] Following from the above, the situation may arise where certain arrangements within an assembly (e.g. compartment layouts, component configurations) cannot be directly equated to similar arrangements in the representative type-tested samples and/or cannot be considered to be insignificant deviations within the context of a TTA. Here, the standard allows such deviations but only provided they are themselves derived from type-tested arrangements! This means design criteria and calculation methods have been proven by type-tests. However, since they have not actually been type-tested, they then have to be subject to the alternative considerations listed in Table 7 of the standard. Such an assembly is designated a "partially type-tested assembly (PTTA)". It is very important to note, therefore, that the description "partially type-tested assembly (PTTA)" does not indicate that an absence of type-test background for some arrangements within the assembly is permissible. On the contrary, all design aspects of a PTTA must be based, directly or indirectly, on the results of type tests. 1.3 Performance requirements The standard lays down a comprehensive package of performance requirements. These come under 8 basic headings (see Figure 2) and all must be verified irrespective of whether an assembly is to be offered as a TTA or as a PTTA! Temperature-rise limits Dielectric properties Short-circuit withstand strength of main circuits Effectiveness of protective circuit Short-circuit withstand strength of protective circuits Clearances and creepage distances Mechanical operation tests IP degree of protection Figure 2. Performance requirements to be verified - for TTA and PTTA The correlation between these performance requirements and the safety objectives of the Low Voltage Directive should be evident. Summary The principal standard for LV switchgear and controlgear assemblies is BS EN , but it only covers those which can be designated as TTA or PTTA. Assemblies conforming to this standard are deemed to satisfy the essential safety requirements of the Low Voltage Directive and can bear the CE marking accordingly. 4

8 From 1 January 1997 all new assemblies intended for use in Europe must be so marked. The TTA and PTTA concepts are closely related and are not opposite extremes - they are both based on a full background of type testing. To comply with the standard, a full programme of verifications must be completed. Users should ask to see documentary evidence of this. The TTA approach is the most straightforward way of ensuring full compliance with the standard and with UK legislative requirements. 2. Type testing Introduction BS EN details eight type tests in Table 7 (see Figure 2) which are carried out to verify equipment designs. All eight type tests must be carried out on each design of TTA and in the majority of cases, type tests on similar designs must form the basis of design verification for PTTAs. At first sight, all type tests may appear to be associated with constructional and key performance aspects of the assembly. Careful examination shows that most are also very much related to safety, as will be identified. Type tests are expensive to conduct but they are vital and the only effective means of verifying the design of a TTA and of providing the design basis for a PTTA. Ironically, the most significant tests are also the most difficult and expensive to carry out. 2.1 Enclosure and degree of protection (Design and Construction: Clause 7.2/Type tests: Clause 8.2.7) The usual interpretation of the IP code of an assembly is in terms of external protection. Dust or particles and moisture entering the interior or protected space must have no harmful effect. When the assembly is in its normal service condition, personnel outside the equipment should not be able to touch any dangerously live parts. If the degree of protection of part of the assembly,for example the operating face, differs from that of the remainder, then the manufacturer must indicate the degree of protection of the various parts. The other use of the IP code is to define the internal degree of separation assigned to the assembly (see Chapter 5). This includes personal protection, and protection of the assembly from the transfer of objects from one compartment to another. Type testing is carried out in accordance with BS EN For a PTTA, no IP code can be given unless the appropriate verification has been made or tested prefabricated enclosures are used. Although the design and construction requirements for protection against electric shock are treated as a separate issue in the standard, verification of protection against electric shock by direct contact is embedded within the section dealing with degrees of protection. 5

9 2.2 Temperature rise (Design and Construction: Clause 7.3/Type tests: Clause 8.2.1) Testing in this category is one of the most critical in determining the reliability and long service capability of an assembly and must not be overlooked. Excessive temperatures result in premature ageing of components and insulation and ultimately failure. Current ratings of components are valid only when the temperature around them is within the limits specified by the component manufacturer. Safety aspects are also of significance although they may arise mainly as a secondary effect through the touching of hot covers or operating handles. The design of an assembly should take into account a number of factors which will affect the assembly s ability to meet the temperature rise limits set by the standard. The limits for the various parts of an assembly are summarised in Figure 3, which is based on a table appearing in the standard (BS EN Table 3 - Temperature Rise Limits). Parts of assemblies Built-in components Temperature rise (K) In accordance with the component manufacturer s instructions taking into account the temperature in the assembly Terminals for external insulated conductors 70 Busbars and conductors, plug-in contacts of removable or withdrawable parts which connect to busbars Limited by: mechanical strength of conducting material possible effect on adjacent equipment permissible temperature limit of the support insulating material the effect of the conductor temperature on equipment connected to it nature and treatment of plug-in contact material Manual operating means: of metal 15 of insulating material 25 Accessible external enclosures and covers metal surfaces 30 insulating surfaces 40 Discrete arrangements of plug and socket-type connection Determined by the limit for those components of which they form a part Figure 3. Temperature Rise Limits 6

10 From Figure 3, it is clear that temperature rise and therefore temperature limits are set for the external interfaces, cable terminals, covers and handles. There is a differentiation between operating handles which need to be held and covers, and a recognition of the effects of different materials. Top temperatures are however quite high, for example a plastic cover at a temperature of 75ºC (temperature rise of 40 K plus daily ambient temperature of 35ºC) is considered acceptable. Other parts within the assembly are essentially limited to temperatures which will have no detrimental effects. This does not mean temperatures are unlimited. The manufacturer must ensure the temperatures within the assembly do not exceed the specified ratings of the components, materials and in particular the insulation, used. Predicting or calculating temperatures within an assembly is difficult and has to take into account load and component operating temperatures. Each component may have a different temperature operating capability. Due to close coupling one component may transfer heat to another. Adjacent circuits will have a mutual heating effect. Different levels of ventilation have significant effects. As the level of ingress protection increases (see previous section) so does the potential for overheating and derating of components may need to be applied in order to overcome the problem. Temperature rise tests cannot be avoided if confirmation of performance is required. To carry out a temperature rise test all devices are closed and control circuits energised as in normal service. Current is applied to the incoming circuit and shared between the outgoing circuits, loading each to its rated current multiplied by the diversity factor applicable to the application. Where no other information is provided, the standard details the diversity factor to be used in Table 1, with values ranging between 0.6 and 0.9. Temperature rise tests are time consuming. Current is applied until conditions stabilise, usually around eight hours, and in the final hours temperatures are monitored, normally with thermocouples. Critical areas for temperature measurement are covers, operating handles, busbars and joints, insulators, cable terminals, device and/or internal air temperatures. As a guide to the potential power loss within an assembly some typical test results for Watts loss are shown in Figure A Horizontal busbar (per section) 114W 15kW DOL Starter - fuse protected 32W 315A Fuse Switch Disconnector 130W 2000A Air Circuit Breaker 365W Figure 4. Typical Watts losses A section of a typical MCC may have an available capacity to dissipate 400 Watts. With circuits having losses as detailed in Figure 4, this can be readily exceeded. 7

11 If circuits which generate a lot of heat are being incorporated, or the section s ability to dissipate heat is reduced, e.g. as a result of a high ambient temperature, the problems are even more acute. Natural ventilation of the section may not be sufficient and forced ventilation may be the only alternative. Temperatures directly affect equipment life. The key to ensuring long life and reliability in this respect is temperature rise type testing. 2.3 Short-circuit withstand strength (Design and Construction: Clause 7.5.2/Type tests: Clauses and ) Short-circuit testing is necessary to verify the ability of electrical equipment to withstand the forces and thermal effects produced by short-circuit currents. It is often carried out at an accredited third party testing station which will issue a certificate detailing the tests that have been completed satisfactorily. The tests must be carried out in accordance with the requirements laid down in BS EN They are type tests which are not repeated for every piece of equipment supplied and are thus carried out on equipment manufactured specifically for testing purposes. For subsequent equipment to be covered by the test certificate it must be without significant variation in design (see 1.1). In order to completely type-test an assembly, it is necessary to test a sample of each significant variation of circuit and busbar system within that assembly. Individual component parts such as contactors and MCCBs should have been tested by the component manufacturer. Within a range of equipment, this normally includes short-circuit tests on each type of: Main busbar system Distribution bars (risers or droppers) Outgoing circuits Incoming and bus section unit. In order to achieve the above, the testing is carried out in accordance with clause of the standard and would typically comprise the following: Outgoing circuits Each basic type of outgoing circuit which includes a component (connections being considered a component) which has not previously been tested is through-fault tested in turn. For this test the circuit being considered is closed and a short-circuit applied to its outgoing terminals. A 3 phase test supply having a voltage equal to 105% of the operational voltage of the equipment being tested, and capable of delivering the specified short-circuit current, is connected to the incoming terminals of the assembly. Usually with outgoing circuits this prospective short-circuit current is allowed to flow until interrupted by a short-circuit protective device (fuse or circuit-breaker). If the circuit includes a neutral, the procedure is repeated considering the neutral and adjacent phase, but with a prospective fault current equal to 60% of the 3 phase value. 8

12 Incoming circuits and busbars Generally, the incoming circuits and busbars (plus bus-section units) are tested together. The test supply is connected to the incoming terminals and a short circuit applied to the remote end of the busbar system being considered. If the incoming circuit contains a short circuit protective device, then the fault current may be interrupted after a short duration as described for outgoing circuits above. Alternatively, and more likely with larger rating assemblies, the fault current will be required to persist for a definite time (short time withstand current). As for outgoing circuits, tests are carried out for 3 phase and single phase and neutral, again with the prospective neutral current equal to 60% of the 3 phase value. Where different busbar designs (horizontal and vertical) are included within the assembly, each must be tested. On completion of the short circuit tests, minimum of IP protection, creepage and clearance distances, insulation integrity and mechanical capability must be maintained. Slight deformation of enclosures and busbars is acceptable. 2.4 Effectiveness of the protective circuit (Design & Construction: Clause /Type Tests: Clause 8.2.4) Adequate protective circuits within an assembly are vital. Their principal function is to protect personnel should non-current carrying parts accidentally become live. Generally, the basic protective circuit is the metal structure of the assembly. Usually, but not essential by according to the standard, for multi-section units this is supplemented by a protective conductor (earth bar) running the full length of the assembly. To this are connected supplementary earth bonds from instruments, cable glands, etc. where appropriate. Verification of the effectiveness of the protective circuit is achieved by examination and by tests. Examination An examination of the assembly is carried out to confirm that the constructional requirements to ensure an effective protective circuit have been met e.g. (i) All exposed conductive parts greater than 50 mm by 50 mm and which can be touched are connected to the protective circuit. (ii) Manual operating handles etc. are effectively connected to the protective circuit or adequately insulated. (iii) The removal of a part from an assembly does not interrupt the protective circuit for other parts. (iv) Doors and covers are effectively bonded. (v) Protective conductors are sized in accordance with the standard. 9

13 Test:Verification of the effective connection between the exposed conductive parts and the protective circuit. Clause of the standard requires that it must be verified by a specified test method that the resistance between the incoming protective conductor and exposed conductive parts does not exceed 0,1 ohms. Test:Verification of the shortcircuit strength of the protective circuit Requirements for short-circuit withstand testing of the protective circuit are covered by clause of the standard. This test verifies that the assembly enclosure and its protective circuit (earthing system) are capable of withstanding the thermal and electrodynamic stresses resulting from shortcircuit currents up to their rated values. The various component parts of the protective system need to be considered as it is important that all parts of the system are adequately rated. Particular areas which require full consideration are: Short-circuit rating of the earth bar The connection between the outgoing protective conductor terminal and the earth bar. Effectively the tests involve a repeat of the single phase and neutral short-circuit tests but using one phase and the protective circuit. Again, they are carried out with a prospective short-circuit current equal to 60% of the 3 phase value. 2.5 Dielectric properties (Design and Construction: Clause /Type tests: Clause 8.2.2) The standard gives a choice of dielectric type tests. If the manufacturer has declared an impulse withstand capability, then impulse tests are carried out. Table G1 in the standard gives guidance on the appropriate value for a particular voltage and place in the system. More usually the second option of power frequency dielectric tests is carried out. These are popularly referred to as "Flash Tests" and involve the application of specified test voltages between all live parts and the interconnected exposed conductive parts of the assembly. The standard sets out values for the test voltage in sub-clauses and For main power circuits, for example, the dielectric test voltage should be 2500 V ac when the operational voltage of the equipment is between V. Auxiliary circuits typically have a test voltage of 1500 V minimum. The test voltage is required to have a practically sinusoidal waveform and a frequency between 45 Hz and 62 Hz. At the moment of application of the test voltage, its value should not exceed 50% of the values given in the above sub-clauses. It is then steadily increased to the full value specified in the sub-clause and the level is maintained for five seconds. Care has to be taken to ensure that the a.c. power source is capable of maintaining the test voltage irrespective of any leakage currents. 10

14 The succinct definition of a successful result contained in the standard is: The test is considered to have been passed if there is no puncture or flash-over. [BS EN Clause ] 2.6 Clearances and creepage distances (Design and Construction: Clauses and /Type tests: Clause 8.2.5) When considering this particular type test, the requirements of the standard are fairly brief: It shall be verified that clearances and creepage distances comply with the values specified in If necessary, these clearances and creepage distances shall be verified by measurement, taking account of the possible deformation of parts of the enclosure or of the internal screens, including any possible changes in the event of a short-circuit. If the ASSEMBLY contains withdrawable parts, it is necessary to verify that both in the test position (see 2.2.9), if any, and in the disconnected position (see ), the clearances and creepage distances are complied with. [BS EN Clause 8.2.5] In other words, clearances and creepage distances shall not be less than given in Tables 14 and 16 respectively of the standard. The values detailed however are no longer simple arbitrary values. Clearance distances are based on the degree of pollution anticipated. If the user does not indicate otherwise, the manufacturer will assume pollution degree 3 (i.e. conductive pollution occurs or dry, non-conductive pollution occurs which becomes conductive due to condensation). Similarly, creepage distances are a function of pollution degree and the tracking index of insulation material (Material group) being used. Creepage and clearance distances of 8mm or less for main busbars and connections may well be acceptable in many applications. 2.7 Mechanical operation (Type tests: Clause 8.2.6) When considering this particular topic, the standard is brief but its text, as follows, says it all: This type test shall not be made on such devices of the ASSEMBLY which have already been type tested according to their relevant specifications provided their mechanical operation is not impaired by their mounting. For those parts which need a type test, satisfactory mechanical operation shall be verified after installation in the ASSEMBLY. The number of operating cycles shall be 50. Note: In the case of withdrawable functional units, the cycle shall be from the connected to the disconnected position and back to the connected position. At the same time, the operation of the mechanical interlocks associated with these movements shall be checked. The test is considered to have been passed if the operating conditions of the apparatus, interlocks, etc., have not been impaired and if the effort required for operation is practically the same as before the test. [BS EN Clause 8.2.6] 11

15 3. Routine Tests This chapter gives a brief introduction to routine tests, with specific reference to LV Motor Control Centres. The introductory text in the standard sets out the basic requirements, as follows: Routine tests are intended to detect faults in materials and workmanship. They are carried out on all parts of each new ASSEMBLY. Another routine test at the place of installation is not required. ASSEMBLIES which are assembled from standardised components outside the works of the manufacturer of these components, by the exclusive use of parts and accessories specified or supplied by the manufacturer for this purpose, shall be routine-tested by the firm which has assembled the ASSEMBLY. Routine tests include: a) inspection of the ASSEMBLY including inspection of wiring and, if necessary, electrical operation test (8.3.1); b) dielectric test (8.3.2); c) checking of protective measures and of the electrical continuity of the protective circuit (8.3.3). These tests may be carried out in any order. Note: The performance of the routine tests at the manufacturer s works does not relieve the firm installing the ASSEMBLY of the duty of checking it after transport and installation. [BS EN Clause 8.1.2] The requirements are further amplified in clause 8.3 of the standard but as the standard covers a broad range of equipment, it is very general. It is up to each manufacturer to establish, within their ISO 9000 Quality Assurance System, procedures which will ensure they are fully satisfied the equipment they are providing complies with the standard, and that the equipment is fit for purpose. In addition, the user may also have included within the contract further specific routine tests they believe are necessary to confirm the assembly is suitable for their applications, e.g. measuring the resistance of busbar joints. In order to carry out routine tests in a controlled, logical and efficient manner it is usual for a manufacturer to have detailed procedures with limits of acceptance for test results. It is now general practice for the results obtained during routine tests to be formally recorded on documents that form part of the supplier s Quality Plans and, as such, an inherent element in their quality assurance procedures. Within the standard there is only one allowable difference between TTA (typetested assemblies) and PTTA (partially typetested assemblies), this being with respect to dielectric testing. TTAs must be subject to a power frequency dielectric ( flash ) test or impulse test. PTTAs may be subject to the same test, or alternatively, an insulation resistance ("Megger") test at a minimum of 500 V. A manufacturer can, of course, if they feel it advantageous carry out both tests on all types of assembly. 12

16 Any insulation resistances measured in the various tests will depend on the size of the assembly, its contents and the climatic conditions. Usually a manufacturer will set their own minimum values. This may be of the order of 10 megohms but the standard 4. The so-called fault-free zone only requires a minimum of 1000 ohms/volt. It is also important to recognise that values obtained in a clean, dry factory can be considerably reduced when the equipment is installed on site. Introduction The main busbars and the connections between them and the supply side of functional units share the same upstream short-circuit protection. Therefore, in the absence of any relaxation to the contrary, these connections would need to have the same short-circuit withstand strength as the main busbars themselves. However, in practice it may be impossible or, at least, impractical or uneconomic to achieve this short-circuit strength because the rated currents of some functional units may be of a much lower order than that of the busbars and this may have to be reflected in the dimensioning of the associated conductors and, indeed, in the dimensioning of related circuit parts such as plugs and sockets (e.g. in the case of withdrawable functional units). The solution In such cases, the dilemma is resolved by clause of the standard which allows that the conductors between the main busbars and the supply side of functional units may be rated on the basis of the reduced short-circuit stresses occurring on the load side of the short-circuit protective devices in the functional units.this relaxation is subject, of course, to certain provisos. For example, the conductors have to be so arranged that under normal operating conditions, an internal short-circuit... is only a remote possibility. The term fault-free zone is used colloquially to describe this interface zone between the main busbars and the functional units, although it is not actually a term which is used in the standard, which refers to nonprotected active conductors. SCPD in functional unit Main busbars Figure 5. The fault-free zone The fault-free zone A preference for solid rigid conductors led to the inclusion in clause of distribution busbars as particular examples of conductors which may be rated on the basis of down-stream short-circuit protective devices. The standard defines a distribution busbar to be a busbar within 13

17 one section (of the assembly) which is connected to a main busbar and from which outgoing units are supplied. The 1999 edition of the standard introduced a new clause to provide guidance on the selection and installation of conductors in order to satisfy the objective that a short-circuit between the conductors, or between the conductors and earth, should be only a remote possibility. At the same time it encompasses certain other circuits in an assembly where it may not be possible to provide upstream shortcircuit protection. For example, auxiliary circuits, where danger could arise if the supply were to be interrupted through the operation of an upstream protective device. Clause provides this guidance in the form of a table (see Figure 6). It gives examples of conductor types and associated installation requirements. Conductors installed as in this table and having a short-circuit protective device connected on the load side may be up to three metres in length. Type of conductor Bare conductors, or single-core conductors with basic insulation, for example cables according to IEC Single-core conductors with basic insulation and a maximum permissible conductor operating temperature above 90ºC, for example cables according to IEC , or heat-resistant PVC-insulated cables according to IEC Conductors with basic insulation, for example cables according to IEC , having additional secondary insulation, for example individually covered with shrink sleeving or individually run in plastic conduit. Conductors insulated with a very high mechanical strength material, for example FTFE insulation, or double-insulated conductors with an enhanced outer sheath rated for use up to 3 kv, for example cables according to IEC Single or multi-core sheathed cables, for example cables to IEC or IEC Requirements Mutual contact or contact with conductive parts shall be avoided, for example by the use of spacers. Mutual contact or contact with conductive parts is permitted where there is no applied external pressure. Contact with sharp edges must be avoided. There must be no risk of mechanical damage. These conductors may only be loaded such that an operating temperature of 70ºC is not exceeded. No additional requirements if there is no risk of mechanical damage. Figure 6. Examples of conductor types and associated installation requirements (conductors not protected by short-circuit protective devices) 14

18 5. Forms of internal separation (Forms 1-4) Introduction The internal compartmentation of assemblies is dealt with in Clause 7.7 of the standard under the heading "Internal separation of ASSEMBLIES by barriers or partitions". This clause is concerned with the ways in which the busbars and "functional units" in an assembly may be separated from one another - either by fitting interposing barriers or by locating them in separate compartments, and classifies some typical arrangements into four groups - the socalled "Forms of internal separation, Forms 1-4" (see Figure 8). The clause is only concerned with this one aspect of internal separation and does not otherwise preclude the use, for whatever purposes, of other barriers, partitions, shrouds or compartmentation. It should already be emphasised at this stage that the standard also does not impose a requirement on the manufacturer to adopt any of the Form 1-4 separation classifications. Indeed, for some assemblies it can be inappropriate or impossible to do so. For example, since each classification (except Form 1) relates to the separation of functional units from busbars, then it would clearly be difficult to declare any of the other classifications in the case of an assembly having no busbars. Nevertheless, such an assembly can still conform fully with the standard and the manufacturer is entirely free to discuss alternative or synonymous separation arrangements with the client. This is just one reason why the standard makes it clear that the form of separation has to be subject to agreement between the manufacturer and user. The elements to be separated The clause is concerned solely with the separation of busbars and functional units. By busbars is meant the main busbars as well as any associated risers and distribution busbars (droppers). A functional unit is defined as "a part of an assembly comprising all the electrical and mechanical elements that contribute to the fulfilment of the same function." Typical examples of functional units would be incomers, distribution outgoers, individual starters, and the like. The clause also gives special consideration to those terminals which are required for the connection of external conductors to a functional unit and which are treated as an integral part of that unit. These may have to be separated from the terminals of other functional units and/or from the busbars; and in some cases there may be advantages in separating them from the main body of the associated functional unit. Conductors which are connected to a functional unit but which are external to its compartment or associated individual terminal box (e.g. control cables to a common marshalling compartment) would not be considered to form part of the functional unit. 15

19 Separation objectives The standard only considers the two objectives detailed in Figure 7. Either one or both of these objectives may be used as the basis of a classification of the Form of internal separation. Protection against contact with live parts belonging to the adjacent functional units. The degree of protection shall be at least IP2X or IPXXB. Protection against the passage of solid foreign bodies from one unit of an assembly to an adjacent unit. The degree of protection shall be at least IP2X. Figure 7. The possible objectives of internal separation The purpose of the first of these objectives is to ensure that there will be at least fingerproof (IP2X or IPXXB) protection between adjacent functional units. This is to enable an individual functional unit to be disconnected (isolated) from the supply and for its interior to be accessed while the rest of the assembly remains in service. The aim here is to reduce the risk that accidental contact could be made with the live parts of adjacent units or busbars. Similarly, if the functional unit in question is of the removable or withdrawable type and has been removed from the assembly, then access to live parts via the vacated compartment interior must also be restricted. The second objective is to reduce the risk that loose parts, tools or debris, for example, could fall or otherwise pass into an adjacent compartment. This could occur either while the assembly is in service or, as above, while work is being carried out on an individually isolated functional unit. The requirement that the degree of protection must be at least IP2X means that objects greater than 12.5 mm diameter are unable to pass into adjacent functional units. The requirement for at least IP2X or IPXXB protection also makes it clear, subject to certain provisos, that gaps between compartments can be allowed. This verification was lacking in earlier editions of the standard and led to conjecture about the acceptability of gaps. Nevertheless, higher degrees of protection may be demanded for certain applications and these are permitted by the standard subject to agreement between the manufacturer and user. It should be clearly noted that these two objectives are not concerned with separation in terms of arc fault containment. Arc fault containment is not specifically addressed in the standard and might need to be the subject of a special agreement between the manufacturer and user. The means of separation The standard states that an assembly may be divided by means of partitions or barriers (metallic or non-metallic) into separate compartments or barriered sub-sections. In other words the standard is not solely concerned with compartmentation in the strictest sense, i.e. the confinement of 16

20 functional units within their own discrete and virtually sealed housings, but also with their separation through the interposition of simple partitions or barriers. A barrier is defined as a part intended to provide protection against direct contact from any usual direction of access (minimum IP2X) and against arcs from switching devices and the like, if any. Because of the wide variety of assemblies which this standard covers - from small control panels to large motor control centres and distribution boards, and not least because of the variety of constructional possibilities which may be available even within a given assembly type - the standard does not attempt to impose detailed design requirements. It does not dictate, for example, how the partitions or barriers are to be constructed or from what materials they are to be made. Nor has it been found practicable to lay down performance criteria apart from the IPXXB or IP2X verification requirements. The manufacturer is, therefore, allowed considerable freedom in his choice of constructional techniques and materials and may indeed be able to offer the user several possible solutions, even for a given assembly type. This freedom is yet another reason for the standard to state that: the form of separation...shall be the subject of an agreement between the manufacturer and user. [BS EN Clause 7.7] The Forms of separation (Forms 1-4) The basic descriptions of the Forms of separation are set out in Figure 8. Form 1 No separation. Form 2a Separation of the busbars from the functional units. The terminals for external conductors are not separated from the busbars. Form 2b As 2a but the terminals for external conductors are separated from the busbars. Form 3a Separation of the busbars from the functional units and separation of all the functional units from one another. The terminals for external conductors are separated from the functional units, but not from each other or from the busbars. Form 3b As 3a but the terminals for external conductors are separated from the busbars. Form 4a Separation of the busbars from the functional units and separation of all functional units from one another, including the terminals for external conductors which are an integral part of the functional unit.the terminals for external conductors are in the same compartment as the associated functional unit. Form 4b As 4a but the terminals for external conductors are not in the same compartment as the associated functional unit, but are in individual, separate, enclosed protected spaces or compartments. Figure 8. The Forms of separation 17

21 The standard explains that these Forms of separation are to be regarded as typical only. In other words, it is not an exhaustive list and does not preclude other arrangements. Furthermore, just as there is flexibility in the choice of construction techniques and materials for the barriers and partitions, so there is also wide flexibility in the design solutions which can be offered for each Form of separation. There are no definitive solutions! For a given Form, the actual design solution which is chosen is likely to depend on several factors. For example, if the intention of deciding on a Form 4a or 4b arrangement is to allow maintenance staff to access the interiors of individually isolated functional units or their terminals while the rest of the assembly remains live, then the decision may also depend on the likely frequency of access and the qualifications/experience of the persons involved in such activities. It will depend also on the solution possibilities offered by the particular assembly type and, last but not least, it will almost certainly depend on the cost! For example, the user may wish for a Form 4a arrangement in which the terminals are accommodated in the same compartment as the associated functional unit. However, space and other factors may hinder or preclude this option. If, therefore, consideration has to be given to a Form 4b solution in which the terminals are separately accommodated, then a choice of constructions for the terminal housings may be available for a given switchboard type ranging from simple flexible shrouds to individual rigid insulated or steel casings. Each solution has its place - and this is yet another reason for the form of separation to be the subject of agreement between the manufacturer and user. 5.1 National annex to BS EN : 1999 A UK National Annex to the standard was published in order to provide manufactures, specifiers and users with a means of specifying some of these possible solutions. For ease of reference this describes and classifies various basic solutions based on typical UK practice. They are included, where appropriate, against the Forms 1-4 under the heading "Types of construction". The National Annex, first published in 1995, was not updated in the 1999 edition of the standard. It was eventually correctly implemented by Corrigendum No.2, which is taken into account in this edition of the guide. It should be noted, however, that these Types of construction do not preclude other constructional arrangements, nor is it necessary to adopt any of the included Types in order to comply with the requirements of the standard. Nevertheless, adoption of an included Type may assist in the process of achieving agreement between manufacturers and users. Essentially, the "Types of construction" consider three aspects of the assembly design: the arrangement of the terminals for external conductors, i.e. whether or not accommodated in the same compartment as the associated functional unit 18