Session Twelve: Protective Earthing, Impacts, Myths and Verification Tests

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1 Session Twelve: Protective Earthing, Impacts, Myths and Verification Tests Paul Lo MIE Aust., Chartered Professional Engineer Abstract Protective earthing is essential for electrical safety and power system reliability and stability. The effectiveness of protective earthing design and installation affects not only the adequate clearance of the life critical electrical faults, but also the power system reliability and availability. The ineffective protective earthing will lead to the potential hazardous ground voltage (potential ) rise during the ground fault and the prolonged ground potential rise (GPR) will have significant impacts on: 1. Dangerous touch and step voltages 2. Unreliable voltage stability 3. The LV telecommunications insulation breakdown 4. The hazards in the interconnected installations, e.g. equipotential bonding equipment, plumbing, etc. The severity of potential rise and the safety protection with satisfactory fault clearance are dependent on the electrical earthing design and verification testing. The myths of protective earthing in Electrical design: Protective Earthing safety design beyond the limits of MEN Earthing system. Earth leakage disconnection Insulation breakdown of the telecommunications Protective earthing for ground fault damages control and system reliability Practical review of the most common verification tests of the protective earthing: Simple grounding Impedance measurement Traditional current injection tests Off-frequency current injection test. 1

2 Introduction Protective earthing is essential for electrical safety and power system reliability. The effectiveness of protective earthing design and installation affects not only the adequate clearance of the life critical electrical faults but also the power system reliability, stability and availability. 2.0 The ineffective protective earthing will lead to the potential hazardous ground voltage (potential ) rise during the ground fault and the prolonged ground potential rise (GPR) will have significant impacts on: 2.1 Dangerous touch and step voltages 2.2 Unreliable voltage stability 2.3 The LV telecommunications insulation breakdown 2.4 The hazards in the interconnected installations, e.g. plumbing 3.0 The severity of potential rise and the safety protection with satisfactory fault clearance are dependent on the electrical earthing design and verification testing. 3.1 In Australia, the best illustration of our MEN system can be found in Fig B5, AS When the normal (healthy) MEN installation with most of the fault current is conducted by the active and the neutral (return) conductor, the fault current through the earth electrode is minimal and the earth electrode service mainly as the zero reference point for earth potential. 3.3 However when the fault current through the earth electrode is significant due to unhealthy MEN installation (or excessive long feeder with high neutral impedance) the earth potential rise becomes critical to the safety. For illustration the earth fault dynamic with 100A circuit and Type D Circuit breaker, The fault current can be 1250 Amp with 240V and 0.19 ohm ELI, before the CB trips to protect the fault damages. Then, if the fault impedance of neutral conductor diverts 2% maximum fault current to the main earth conductor and the earth electrode with 25 Amp.. The earth grid and main earth conductor with traditional expectation of 2 ohm impedance will have 50 volt GPR,; whilst 5 ohm impedance due to installation degradation will have 125 volt GPR, well above the 50 volt safety limit voltage. 3.4 The HV motor installation and Harmonic dynamic of local Earth potential rise can further endanger the public and any live in contact. 4.0 The Protective Earthing Installation is dictated by various earthing connection systems, and forms the most critical integration of safety earthing protection. AS3000 clause 5.1.4, stipulates various earthing connection systems as alternative to achieve the electrical safety design for mining, direct earthing to IEC60364, direct earthing etc. 4.1 The most common earthing connection system includes IT, TT and TN systems TN System= One of the points in the generator or transformer is connected with earth, usually the star point in a three-phase system. The body of the electrical device is connected with earth via this earth connection at the transformer. TN-C-S system is the most common MEN system with additional configuration of: 2

3 C= Combined Neutral and protective conductor in consumer mains, submains and feeders. S= Separate neutral and earth protective conductors within the electrical services installation TT System - The protective earth connection of the consumer is provided by a local connection to earth, independent of any earth connection at the generator IT system - The electrical distribution system has no connection to earth at all, or it has only a high impedance connection. It use is limited for some special electrical installation when double insulation and /or the insulation monitoring device to monitor the impedance are used. 4.2 Comparison of Earthing Connection Systems Earth fault loop impedance High Highest Low RCD preferred? Yes Yes Yes PE conductor cost Low Low High Risk of broken neutral No No High Safety Safe Less Safe Safe Electromagnetic interference Least Least Low Safety risks Advantages TT High loop impedance (step voltages) Safe and reliable IT Double fault, overvoltage Continuity of operation, cost TN-C-S(= MEN) Broken neutral Safety and cost 2 Protective Earthing installation is made to: safely carry a fault-to-ground; provide a low impedance path-to-ground; eliminate step and touch potential; or Provide a stable reference voltage. Myths of protective Earthing in electrical design: As the Earthing protection installation is critical to the effective safety protection of the electrical services, more attention is therefore required on the risk control and validation of the effective Earthing installation. The following myths of protective Earthing installation must be taken into account: 2.1 MEN Earthing system limits and beyond Protective earthing system safety design is generally guided by AS3000 on MEN system. However there are limits of MEN application, and the effectiveness of protective earthing is subject to the earth loop impedance measurement is low 3

4 enough to clear any earth fault with the circuit protection device, as per Section 8 of AS3000. A long final sub-circuit will most likely defeat the safety protection of the MEN earthing installation when the distance exceeds the Table B, AS3000. It is not uncommon to have more than 200m final sub-circuits to serve the boundary lighting and control devices, in a bigger industrial compounds or infrastructure installations. Protective MEN earthing system must have a low earth loop impedance, that can activate the automatic circuit protection device to clear the earth fault within the time threshold limits of electrical safety. To ensure the effective earthing protection, either MEN earthing system has to be modified or replaced with non-men earthing system. In most cases, the MEN system with additional MEN link(s) in accordance with the Out-building Earthing design will be able to compensate the earth loop impedance to satisfy the time limit of the safety clearance with the time curve of the Circuit Protection Device. If the earth loop impedance is so high, that MEN cannot provide effective protection for electrical faults, then, TT or IT protective earthing design my be required, as stipulated in AS Protective earthing design with Earth Leakage Protection Relay. When MEN earthing system cannot provide effective protection, other earthing system with earth leakage disconnection can be used for safety protection to clear any earth fault and protect human from electrical shocks. It can be either voltage or current based. Voltage based earth leakage protection relay disconnects the power system when dangerous voltage is sensed on the protection earth conductor, immediately. Current base earth leakage protection relay is the famous RCD that protects not only earth fault but also the inadvertent direct contact of human with the live conductors. 2.3 Risk of insulation breakdown of telecommunications The ground potential rise due to power system ground fault may cause telecommunications insulation breakdown and endanger the worker on the affected circuit. Telecommunication installations are design for safety LV and it insulation may be far below the integrity of power system earth fault; thus shall be gauged with the severity of the GPR during the real power system ground fault. 4

5 Physical segregation and/or earthed effective equipotential bonding barrier are necessary to keep the telecommunications safety and integrity, as per AS/S Protective earthing design for Ground fault damage control The Protective earthing design shall construct a sound grounding system with earthing electrode (and grid) to eliminate any potential hazardous ground voltage (potential) rise (GPR) during the ground fault and control the ground fault damages to human and equipment. The effective grounding system shall anchor the earthing reference to the power system for safety and power system stability; under all available electrical faults. Touch and step voltage shall be effectively controlled for site safety. The grounding system must be effective over time without aging degradation to endanger people and equipment, in case of the earth fault. The grounding system must allow the circuit protection device to clear the earth faults timely for electrical safety and selectively discrimination for the highest power system reliability and stability. 2.5 Effect of Leakage stray current and voltage on Earthing installations beyond the scope of safety earthing protection Stray voltage and current is another issues with the power system earthing installations. Stray voltage is the occurrence of electrical potential between two objects that ideally should not have any voltage difference between them, due to small leak current between two grounded objects in distant locations, commonly due to normal stray current flow in the power system. However the most significant stray current damage of electrolysis (galvanic) corrosion is mainly limited to Direct Current only. However, the electrical earthing has side effect with stray current and voltage, that is not directly applicable to electrical safety, hence not further pursued in this paper. 3 Practical review of the most common verification tests of protective earthing grounding system: The severity of potential rise and the safety protection with satisfactory fault clearance are dependent on the electrical earthing design and verification testing. Protective earthing system shall be able to conduct the maximum fault current and the touch and step voltage does not exceed the safety limit. However, soil is not a very good conductor. Only when the area of the earthing path for current is large enough and earth grids/ electrodes are effectively installed and connected, resistance can be quite low and the earth can be a good conductor. 5

6 The Grounding system testing provides a valuable feedback for design verification. The ground system testing does not only ensure that the ground faults can be adequately detected and cleared by the protective device but also control the risk of hazardous high ground grid voltage rise and consequently dangerous touch and step voltages. The impedance of a grounding system is the resistance of soil to the passage of current and tested to: ensure the earth electrode is adequate for the grounding design of electrical safety and power system reliability; monitor the condition of the grounding system over time Check the grounding is operating correctly and no voltage hazard exist both for the people and equipment. Various Earthing impedance measure methods for safety verification and validation are available at different price and the effectiveness of Earthing impedance measurement depends on various applications/influences from harmonic interferences and surges. 3.1 Simple Impedance measurement with 2 point and 3 point methods The simplest and most common method of the ground system test is the earth fault loop impedance measurement with 2 and 3 point methods. In MEN system, the earth fault loop impedance can then be measured by a simple portable tester. However, portable tester may not work for measuring touch and step voltages risks, because long testing leads are required between the ground grid, meter and the test point and also the followings may limit the success of the portable testers: Electrical noise on the ground grid from power system equipment Very low grid impedance (less than 5 ohm) Very large and or extensive ground grid. Ground systems have residual voltages not only at the power system foundation but also harmonic frequencies, due to unbalanced loads, electrostatic and electromagnetic inductions. This frequently leads to noise of stray voltage and current on the protective earthing system. 3.2 Traditional Current injection tests Current injection at the broad band can be used to test the grounding system where the portable ground testers are unsuitable to distinguish between the test signal and any unwanted noise. This traditional high current injection method swamps any noise on the system and can use the traditional RMS wide band multi-meters with a significant power source 6

7 that can dominate the NOISE of the grounding system at the power system foundation frequency. However the required power source for broad band ground system current injection test can be prohibitively high and unrealistic in some electrical installations. The high injection current may also heat up the earthing electrodes/ grid and dry out the grounding surrounding; hence the voltage measurement will not be consistent for the duration of the test. 3.3 Off-frequency current injection test Off-frequency injection using a unique signal is a discrimination method against the interference signals. Off-frequency injection method can measure low injected signal levels with much higher background noise. Off-frequency injection testing requires a frequency locked current injection generator that can tolerate limited induced current and inject constant current (typically less than 100A) for ground system voltage measurement. The injection test frequency is locked away from the foundation and harmonic frequencies to test with an unique signal. A tuned voltmeter shall measure the ground voltage (GPR) due to the current injection but reject the power system noise on the ground system. The remote ground, free from earth fault impacts, is determined when the GPR measurement of the current injection test is relatively constant with the distance away from the injection electrode, as the rate of increase in GPR will decrease with the separation increases from the grounding system. 4 Conclusion AS3000 requires the protective earthing fault-loop impedance must be low enough to permit the passage of current necessary to operate the circuit protective device. The protective earthing system must be maintained properly and verified effectively for safety and risk control. This paper focuses on the verification and validation of effective earthing installation. References AS3000 BS7430 Code of electrical installation Code of Practice for Earthing, British Standards Institute 7