Session Twelve: Protective Earthing, Impacts, Myths and Verification Tests
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.
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Session Twelve: Protective Earthing, Impacts, Myths and Verification Tests
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,
AS3000.
3.2 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,
3.3.1 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.
4.1.1 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:
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Session Twelve: Protective Earthing, Impacts, Myths and Verification Tests
C= Combined Neutral and protective conductor in consumer mains, submains and
feeders.
S= Separate neutral and earth protective conductors within the electrical services
installation.
4.1.2 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.
4.1.3 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
TT
IT
TN-C-S(=
MEN)
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
Least
Low
Electromagnetic interference Least
Safety risks
High loop impedance
(step voltages)
Double fault,
overvoltage
Broken neutral
Advantages
Safe and reliable
Continuity of
operation, cost
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
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Session Twelve: Protective Earthing, Impacts, Myths and Verification Tests
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 AS3000.
2.2
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.
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Session Twelve: Protective Earthing, Impacts, Myths and Verification Tests
Physical segregation and/or earthed effective equipotential bonding barrier are
necessary to keep the telecommunications safety and integrity, as per AS/S009.
2.4
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.
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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.
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Session Twelve: Protective Earthing, Impacts, Myths and Verification Tests
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
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Session Twelve: Protective Earthing, Impacts, Myths and Verification Tests
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.
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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
Code of electrical installation
BS7430
Code of Practice for Earthing, British Standards Institute
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