Published:
15 July 2011
Category:
Other
Earth fault loop impedenance test instruments using techniques other than conventional high-current testing are required to prevent tripping of RCDs and certain types of circuit-breaker
Many earth fault loop impedance test instruments use a test measuring current of between 20 A and 25 A. This test current can flow in the earth fault loop for, typically, 40 ms. This relatively high current will usually trip any RCD protecting the circuit and may also trip some circuit-breakers, such as Type B overcurrent circuit-breaker to BS EN 60898 having a rated current less than 10 A.
This unwanted operation of a protective device will prevent the measurement of earth fault loop impedance from being taken and, perhaps more importantly, result in an unwanted disconnection of the supply to the circuit under test.
Regulation 612.9 indicates that the value of Zs may be determined by direct measurement or by an alternative method.
Instrument manufacturers can supply earth fault loop impedance test instruments that are less liable to trip RCDs and circuit-breakers than is the case with a conventional high current earth fault loop impedance test instrument. There are two common techniques that manufacturers employ:
This unwanted operation of a protective device will prevent the measurement of earth fault loop impedance from being taken and, perhaps more importantly, result in an unwanted disconnection of the supply to the circuit under test.
Solution
The value of earth fault loop impedance (Zs) is required to be determined for every distribution circuit and final circuit having Automatic Disconnection of Supply (ADS) as its protective measure against electric shock. The value of Zs is also one of the values required to be recorded on the Schedule of Test Results in the Electrical Installation Certificate and Periodic Inspection Report.Regulation 612.9 indicates that the value of Zs may be determined by direct measurement or by an alternative method.
Instrument manufacturers can supply earth fault loop impedance test instruments that are less liable to trip RCDs and circuit-breakers than is the case with a conventional high current earth fault loop impedance test instrument. There are two common techniques that manufacturers employ:
- d.c. biasing the RCD: this technique users a d.c. bias current to saturate the core of the RCD prior to the test such that the actual Zs measuring test current is undetected by the RCD. (It is important to remember that this technique is not effective for all types of RCD)
- testing a reduced current: this technique uses a test current at or below 15 mA, such that RCDs having a rated residual current of 30 mA and greater will not trip. This technique will also mean that overcurrent circuit-breakers of Type B with a rated current less than 10 A should not trip when measuring Zs.