Measurement of the earth fault loop impedance
According to IEC/HD 60364-6:2016, the protective measures of electrical installations must be verified.
One requirement is the measurement of the earth fault loop impedance. What is the background or reason of this measurement?
In the event of a earth fault (between the live conductor and the protective conductor), a residual current will flow, which causes to trip the protective device.
In a voltage circuit (with e.g. 230 V) within a closed loop, according to Ohm's law, the resistance/impedance of this circuit will limit the short circuit current.
In the event of an earth fault, the protective device should trigger as fast as possible, for this the short circuit current should be as high as possible.
Conversely, this will require a loop resistance/impedance as low as possible.
This short circuit current must be high enough this to provide the protection by automatic disconnection of supply.
The data sheets of the protective devices as breakers or fuses will state that this protection devices will trip within 0.2 s at the following cut-off currents.
Here some examples:
- Breaker type B at 5x of nominal current (e.g. B16A at 80 A)
- Breaker type C at 10x of nominal current (e.g. C16A at 160 A)
- Breaker type K at 12x of nominal current (e.g. K16A at 192 A)
- Fuse type gG (D01, D02) at ~8...10x of nominal current (e.g. gG16A at 130 A)
When this cut-off currents are met, this will fulfil the compliance with the rules of IEC 60364-4-41 with the maximum disconnection times in table 41.1
Fluke installation testers will display both.
- Loop impedance
- Prospective earth fault current (PEFC), or
prospective short circuit current (PSC)
The lower the loop impedance, the greater the resulting short-circuit current.
The inspector must decide if the measured values meet the requirements of the protective device to comply the automatic disconnection of supply.