What is the function of a fault loop impedance meter?

• A. To measure insulation resistance.
• B. To measure leakage current from appliances.
• C. To measure fault loop impedance (Zs) and verify protective devices will trip in the event of a fault.
• D. To measure earth electrode resistance.

Answer: (C)

The key idea is that safety in fault conditions depends on how quickly the protective device will interrupt current. The fault loop impedance, Zs, represents the total resistance the fault current must overcome from the supply, through the circuit conductors and protective conductor, back to the source. A fault loop impedance meter measures this total Zs so you can confirm that, if a fault occurs, the current will be high enough and the protective device (fuse or breaker) will trip within the required time. It does this by injecting a known test current and measuring the resulting voltage, then calculating Zs from those readings. This helps ensure the installation will disconnect fast enough to protect people from shock.

Other options don’t fit because they test different things: insulation resistance testers check how well insulation blocks current, not the fault-current path; leakage current testers measure stray currents during normal operation; earth electrode testers measure the resistance of the earth rod itself. None of those directly verify that a fault will cause the protective device to trip within the safe time frame.