Circuit consists of a domestic supply of 230V ac @ 50Hz with a TN-C-S earthing arrangement.
Ze = 0.35Ω. This is the maximum impedance external to a domestic installation with TN-C-S earthing in the UK. Ze equal to or less than this allows the protective device to operate within the permitted time for safety (0.4s for a consumer unit, 5s for a distribution board).
Switch simulates a line-to-earth fault. In a TN-C-S system under fault conditions, fault currents could be as high as 30,000A. A UK consumer unit with a 16kA conditional fault current rating can only handle up to 16,000A; anything higher will blow up the protective devices.
In the circuit, the simulated line-to-earth fault current is 657A (rms).
