When selecting protection devices, consideration must be given to the prospective short-circuit current (PSC) at the location the device is to be installed in (AS/NZS 3000:2018 clause 2.5.4.1). The PSC can be determined by measurement or by calculation. Many multi-function testers are able to measure the PSC.

PSC can also be calculated by undertaking a Live Fault Loop Impedance measurement (in ohms) and dividing the voltage by this measurement.

Example

Voltage tests as 240 volts and the measured fault loop impedance between incoming line and neutral is 0.08.

Max PSC = E/R

240/0.08 = 3000 A or 3kA

Three Phase Supplies
Where there is a three phase supply, the PSC is likely to be between line conductors. In this case, the PSC can be calculated by multiplying the single phase reading by the square root of 3 or 1.73. Therefore in the example above, the PSC would be 3000 x 1.73 = 5190 A or 5.19kA.