Residential
In designing domestic power installations the short-circuit current available on the electrical outlets should not be too high or too low. The effect of too high short circuit current is discussed in the previous paragraph. The short circuit current should be around 20 times the rating of the circuit to ensure the branch circuit protection clears a fault quickly. Quick disconnecting is needed, because in live-ground short circuit the grounding pin potential on the power outlet can rise compared to the local earth (concrete floor, water pipe etc.) during short circuit to a dangerous voltage, which needs to be shut down quickly for safety. If the short circuit current is lower than this figure, special precautions need to be taken to make sure that the system is safe; those usually include using a residual current device (sometimes called a ground fault interrupter) for extra protection.
The short circuit current available on the electrical outlets is often tested when inspecting new electrical installations to make sure that the short circuit current is within reasonable limits. A high short circuit current on the outlet also shows that the resistance from the electrical panel to the outlet is low, so there won't be an unacceptably high voltage drop on the wires under normal load.
The resistance path is the total resistance back through the supply transformer therefore to measure this an engineer will use an "Earth fault loop impedance meter". The application of a low voltage allows a small current to pass from the socket back through earth to the supply transformer and distribution board. The resistance measured can be used to calculate the short-circuit current.
Read more about this topic: Prospective Short Circuit Current
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