A fault current is an abnormal current in an electric circuit due to a fault (usually a short circuit or abnormally low impedance path).

In terms of installation wiring, the prospective short-circuit current must be known as it influences the choice of protective device. If a circuit is to be properly protected, the fault current must be high enough to operate the protective device within as short a time as possible; also the protective device must be able to withstand the fault current and extinguish any resulting arcs without itself being destroyed or sustaining the arc for any significant length of time.

Fault current comes in three varieties: Phase to neutral, phase to phase and phase to earth. These differ widely depending on the type of earthing system used, the installation's supply type and earthing system, and its proximity to its substation. Typically, for a domestic UK 230 V, 60 A TN-S supply, fault currents may be in the thousands of amps. Prior to selecting protective devices, prospective fault current must be measured reliably at the origin of the installation and at the furthest point of each circuit, and this information applied properly to the application of the circuits.

For the exact requirements concerning UK wiring systems consult the most recent edition of BS7671 and the Building Regulations.

Batteries

The prospective fault current of larger batteries, such as deep-cycle batteries used in stand-alone power systems, is often given by the manufacturer.

In Australia, when this information is not given, the prospective fault current in amps "should be considered to be 6 times the nominal battery capacity at the C A.h rate," according to AS 4086 part 2 (Appendix H).

See also

• Ground (electricity)
• Residual-current device
• Electrical wiring

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