Typical examples of transient faults include: Transmission and distribution systems use an automatic re-close function which is commonly used on overhead lines to attempt to restore power in the event of a transient fault.
It would be unusual in a well-designed power system to have a metallic short circuit to ground but such faults can occur by mischance.
[citation needed] Such faults can cause objectionable circulating currents, or may energize the housings of equipment at a dangerous voltage.
Some special power distribution systems may be designed to tolerate a single ground fault and continue in operation.
Wiring codes may require an insulation monitoring device to give an alarm in such a case, so the cause of the ground fault can be identified and remedied.
Such an arc can have a relatively high impedance (compared to the normal operating levels of the system) and can be difficult to detect by simple overcurrent protection.
Utility, industrial, and commercial power systems have additional protection devices to detect relatively small but undesired currents escaping to ground.
Electric motors can also be considered to be generators, because when a fault occurs, they usually supply rather than draw power.
However, due to the linearity of power systems, it is usual to consider the resulting voltages and currents as a superposition of symmetrical components, to which three-phase analysis can be applied.
In the method of symmetric components, the power system is seen as a superposition of three components: To determine the currents resulting from an asymmetric fault, one must first know the per-unit zero-, positive-, and negative-sequence impedances of the transmission lines, generators, and transformers involved.
The individual circuits are then connected together in a particular arrangement that depends upon the type of fault being studied (this can be found in most power systems textbooks).
The solution results in voltages and currents that exist as symmetrical components; these must be transformed back into phase values by using the A matrix.
Analysis of the prospective short-circuit current is required for selection of protective devices such as fuses and circuit breakers.
For example, for a domestic UK 230 V, 60 A TN-S or USA 120 V/240 V supply, fault currents may be a few thousand amperes.
Terminal methods can be used to locate the general area of the fault, to expedite tracing on a long or buried cable.
While this test contributes to damage at the cable site, it is practical because the faulted location would have to be re-insulated when found in any case.
In Australia, when this information is not given, the prospective fault current in amperes "should be considered to be 6 times the nominal battery capacity at the C120 A·h rate," according to AS 4086 part 2 (Appendix H).