Fault current is the current driven through a short circuit or earth fault, limited only by the impedance of the source, the network and the fault itself. It can be tens of times normal load current, from a few kiloamperes on a distribution feeder to over 60 kA on a large station busbar. Its value sets breaker breaking capacity, busbar and cable withstand ratings, relay settings and arc flash energy.
Max fault current at the LV board is 48 kA, so those 36 kA breakers in the old panel are underrated and have to go.
Fault level is often quoted in MVA (1.732 x voltage x fault current), so 'the 11 kV bus is 250 MVA' is a statement about fault current, not load.
Also heard as
- short-circuit current
- fault level
- available fault current
Related terms
Short circuit
Unintended low-impedance path between conductors at different potentials, bypassing the load.
Breaking capacity
Highest fault current a circuit breaker or fuse can interrupt safely at its rated voltage.
X/R ratio
Ratio of reactance to resistance in the fault path, which sets how large and slow the DC offset in fault current is.
Arc flash
Explosive release of heat, light and pressure when an electric arc forms through air between energised parts.