When there is a fault in an electrical system, such as a short circuit, fault current is the immense current (in amperes) that briefly flows before the overcurrent device (a fuse or circuit breaker) trips/blows. Understanding fault current is important when designing electrical systems because a system that is not properly rated for high fault current can overheat enough to start a fire or be permanently damaged before the overcurrent device trips.

For example, a typical load center panel has particular fault current rating that must be matched with its overcurrent devices and the upstream power supply.

Fault current can lead to impressive fireworks displays and a lot of noise, particularly in high voltage, high current systems. Utility workers will put on hearing protection before closing a high voltage dropout fuse on a utility pole after replacing it, so that if the fault that blew the fuse is still there, the resulting *BANG* of the fuse blowing again won't damage their hearing.