Fault Types and Fault Level

Types of fault

• Three-phase fault — all three phases shorted. Rare, but usually gives the highest fault current and is the basis for equipment short circuit ratings.
• Phase to phase — two phases shorted.
• Phase to earth — by far the most common fault, typically from insulation failure, moisture, contamination, mechanical damage or an animal.
• Double phase to earth, and open circuit faults such as a broken conductor or a blown fuse causing single phasing.
• High impedance faults — a conductor touching a poorly conducting surface, drawing a current that may be below normal load and is therefore very hard to detect.

Fault level

The prospective fault current at a point is set by the source capacity and the impedance between the source and that point. It matters for three reasons: every device must be able to interrupt it, every busbar and cable must be able to withstand its mechanical and thermal effects, and protection settings are derived from it.

Fault level changes when the system changes. Adding a generator, changing a transformer, running with a bus coupler closed, or connecting to a stronger supply all raise it. Fault level study must be revisited after any such change, and equipment ratings checked against the new figure. Operating with the bus coupler closed on a system designed for split operation is a common way of exceeding equipment ratings without anybody noticing.

Fault current behaviour

Fault current is not constant. It starts with a decaying DC offset superimposed on the AC component, and for faults near generators the AC component itself decays from a high subtransient value through a transient value to a lower steady state. Protection and equipment ratings account for these separately, and a generator's steady-state fault contribution can be low enough that overcurrent protection alone will not detect a fault reliably — which is why generators need voltage-restrained or voltage-controlled overcurrent protection.