Fault Currents and Short Circuit Decrement Curves

A generating set operating in service will be connected to a distribution system serving the connected load[s]. This distribution system has impedance, and so will introduce a value of Z between the alternator terminals and the point at which the ‘fault’ has occurred. The ‘fault’ will also have a value of Z, and if the nature of the ‘fault’ is an ‘earth-fault’, then the ‘earth-return’ path will also have a level of impedance. The total ‘fault’ circuit / loop impedance value will much modify the actual level of alternator fault current displayed on the alternator’s published Short Circuit Decrement Curve.

Curves are drawn for Star (Wye) connected machines. For other connections the following multipliers should be applied to current values:

Parallel Star = Curve current value x 2

Series Delta = Curve current value x 1.732

Maximum sustained duration. Note 2 also details the maximum sustained duration of a fault condition that the alternator can withstand. The disconnection time from the fault is critical to prevent damage to the alternator winding. Safe guidance is considered to be: < 10sec for a fault across 3 phases (L-L-L), < 5sec for a fault across 2 phases (L-L) and < 2 sec for single phase to Neutral fault (L-N)