8.2.3 Instantaneous relays
There are several constraints imposed upon the use of instantaneous relays; some of them depend
upon the design of the relay. In all cases, of course, the relay must not operate on inrush, or for
low-side faults. Peak magnetizing current in a transformer can be as high as 8–10 times peak
full-load current. Since the relay will see low-side faults, one must consider these faults when they
are fully offset. Some relay designs – for example electromechanical plunger-type relays – respond
to the actual instantaneous value of the current, which includes the DC offset. Such a relay must
be set above the low-side fault currents with full DC offset. Disc-type relays respond only to the
AC portion of the current wave. Solid-state or computer-based relays may or may not respond to
the DC offset, depending upon their design.