setting. Also, for an electromechanical instantaneous relay, the distinction between ‘go’ and ‘nogo’
is much better defined than for a time-delay, particularly an induction-disc, relay. As a result,
there is no need to set an instantaneous overcurrent relay with margins such as 200 % of load and
one-third of fault current. However, in addition to the inaccuracies of the relay itself, there is a
factor called ‘transient overreach’ that must be considered. Transient overreach is the tendency of
a relay to instantaneously pick up for faults farther away than the setting would indicate. When
discussing the parameter T in Chapter 3 (section 3.3), we noted that this factor is related to the
time constant of the DC decay of the fault current; the slower the decay, the more overreach is
possible. High-voltage transmission systems are more susceptible to transient overreach than lower
voltage distribution systems because the latter have a lower X/R ratio in their line impedances.
The tendency is also more pronounced in electromagnetic attraction relays than in induction-type
relays. Transient overreach is usually only a concern for instantaneous or zone-1 relays. Their reach
settings are more critical than backup relays, and backup relays have a time delay which allows
the offset to decay. It is therefore common to set an instantaneous relay about 125–135 % above
the maximum value for which the relay should not operate, and 90 % of the minimum value for
which the relay should operate. Solid-state or digital relays can be set closer, e.g. 110 % above the
maximum no-go value.