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 (a) The 0.3-0.4 second margin may be decreased if field tests of relays and circuit
breakers indicate the system still coordinates with the decreased margins. The overtravel of very
inverse and extremely inverse time overcurrent relays is somewhat less than that for inverse
relays. This allows a decrease in time interval to 0.3 second for carefully tested systems. When
solid-state relays are used, overtravel is eliminated and the time may be reduced by the amount
normally included for overtravel. For systems using induction disk relays, a decrease of the time
interval may be made by using an overcurrent relay with a special high-dropout instantaneous
element. This is set at approximately the same pickup as the time element, with its contact wired
in series with the main relay contact. This eliminates overtravel in the relay. The time interval
often used on carefully calibrated systems with high-dropout instantaneous relays is 0.25 second.
The minimum time interval using a high-dropout instantaneous relay could be 0.15 second (that
is, 0.03 second instantaneous reset, plus 0.05 second vacuum circuit breaker opening time, plus
0.07 second safety factor).
4.7.3.2 Relays and Fuses. When coordinating relays with downstream fuses, the relay
overtravel and circuit breaker opening time do not exist for the fuse. The margin for overtravel is
plotted beneath the relay curve, and since a safety factor is desirable above the total clearing time
of the fuse, the same time margin is needed for relay-to-relay coordination. Reduction of the
margin is acceptable, however, when below 1 second. The same margin is used between a
downstream relayed circuit breaker and the damage curve of the fuse.
4.7.3.3 Direct-Acting Trip Circuit Breakers and Fuses. When coordinating direct-acting
trip low-voltage power circuit breakers with source-side fuses at the same voltage level, a 10
percent current margin is sometimes used. This allows for possible fuse damage below the
average melting time characteristics. The published minimum melting time-current curve should
be corrected for ambient temperature or preloading if the fuse manufacturer provides the data
necessary to perform this correction. If the fuse is preloaded to less than 100 percent of its
current rating, however, and the ambient temperature is lower than about 50C, the correction to
the minimum melting time-current curve of the fuse is usually less than 20 percent in time. Since
the characteristic curves are relatively steep at the point where the margin is measured, the
normal current margin applied probably is sufficient to allow coordination without making a fuse
characteristic correction also.
4.7.3.4 Direct-Acting Trip and Relayed Circuit Breakers. When low-voltage circuit
breakers equipped with direct-acting trip units are coordinated with relayed circuit breakers, the
coordination time interval usually used is 0.4 second. This interval may be decreased to a shorter
time as explained previously for relay-to-relay coordination. The time margin between the fuse
total clearing curve and the upstream relay curve could be as low as 0.1 second where clearing
times below 1 second are involved.
4.7.3.5 Direct-Acting Trip Circuit Breakers. When coordinating circuit breakers equipped
with direct-acting trip units, the characteristic curves should not overlap. In general, only a slight
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