UFC 3 -520-01
June 10, 2002
9-2.3.2.3 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.
9-2.3.2.4 If electronic multifunction relays are used, overtravel is eliminated and the
coordination time interval can be reduced by the amount normally included for
overtravel.
9-2.3.2.5 For systems using induction disk relays, a decrease of the time interval can
be made by using an overcurrent relay with a special high-dropout instantane ous
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 circuit breaker opening time, plus 0.07 second safety factor).
9-2.3.2.6 Do not reduce the margin unless needed to resolve a particular coordination
problem. Multifunction relays can be very accurate, which can allow for reduced
margins if needed.
9-2.3.3 Relays and Fuses. When coordinating relays with downst ream 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 because a safety factor is
desirable above the total clearing time of the fuse, the same time margin is needed as
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. A similar process should be us ed for
upstream fuses. The relay should actuate and the associated breaker should clear the
fault before reaching the minimum melting time curve of the fuse. Once again, the time
margin should be provided as for relay-to-relay coordination.
9-2.3.4 Direct-Acting Trip Circuit Breakers and Fuses. When coordinating direct -
acting trip low voltage power circuit breakers or MCCBs with source-side fuses at the
same voltage level, a 10 percent current margin can be 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 and the ambient
temperature is lower than about 50 C (122 F), the correction to the minimum melting
time-current curve of the fuse is usually less than 20 percent in time. Because the
characteristic curves are relatively steep at the point where the margin is measured, the
normal current margin applied is usually adequate to allow coordination without making
a fuse characteristic correc tion also. Refer to IEEE 1015 for additional considerations
regarding coordination of direct-acting trip circuit breakers with fuses.
9-4
Previous Page
