UFC 3 -520-01
June 10, 2002
9-2.3.5 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 should be 0.4 second. This interval can be
decreased to a shorter time as explained previously for relay-to-relay coordination.
9-2.3.6 Direct-Acting Trip Circuit Breakers. When coordinating circuit breakers
equipped with direct -acting trip units, the characteristic curves should not overlap. In
this case, only a slight separation is necessary between the different characteristic
curves. This lack of a specified time margin is based on the incorporation of all the
variables plus the circuit breaker operating times for these devices within the band of
the device characteristic curve.
9-2.4
Pickup Current.
9-2.4.1 Description. The term pickup has acquired several meanings. For many
devices, pickup is defined as the minimum current that starts an action. It is accurately
used when describing a relay characteristic. It is also used in describing the
performance of a low voltage power circuit breaker. The term does not apply accurately
to the thermal trip of a molded case circuit b reaker, which operates as a function of
stored heat.
9-2.4.2 Overcurrent Relay. The pickup current of an overcurrent protective relay is
the minimum value of current that will cause the relay to close its contacts. For an
disk to start to move and ultimately close its contacts. For solenoid-actuated devices
with time-delay mechanisms, this same definition applies. For solenoid-actuated
devices without time-delay mechanisms, the time to close the contacts is extremely
short. Taps or current settings of these relays usually correspond to pickup current.
9-2.4.3 Low Voltage Circuit Breakers. For low voltage power circuit breakers,
pickup is defined as that calibrate d value of minimum current, subject to certain
tolerances, which will cause a trip device to ultimately close its armature. This occurs
when either unlatching the circuit breaker or closing an alarm contact. A trip device with
a long-time delay, short -time delay, and an instantaneous characteristic will have three
pickups. All these pickups are given in terms of multiples or percentages of trip-device
rating or settings.
9-2.4.4 MCCBs. For MCCBs with thermal trip elements, tripping times, not pickups,
are defined. The instantaneous magnetic setting could be called a pickup in the same
way as that for low voltage power circuit breakers.
9-2.5
Coordination Curves.
9-2.5.1 Provide coordination curves as part of the facility design package. On a
coordination curve, time 0 is considered as the time at which the fault occurs, and all
times shown on the curve are the elapsed time from that point. A coordination curve is
arranged so the region below and to the left of the curve represents an area of no
9-5
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