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.
9-2.4.1 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
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
induction disk time-overcurrent relay, pickup is the minimum current that will cause the
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.
9-2.5.1 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