Quantcast Direct-Acting Trip Circuit Breakers and Fuses - mo2010145

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separation is planned between the different characteristic curves. This lack of a specified time
margin is explained by the incorporation of all the variables plus the circuit breaker operating
times for these devices within the band of the device characteristic curve.
4.7.4 Pickup Current. The term pickup has acquired several meanings. For many devices,
pickup is defined as that minimum current which 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 breaker, which operates as a function of stored heat.
4.7.4.1 Overcurrent Relay. The pickup current of an over current protective relay is the
minimum value of current which will cause the relay to close its contacts. For an induction disk
time-overcurrent relay, pickup is the minimum current which 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.
4.7.4.2 Low-Voltage Circuit Breakers. For low-voltage power circuit breakers, pickup is
defined as that calibrated 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 longtime 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.
4.7.4.3 Molded-Case Circuit Breakers. For molded-case circuit breakers with thermal trip
elements, tripping times, not pickups, are discussed. This is because a properly calibrated
molded-case circuit breaker carries 100 percent of its rating at 25C in open air. The
instantaneous magnetic setting could be called a pickup in the same way as that for low-voltage
power circuit breakers.
4.7.5 Current Transformer Saturation. A current transformer produces a current applicable to
standard protective relays and in a specific proportional and phase relationship to the primary
current. This current is used by current meters, power meters, and protective relays. Current
transformer saturation can slow induction disk relay operation. When the current transformer
becomes saturated, due to a high burden or many times full-load current, the actual secondary
relay current is less than it should be. This causes the relay to operate more slowly than it should,
or not at all. Instantaneous elements should be set below the current transformer saturation point
so they will not be affected by a saturation condition. In most industrial systems, current
transformer saturation is a problem only on circuits with relatively low-ratio current
transformers.
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