Pickup
The term pickup has acquired several meanings. For many devices,
pickup is defined as the minimum current that starts an action. This definition
is accurately used when describing a relay characteristic.
Pickup also describes the performance of a low-voltage
circuit breaker with an electronic trip device. However, the term does not
apply accurately to the thermal trip element of a thermal magnetic molded-case
circuit breaker (MCCB), which deflects as a function of stored heat.
The pickup of an overcurrent protective relay has generally
been considered the minimum value of current that causes the relay to close its
contacts. The current (or tap) setting of the relay and the minimum pickup were
synonymous. However, with new technology developments in static overcurrent
relays, this definition needs more clarification.
Electromechanical Versus Static Relays
The pickup value for electromechanical induction disk
time-overcurrent relays is the minimum current that causes the disk to start
turning and ultimately close its contacts. This value is not necessarily the
tap setting on the relay.
The time it takes the contacts to close is a function of the
dynamics of the relay’s magnetic circuits and the manufacturer’s tolerances. At
the pickup value, the time to contact closure is long, and the accuracy is less
than desired.
Any deviation in the applied current results in significant
time changes. As a result, manufacturers generally do not plot their time
curves below 1.5 to 2 times minimum pickup (see Blackburn1).
The tap or current setting of static relays usually correspond
to the pickup current. Also, the trip time is much more accurate, especially in
the range of 1.0 to 2.0 times minimum pickup.
However, manufacturers still do not plot their time curves
in this range to correspond with the electromechanical devices.
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