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.