IMPORTANT TERMS REGARDING RELAYS
(i) Pick-up current - It is the minimum current in the relay coil at which the relay starts to operate when the current in the relay is less than the pick-up value there lay does not operate and the breaker controlled by it remains in the closed position. When the relay coil current is equal to or greater than the pickup value, the relay operates to energies the trip Coil that opens the circuit breaker.
(ii) Current setting – It is often desirable to adjust the pickup current to any value. This is know as current setting and is usually achieved by the use of tapping on the relay coil. The taps are brought out to a plug bridge, which permits to alter the number of turns on the relay coil, this changes the torque on the disc and the hence the time of operation of the relay.
Pickup current = Rated secondary current of CT x Current setting
(iii) Plug setting multiplayer (P.S.M) – It is the ratio of fault current in the relay coil to the pick – up current.
P.S.M = Fault current in relay coil
pick up current
= fault current in relay coil
rated secondary current of CT x current setting
(iv) Time-setting multiplier – A relay is generally provided with control to adjust the time of operation. This adjustment is known as time setting multiplier the time setting dial is calibrated from 0 to 1 in steps 0.05. These figures are multipliers to be used to
convert the time derived from time/p.s.m curve into the actual operating time.
The actual time of operation is calculated by multiplying the time setting multiplier with the time obtained from time/ p.s.m curve of the relay.
(i) Time/ P.S.M– it’s the curve between time of operation and plug setting multiplier
of a relay.