AP is an Auxillary Relay used in the Closing Circuit of the Circuit Breaker for Protection of the Closing Coil and preventing the Hunting Effect in the Circuit Breaker.
1. Protection of Closing Coil: Coils (closing and tripping) used in Circuit Breakers are not designed for continuous supply, because they don’t need to. For a tripping coil, exciting voltage lasts merely secs or less and the same will be cut off by tripping logic itself. Whereas for closing coil, after the breaker is closed (can be manual or automated) there is a possibility that the continuous supply can be applied (in manual operation). Antipumping Relay opens the closing circuit instantly (faster than Bkr Aux contact) and therby prevents the Closing Coil from Burning.
2. Preventing the Hunting Effect – Lets say that a Feeder Circuit Breaker is switched ON with an existing Fault on the Line, and the Breaker TNC Switch spring fails, so that the TNC Switch gets stuck at Close (C) position. Now, as there is a Fault on the line, the Relay will operate and the Breaker will trip. Since the TNC switch is stuck up at Close position, the Breaker will close, again the Breaker will Trip and close , this goes on and on , damaging the Breaker Mechanism.
The Adjacent diagram shows the Breaker Closing Circuit with Antipumping relay and without Antipumping Relay.
The Coil of the Antipumping Relay is placed in Parallel with the Closing Coil.
The Normally Closed (NC) output contacts of the Antipumping Relay are placed in series with the Closing Coil. Being NC contacts, they are closed when the Brekaer is in OFF condition.
While turning ON the Breaker, the TNC switch gets stuck on C position and so a continous closing command is extended and there is fault existing on the Line.
- The AP relay gets energised and its contact opens up. The Auxillary Contact also opens up but the contact of the AP relay is much faster than the Auxillary contact 52b, and so it protects the Closing Coil.
Now lets see what happens, if the AP relay is not used. As the TNC switch is stuck on C, a continous closing command is extended and 52 b is slightly sluggish , the Closing Coil may get burnt as it is not designed to withstand longer periods of supply.
Now, as there is an existing Fault on the Line the Breaker trips. And as soon as the Breaker trips, the Aux Contact 52b closes.
- Now, if there is an Anti Pumping Relay, as continous DC positive is coming from TNC switch (TNC switch being stuck on C), the AP relay will remain energised. As AP relay is energised, its Contact is open and so Closing Circuit is open and so breaker wont close and thus remain in Off position.
- If the AP relay is absent in the Closing ckt, the continous DC positive coming from the TNC Switch will again Close the Breaker. And as the Fault exists, the breaker will Trip. This Trip-close cycle will continue, till either the closing coil gets burnt or the Bkr mechanism gets damaged.