A.C.
A non magnetic metal disc can rotate in a magnetic field between two electro magnets. The disc is restrained by a coil spring. The flux produces a torque on the side which rotates the trip lever away from the trip contacts.
In reverse power conditions the flux from the voltage coil and current coil interact to rotate the disc in the reverse direction. The amount of torque/current (and hence power) is set on the current coil tapping.
A permanent magnet is provided on the disc to provide damping. A 3 to 5second delay is incorporated into the trip circuit to allow for transients when paralleling.
D.C.
2a and 2b are fitted in case of circulating current via the equalising connection.
Under normal running , fields of '3' and '4' act together to hold the trip contact down. With reverse current fields are in opposition and a spring pushes the plunger against a trip bar to open the reverse current trip relay.
