Ques. What is the objection to alternating current trip coils?
Ans. They have relatively high impedance and impose a heavy volt ampere load on the transformers.
Circuit Closing Relays.—The duty of a circuit closing relay is to close the auxiliary circuit at the time when the predetermined abnormal condition is reached in the primary circuit. The closing of the auxiliary circuit energizes the trip coil and opens the breaker.
Figs. 2,283 to 2,291.—General Electric instantaneous overload circuit opening relays, covers removed. Circuit opening relays are used chiefly in those cases where direct current for the tripping circuit is not available. Alternating current trip coils have relatively high impedance and impose a heavy volt ampere load on the current transformers. To reduce this load during normal operation the circuit opening relay is frequently used and is usually necessary where instruments and meters are to be operated on the same current transformers as the trip coils if the greatest accuracy be required. The relay contacts in the normal, closed position, short circuits the trip coil. When the relay operates on overload or other abnormal condition the contacts are opened with a quick break, sending the current through the trip coil circuit momentarily and tripping the switch. With circuit opening relays, the trip coils of the oil switch must be set to trip somewhat lower than the setting of the relay. In construction the relay consists of a solenoid with iron frame forming the support for the relay; a central plunger or armature of special construction which is picked up or released by the magnetic action of the solenoid; a plunger rod which actuates the relay contacts, which are mounted on an insulated base usually above the solenoid; a tube or plate for the calibration marking and adjustment; covers of glass or metal to keep out dust; terminal boards with points corresponding to tagged leads from relay coils and external wiring diagrams. The relay contacts are of two kinds, circuit opening, as shown above, and circuit closing, as shown in figs. 2,292 to 2,300.
Ques. What kind of current is generally used for the auxiliary circuit of a circuit closing relay?
Ans. Direct current.
Ques. At what pressure?