Fig. 2,336 and 2,337.—Diagram showing three phase no voltage connections for I-T-E circuit breaker. The no voltage coils for three phase circuits are connected in Δ by means of binding posts A, B, C and D on the face of the base, and from the A and B of the no voltage coils, connections are made respectively to spring contacts 1 and 2 of the small disconnecting switch. Each of the contacts a and b of the disconnecting switch is connected respectively through resistance R2 and R1 to one of the mains at aa and bb. The terminal C is connected through resistance R3 on the back of the board to the middle main as shown at point 3. The terminal D is linked on the back of the board to terminal B to complete the [Greek: D
connection. The terminals aa, bb and 3 of the circuit breaker must, in all cases, be so connected that they will be subject to full voltage of the circuit irrespective of the position of the starting switch. Each no voltage coil is supplied with two terminal wires, one covered with green and one with black insulation. In replacing these coils particular care should be taken to see that the terminal wires connected to any one binding post are of unlike color.]
In plants in which two or more banks of transformers are operated in parallel between high and low voltage busses, it is desirable to have for each transformer bank, an automatic circuit breaker equipment which will act selectively and disconnect only the bank in which trouble may occur. With a circuit breaker on each side of transformer bank, selective action may be secured in two ways as follows:
1. By means of an instantaneous differential relay connected in the secondaries of current transformers installed on both the high and low voltage sides of each transformer bank.
The relay operates on a low current, reversal on either side of the bank.
2. By means of one inverse time limit, secondary or series relay installed on that side of the transformer bank which is opposite the source of power, the relay being arranged to trip both the high and low voltage circuit breakers.
The first method has the disadvantage of high first cost due to the high voltage current transformers required, but is more positive than the second method and is independent of the number of transformer banks in parallel.