Considering first alternating current circuits, the prevailing practice is to make the circuit breakers by which the alternators are connected to the low tension bus non-automatic, in order to insure minimum interruption of alternator service. The chance of trouble in this part of the circuit is remote, but should it occur, the station attendant could generally open the circuit breaker before the machines would be injured.
Reverse current relays of instantaneous or time limit types are often connected to the secondaries of current and of pressure transformers to indicate by lamp or bell any trouble that may occur in the generator circuit.
These relays operate with a low current reversal at full pressure and conversely with a proportionally greater current at voltages less than normal. At zero pressure, the relay would act as an overload one, set for high overload. At zero current, a voltage considerably in excess of normal would be required to operate it.
Fig. 2,330.—Diagram showing two phase motor or feeder circuit protected by double pole double coil, overload circuit breaker (or two single pole breakers interlocked) with bell alarm switch.
Specifications sometimes call for automatic generator circuit breakers: in this case definite time limit overload relays are used. They are connected in the secondaries of current transformers and are designed to give the same time delay for all trouble conditions; they allow the defective circuit to be opened, if possible, at a point more remote from the generator than the generator circuit breaker.
Fig. 2,331.—Condit 600 volt, 1,500 ampere, single pole type K circuit breaker pneumatically operated. It is the same as the electrically operated circuit breaker, except that a pneumatic cylinder mechanism is supplied in place of either the electromagnet or the motor. This cylinder mechanism is so arranged that the air pressure is only on the cylinder at the instant of operation. At all other times the air pressure is shut off by means of a control valve. The kind of remote control to be used depends on local conditions. In general, the hand operated remote control device is preferable where conditions are such that it can be used, and where it is necessary to use electrically operated, the motor operated type is recommended if conditions be such that slow closing is not objectionable.
When the total generator capacity exceeds the rated rupturing capacity of the circuit breakers, one or more sectionalizing circuit breakers are placed in each bus.