Fig. 2,348.—Diagram showing three wire dynamo protected by double pole double coil overload circuit breaker (or two single pole breakers with interlock) with bell alarm switches. Complete protection is secured as breaker is connected between armature and series field.
Operating conditions sometimes make it advisable for the generator circuit breakers to open only after the auxiliary and feeder circuit breakers have failed to isolate the trouble.
This is accomplished by using direct current series inverse time limit relays to trip the generator circuit breakers.
Instantaneous reverse current relays are used to trip the machine circuit breaker of battery charging sets, rotaries and motor generator sets to prevent their running as a motor on the charging or direct current end. These relays can act only in case of current reversal.
To prevent serious unbalancing of voltages in Edison three-wire systems causing trouble, differential balance relays are used to trip the circuit breakers on a small percentage of unbalancing.
CHAPTER LIX
LIGHTNING PROTECTION DEVICES
Lightning protection devices, or lightning arresters, are devices for providing a path by which lightning disturbances or other static discharges may pass to the earth.