Ans. Simple knife switches are suitable for use when the circuit is not liable to be opened while carrying large current. A circuit breaker, operated automatically or by hand should be used for interrupting heavy currents.
Figs. 479 and 480.—D & W fuse indicator. The operation is illustrated in the figures which show appearance of the label before the blowing of the fuse, fig. 479, and the same fuse blown, as indicated by the appearance of the black spot within the circle fig. 480.
Circuit Breakers.—A circuit breaker is a switch which is opened automatically when the current or the pressure exceeds or falls below a certain limit, or which can be tripped by hand.
Ques. What is the construction of a circuit breaker?
Ans. It is composed of a switch and a solenoid in the main circuit. When the current, flowing through the circuit, exceeds a certain value, the core of the solenoid is drawn in and trips a trigger which allows the switch to fly open under the action of a spring.
Figs. 481 to 486.—Various open fuses. Fig. 481, fuse for main and branch blocks; fig. 482, standard railway fuse; fig. 483, Edison main style; fig. 484, W. U. pattern; fig. 485, Bell telephone style; fig. 486, sneak current fuse. When an open fuse "blows" as a result of overloading, the rupture is accompanied by a flash, and by spattering of the fused material. With large currents this phenomenon is a source of danger, and the use of enclosed fuses is accordingly recommended whenever the rating of the fuse exceeds 25 amperes. Various types of enclosed fuse are shown in figs. 468 to 470.
There are numerous kinds of circuit breaker to meet the varied conditions of service of which may be mentioned the following:
1. Maximum circuit breaker;
2. Minimum circuit breaker;
3. Reverse current circuit breaker;
4. Maximum and reverse circuit breaker;
5. No voltage circuit breaker.