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A good type of circuit breaker.

breaker with independently operated arms, the usual lever switch in series may be dispensed with, since such a circuit breaker performs the functions of both switch and single pole breaker. Either arm may be closed and is then free to trip, and will trip instantly in response to an overload or short circuit (should such exist) when the second arm is closed.

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Figure 137. General Electric Co.'s breakers.

Circuit breakers of the type shown in Fig. 137 are suitable for every class of service requiring automatic protective devices, and are used extensively in steel mills, machine shops employing electrically driven tools, office buildings, etc. They are indispensable for general motor

Details of Working.

work, automobile charging sets, cranes, and electrically driven machinery in general.

The laminated brush is a feature of this circuit breaker. In closing the breaker a rubbing motion is imparted to the brushes which insures clear contacts.

The circuit is finally broken and the arc ruptured by

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Figure 138. General Electric Co.'s circuit breaker.

means of carbon contacts of simple construction securely fastened to metal arms. The carbon blocks may be quickly and easily replaced if burned.

Extreme ease of operation is secured by the use of a toggle movement, and the breaker will not jar out, nor will it open unless purposely tripped by hand or actuated by the automatic tripping device in the event of an overload or short circuit.

The tripping point may be varied by adjusting the air

Another type circuit breaker.

gap between the electro-magnet (overload coil) and its armature. With the exception of those in the underload circuit breaker, no springs are used in the tripping device, the force on the armature being opposed to gravity. The trip is positive, the pull on the armature increasing as the air gap decreases, consequently when the armature begins to rise it will continue to do so with increased force until

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Figure 139. General Electric Co.'s circuit breaker.

the breaker trips. With this construction any danger from "creeping loads" is obviated.

Two other types are shown in Figs. 138 and 139.

It will be noticed that in this circuit breaker each switch blade is held by an independent catch; on this account it can be used as a main switch at the same time that it acts as a circuit breaker.

Why this is the case will be understood from the following: A circuit breaker may act because the current

Magnetic blow out.

is increased beyond the maximum strength on account of an abnormal load upon some motor, or because there is an excessive drain of any kind. It may also act, however, on account of the insulation in some part of the cir cuit giving out and thus establishing a short circuit. If the abnormal increase in current arises from the first

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Figure 140. General Electric Co.'s circuit breaker.

named causes, it is more than probable that by the time the attendant replaces the switch the demand will have reduced to the normal. If the circuit has acted on account of the current increasing through a short circuit, then it will rise to the same, when the breaker is reset, for the short circuit will still exist. Under such conditions it is injurious, more or less, to every device in the circuit to keep the circuit breaker closed for any length

Using a main switch.

of time. If each side of the breaker operates independently of the other, there is no danger of closing the circuit when it is grounded-that is, if the device is handled properly.

In setting the breaker the proper procedure is to close one switch at a time. Closing the first one does not complete the circuit. As soon as the second switch is closed, if the circuit is short circuited, the current will rise beyond the maximum, and the first switch being free to act, will at once fly open again. If through stupidity the attendant tries to replace the first switch after having secured the second one, then the latter will fly open, and by that time he will come to the conclusion that there is something wrong. If both sides of the circuit breaker are tied together, it is possible to do considerable damage by holding the switch blade in position when the circuit is grounded.

To avoid such contingencies, a main switch is used, with such breakers, and in the proper handling of the devices the circuit breaker is closed first. Then when the switch is closed, if the current rises above the normal, the

circuit breaker will fly open. The independent-action circuit breaker can take the place of the main switch, because when the second handle is raised into position the first one is free to swing back if the current rises to a dangerous strength.

Another way in which the disastrous effects of the arc can be reduced is by using a magnet to blow out the spark. A circuit breaker of the magnetic blow out type is illustrated in Fig. 140, showing the position of the operative parts when closed.

Fig. 141 shows how this circuit breaker is connected.
In the chapter on switches we showed designs in-

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