Installing Circuit
Protection Devices

[lesson title]

INSTALLING CIRCUIT BREAKERS

Circuit breakers combine the functions of manual disconnect and overcurrent protection in a single device. Circuit breakers are available in ratings of 15 to 200 ampere for residential use (Figure 1-13). Larger sizes are made for commercial or industrial application. Circuit breakers, like fuses, are rated in amperes.

Figure 1-13.   Typical 20-ampere circuit breaker

Internal Mechanisms

The internal mechanisms of residential-type circuit breakers consist of a bimetallic strip and spring-loaded contacts as shown in Figure 1-14.

Figure 1-14.   Circuit breaker

Bimetallic strip. This strip is made of two different metals, such as steel and bronze, fused together. It acts as a latch to hold the contacts together. When more than the rated current flows through the breaker, the heat makes the two metals expand at different rates, causing the strip to bend (Figure 1-15).

Figure 1-15.   Bimetallic strip

Spring-loaded contact. When the bimetallic strip bends, the spring-loaded contacts are released and current flow is interrupted. The contacts can also be opened by moving the switch to the OFF position.

Time-delay feature. The bimetallic strip requires time to heat up and trip the breaker. This provides a time-delay feature. Most breakers will carry one and one-half time their rated load for about one minute and as much as three times their load for about five seconds. This provides enough delay to allow a motor-driven appliance to reach normal operating speed without tripping the breaker.

Positions

Most circuit-breaker switches have three positions.

• ON. The first is the ON position used during normal operating conditions.
• OFF. The second is the OFF position. To turn the power off in the circuit, simply move the switch to OFF.
• NEUTRAL. The third is the NEUTRAL position. If the breaker has been tripped by an overcurrent or overload, it will automatically switch itself to NEUTRAL and turn off the power.

Reset. In order to turn the power on again or to reset the breaker, move the switch to the OFF position first and then to the ON position as shown in Figure 1-16.

Position indicators. The NEC requires that circuit breakers clearly show whether they are on or off. The ON and OFF positions of switch-type circuit breakers can be seen on or near the switch. Push-button types have on and off indicators visible through an opening on the front of the breaker.

Figure 1-16.   Reset action

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