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Circuit breakers have replaced fuses as the preferred type of circuit protection. Technically, they are called molded-case circuit breakers, or MCCBs. Circuit breakers use a two-part system for protecting circuit wiring.
When a small overload is on the circuit, a thermal strip will heat up and open, or trip, the circuit. When a massive amount of current comes through very quickly, as in a ground fault or short circuit, an electromagnet gives the thermal strip a boost. The greater the amount of trip current, the faster the breaker will trip.
The most important advantage circuit breakers have over fuses is that they can be easily reset; you don't have to buy a new one every time an appliance draws excessive current. When a breaker is tripped, it won't work unless you throw it all the way to the off position before you turn it back on again. Another characteristic of circuit breakers is that they are air-ambient-compensated-the hotter the air around them gets, the sooner they will trip. For example, if all the circuit breakers around a specific 20-amp breaker are running hot, because of an excessive flow of current, the 20-amp breaker may trip at only 18 amps.
Residential circuit breakers typically range in size from 15 to 60 amps, increasing at intervals of 5 amps. Single-pole breakers rated for 15 to 20 amps control most 120-volt generalpurpose circuits. Double-pole breakers rated for 20 to 60 amps control 240-volt circuits.
Standard circuit breakers are universal and have clips on the bottom that snap onto the hotbus tabs in the panel box. Contact with the hot bus brings power into the breaker. Be aware, however, that some manufacturers make breakers with wire clips that mount on the side. These clips slide over the tab on the hot bus, requiring you to remove one or more of the other breakers to get at the one you want.
Common Breaker Types
In addition to single- and double-pole breakers, quad breakers, GFCI breakers, and surge-protection devices are also available. Single-pole breakers supply power to 120-volt loads such as receptacle and light circuits. A hot black or red wire is usually connected to the breaker. Single-pole breakers come full size or in a twoin-one configuration (twin). The latter type will only fit into a panel having a split-tab hot bus.
Double-pole breakers provide power to 240volt appliances such as electric water heaters and dryers. If a standard NM cable is used as the conductors, both the black and the white wire are connected to the breaker. The white wire must be marked with black tape at both ends. Larger double-pole circuits have two black conductors in the circuit.

Specialty Breakers
A quad breaker falls within the half-size breaker family and can contain several configura
tions within one unit. It may, for example, contain two double-pole circuits, such as a doublepole 30-amp and a double-pole 20-amp circuit; it may have two single-pole circuits and one double pole; or it may provide power to some other combination of circuits. The advantage of a quad breaker is that it takes up half the space of a standard breaker. The panel, though, must be specially designed to accept quad breakers. Furthermore, if the panel is too small, it may end up resembling a tightly interwoven nest of wires.
A GFCI circuit breaker fits into the main panel just like a standard circuit breaker. On its face is a test button but no reset. If properly installed, pressing the test button places a deliberate, preset current imbalance (6 milliamperes) on the line to verify that the breaker will trip when there is an unintended imbalance. When tripped, the breaker arm will go to a halfway off position, cutting power to the circuit. The circuit cannot be reset unless someone first turns the breaker arm completely off.
At first glance, a surge-protection device can be confusing. You will see a device that looks like a double-pole body. This type of device also has two lights that glow when power is applied to the panel. Nevertheless, a surge-protection device connects to the buses in the same way as any other circuit breaker.

Arc-Fault Circuit Interrupters
The National Electric Code requires an arcfault circuit-interrupter (AFCI) breaker be installed to protect branch circuits that supply 125-volt, 15- and 20-ampere outlet(s) in bedrooms. The requirement applies to all new construction, but switching existing circuits will supply this protection to existing buildings as well.

Circuit breakers are limited in protecting wires, and therefore life and property. Breakers other than GFCI cannot prevent electric shock, for instance. Although breakers trip at 15 amps and above, it only takes about 0.06 amp to electrocute someone. Circuit breakers cannot prevent overheating of a fixture or appliance or other device, and they can't prevent low-level faults. For a breaker to trip, a fault must occur when enough current is being demanded to exceed the trip current of the breaker. Breakers cannot trip fast enough to completely block lightning surges from entering the house circuits. They cannot prevent fires within appliances. Circuit breakers are meant to save the wiring to the appliance-not the appliance itself.

Testing Fuses
In homes with older wiring systems, you may need to test the conditions of fuses. To perform a continuity test on a glass fuse, left, touch one probe of a multi-tester to the center contact and the other to the screw shell. A zero reading means the fuse is working properly. Test a cartridge fuse, right, by touching one probe of a multi-tester to each end terminal on the fuse. The knob setting shown will give an audible signal as well as a meter reading.

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