Electrical circuits in structures are designed
to operate at or below a specific current (ampere rating). Each electrical
circuit should be protected, according to the NEC. The wire or conductor of
each circuit should be protected to prevent a higher current than it is
designed to carry. The electrical components in the circuit are also a
consideration when protection is a concern. The standard wire used for
receptacles in most residences is #12 TW. The maximum current protection for
this type of wire according to the NEC is 20 amperes. However, if there is an
electrical component in the circuit that requires protection at 10 amperes, the
circuit protection should be at 10 amperes. If the current in the circuit
becomes greater than the rating of the protective device, the device opens,
disrupting the power source from the circuit.
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| Fig.1 |
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| Fig.2 |
The most common methods of circuit protection
in structures are fuses, as shown in Figure 1, and circuit breakers, as shown
in Figure 2. These devices protect the circuit if the current in the circuit
exceeds the rating of the fuse or circuit breaker. There are many types of fuses
available today with special designs for particular purposes, but the primary
purpose of any fuse is protection. Fuses are made with a short stripe of metal
alloy called an element that has a low melting point, depending on the rating
of the fuse. If a large current flow passes through the fuse than is designed
to pass through the element, the element will melt and open the circuit. Circuit
breakers look a lot like ordinary light switches placed in an electrical panel.
If the current in the circuit that a circuit breaker is protecting exceeds the
breaker's rating, then the switch of the circuit breaker will trip and interrupt
the electrical energy going to the circuit. Fuses and circuit breakers should
be sized for the particular appliance according to the National Electrical Code.
Figure 3 shows a disconnect switch with the results of incorrectly sized fuses.
Technicians should never arbitrarily adjust the size of the fuse or circuit
breaker without following the standards in the NEC for the load of the circuit
according to the NEC to avoid overheating and possible fire.
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| Fig.3 |