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The Function of Circuit Breakers and Fuses
The electrical system in every home features a system of circuits controlled and protected either by circuit breakers or fuses. Most of today's homes now use circuit breakers to offer this control and protection to individual circuits, but older homes that have not had their electrical systems upgraded may use fuses. The circuit breakers or fuses are normally found in a central main service panel. Circuit breakers are lever-operated devices with on-off switches, while fuses are glass and ceramic cylinders with screw-in sockets. You likely already know where your main service panel is located and whether your system uses circuit breakers or fuses.
And you probably also know that when all the lights and fixtures in a portion of the house go dark or dead at the same time, it's because one of those circuit breakers has "tripped" or one of those fuses as blown. These devices are designed to automatically shut off power to the circuit when problems occur. And you probably also know that the solution is to reset the breaker lever to the ON position or replace the blown fuse. In the case of circuit breakers, the immediate answer is to find the breaker that has "tripped" and reset the lever to the ON position. With a fuse, a metal filament inside the fuse has burned through, and you'll need to replace the fuse with a new one.
But it is also important that you understand why the breaker has tripped or the fuse has blown to avoid having it happen again. In rare cases, the breaker may be damaged and need replacing. But in most cases, the breaker or fuse is just doing its job when it pops. Circuit breakers are designed to trip and fuses are designed to blow and turn off the power when any of four dangerous situations occur.Continue to 2 of 5 below.
02 of 05
An overloaded circuit is the most common reason for a circuit breaker tripping. It occurs when a circuit is attempting to draw a greater electrical load than it is intended to carry. When too many appliances or light fixtures are operating at the same time, the internal sensing mechanism in the circuit breaker heats up, and the breaker "trips," usually by means of a spring-loaded component within the breaker. This breaks the continuous pathway of the breaker and renders the circuit inactive. The circuit remains dead until the breaker lever is reset to the ON position, which also rearms the internal spring mechanism.
The circuit breaker or fuse is sized to match the load-carrying capacity of the wires in that circuit. Hence, the breaker or fuse is intended to trip or blow before the circuit wires can heat to a dangerous level. When a circuit breaker regularly trips or a fuse repeatedly blows, it is a sign that you are making excessive demands on the circuit and need to move some appliances and devices to other circuits. Or, it may indicate that your house has too few circuits and is in need of a service upgrade.Continue to 3 of 5 below.
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A short circuit is a more serious reason for a breaker tripping. A "hard short" is caused when the hot wire (black) touches a neutral wire (white). In terms of the physics involved, a short circuit allows for a sudden unimpeded flow of electricity due to lowered resistance, and this sudden increase in current flow within the breaker causes the tripping mechanism to activate.
But sometimes a short circuit occurs not because of the circuit wiring at all, but because of a wiring problem in an appliance or device plugged into an outlet along the circuit. Short circuits, therefore, can be a bit difficult to diagnose and fix.
The presence of a short circuit can be indicated when a circuit breaker trips again instantly after you reset it.Continue to 4 of 5 below.
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A particular type of short circuit, a "ground-fault," occurs if a hot wire comes in contact with a ground wire or a metal wall box or touches wood framing members. Ground faults can be especially dangerous when they occur in areas with high levels of moisture–such as kitchens or bathrooms. There are steps you can take to identify and fix a ground fault, but also essential steps you should take to prevent one from occurring in the first place. For example, in areas where direct contact with the ground or water is possible, building codes may require that outlets be protected with GFCIs (ground-fault circuit interrupters).
As with hard shorts, a ground fault causes an instant reduction in resistance and immediate increase in electrical flow. This causes the internal mechanism of the circuit breaker to heat up and trip. As with hard shorts, if a ground fault is present, the circuit breaker may trip again immediately after you reset it.Continue to 5 of 5 below.
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In recent years, the National Electrical Code, the model code on which most local electrical codes are based, has gradually increased requirements for a special type of circuit breaker, known as an arc-fault circuit interrupter (AFCI).
AFCI breakers, in addition to tripping due to overloads, short circuits, and ground-faults, also sense the power fluctuations that occur when sparking ("arcing") occurs between contact points in a wire connection. This may occur, for example, because of loose screw terminal connections in a switch or outlet. An AFCI breaker, in other words, senses early wiring problems before they can lead to short circuits or ground faults. Neither ordinary circuit breakers nor fuses offer any protection against arc faults.
AFCI breakers are reset in the same manner as ordinary breakers. Repeated tripping is usually an indication that there are loose wire connections somewhere along the circuit, causing arcing.