In electrical terminology, a basic switching device opens and closes an electrical circuit. Electrical circuits must form a continuous loop, and a switch is like a gate in that loop. Therefore, a circuit is "on" when the switch is closed, and the circuit is "off" when the switch is open. The 2017 National Electrical Code book (NEC) does not provide a specific definition for "switching device" or for "switch," but it does define many specific types of switches and disconnects. Let's look at a handful of the most common switching devices found in a home's electrical system.
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The NEC defines a circuit breaker as "A device designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its rating." In other words, a circuit breaker protects a circuit from overload by automatically shutting off the circuit. A breaker can also be shut off manually by flipping its toggle switch.
Every electrical circuit in a modern home's electrical system is protected by a circuit breaker. The breakers are housed in the main service panel, commonly called the breaker box. Older homes often have fuses instead of circuit breakers. Fuses function differently from circuit breakers and must be removed (rather than switched off) to open the circuit manually. When a fused circuit is overloaded, the fuse blows automatically and opens the circuit.
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The NEC defines a general-use switch as "A switch intended for use in general distribution and branch circuits. It is rated in amperes, and it is capable of interrupting its rated current at its rated voltage." In this case, "interrupting" means opening the circuit to stop the flow of electricity through the switch.
This type of switching device includes standard wall switches that control light fixtures and appliances (such as a garbage disposer) throughout a home. Switches on standard 120-volt circuits typically are rated for either 15 or 20 amps.
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The NEC defines a disconnecting means as "A device, or group of devices, or other means by which the conductors of a circuit can be disconnected from their source of supply."
This is a general definition that can apply to many types of switches and disconnecting devices. A good example in a home is a disconnect switch or service disconnect, that is installed between a utility meter and a home's service panel. This switch isolates the service panel from the source of utility power. The main breaker on a service panel is another form of disconnecting means.
A disconnecting means can also be in the form of a standard switch, such as a switch used to shut off the power to a gas furnace. This type of disconnect often is subject to special installation requirements; for example, it may need to be reachable or at least clearly visible from the appliance location.
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The NEC defines a transfer switch as "An automatic or nonautomatic device for transferring one or more load conductor connections from one power source to another."
A transfer switch is a special type of switch in that it's not simply for cutting power to a circuit. Rather, it transfers the path of electricity from one power source to another. Homes that have large generators for backup power usually have a transfer switch that changes the connection from the utility source to the generator. Some solar power systems use a form of the transfer switch to disconnect from the utility grid and connect to the solar power source, and vice versa.