How the NEC looks at switches

A switch seems like a simple, humble device. But actually, the topic quickly becomes complex, especially for switches subject to National Electrical Code provisions. The NEC defines several types of switches:

• A bypass isolation switch is a manually operated device used in conjunction with a transfer switch to provide a means of directly connecting load conductors to a power source and of disconnecting the transfer switch.

• A general-use switch is a switch intended for use in general distribution and branch circuits. It is rated in amperes and is capable of interrupting its rated current at its rated voltage.

• A general-use snap switch is a form of general-use switch constructed so it can be installed in device boxes or otherwise used in conjunction with Code-recognized wiring systems.

• An isolating switch is a switch intended for isolating an electrical circuit from the source of power. It has no interrupting rating, and it is intended to be operated only after the circuit has been opened by some other means.

• A motor-circuit switch is a switch rated in horsepower that is capable of interrupting the maximum operating overload current of a motor of the same horsepower rating as the switch at the rated voltage.

• A transfer switch is an automatic or non-automatic device for transferring one or more load conductor connections from one power source to another.

Then there is a network switch, alternately known as a switching hub or bridging hub. It is used to connect devices to form a computer network, but it is more advanced than a simple network hub, which is essentially a splitter that transfers identical data through each of its output ports. A network switch, in contrast, typically has a larger number of output ports and uses hardware addresses to enable packet forwarding. The most common type of network switch is an Ethernet switch.

Switches come in all different sizes, depending on the amount of current they are designed to handle or the complexity of the data they process. In telephony, the word switch has a specialized meaning. The old-time telephone switchboard, attended by operators in a central office (this term persists) was long ago replaced by computer-controlled electronic switching.

Current telephone central office implementations are entirely digital and automatic, requiring no human intervention to interconnect telephone circuits. Almost all telephone exchanges currently use time-division multiplexing to economize on the amount of cabling and electronic equipment both inside and outside of the central office.