Light switches control the lighting in many homes.
Safety: Remember to make sure the power is turned off to a light switch before opening it or attempting any repairs. Also, make sure the switch is wired to interrupt the live side, not the neutral side.
Types of Switches
There are three basic types of switches, the simplest and most common of which is called the single-pole switch. A single-pole switch has two terminals and is used to control a device, such as a light fixture or receptacle, from one location. It internally connects or disconnects the two terminals to turn the device on or off. A toggle-type switch has on-off markings at the base of its toggle.
A three-way switch is used to control a device from two or more locations. It has three terminals, a specially marked “common” terminal and two “traveller” terminals, and no on-off markings. These switches are always used in pairs, with two live wires, called “traveller” wires connecting the two switches. Each switch internally connects its common terminal to one traveller terminal or the other depending on its position, and the device is on when both switches’ commons are connected to the same traveller wire. Because there are two traveller wires in addition to the neutral, or returning common in a switch loop, three-conductor cable is required between the switches.
A four-way switch is used, in conjunction with a pair of three-way switches, to control a device from three or more locations. It has four terminals, all of them traveller terminals, and no on-off markings. It works by interrupting the traveller wire pair between two three-way switches and internally connects each incoming traveller terminal to one outgoing traveller terminal, the switch position determining which incoming is connected to which outgoing. Any number of four-way switches may theoretically be added to the traveller pair, but there must be three-way switches at either end of the traveller pair.
Then there are also dimmer switches, which are available in single-pole and three-way versions. Only one three-way dimmer switch is used in a multiple switch circuit, the other three-way switch and any four-way switches being regular devices. Dimmer switches are typically made for use with incandescent lighting only. Although as fluorescent bulbs are becoming more common, some are designed to work with dimmers.
When the source cable arrives at a single-pole switch before going to the controlled device, the white neutral wires in the switch box are simply connected together with a wire nut, and the incoming and outgoing black hot wires are connected to the two switch terminals. When the source cable arrives at the light first, however, a switch loop is used. In this setup, the white wire in the cable going from the light to the switch is hot, not neutral, and should be marked as such by recoloring with a black, red or blue sharpie. (Green must never be used.) This recolored white wire is then connected to the incoming black wire with a wire nut, leaving the incoming white wire and outgoing black wire to be connected to the controlled device. At the switch box, the black and recolored white wires are connected to the two switch terminals.