Replacing Light Switches
Frequently used switches may wear out, or their connections may loosen over time. It's easy and inexpensive to replace the switch, but you need to know what you're replacing. The most common switches are single pole, where an outlet light fixture is operated by a single switch, and three-way, which allow an outlet or fixture to be operated from two different switches.
Shut off the circuit breaker or remove the fuse that operates the circuit that the switch is on. Put a sign on the panel to let people know that you're working on the circuit and that they must not restore power to it.
Remove the switch cover plate and the two mounting screws that hold the switch itself to the wall. Carefully pull out the switch. Use a neon circuit tester (place one probe against the metal wall box, and the other probe against each of the switch's screw terminals in turn) to ensure that the power is in fact off (the light should not illuminate). If the power is still flowing to the circuit, shut it off at the main electrical panel (you may have switched off the wrong circuit).
Examine how the switch is wired. There should be two wires connected to the switch, both of which will be live when the switch is on, regardless of their color. You may also have a third wire-a “ground” (green or bare copper wire). Depending on where the switch is located on the circuit, you may see other wires connected together with plastic marrettes, or connectors. Of the wires that are connected to the switch itself, one wire will be connected to the brass screw or the push-in hole closest to the brass screw. The other wire will be connected to the silver screw or the push-in hole closest to it. If there's a ground wire, it should be connected to a green ground screw on the metal wall box or on the switch.
This single-pole switch has two wires (both live) and a ground wire.
Remove the wires from either the screws or the push-in holes of the old switch. (If the ground wire is connected to the metal wall box, it doesn't need to be removed.) Reconnect the wires to the new switch: First, cut as little as possible off the end of the wires to provide you with clean, undamaged wires for the connection. Then, strip the insulation from the last ½ to ¾ inch of the wire and connect the wires to the screw terminals or the push-in holes. The two live wires can be connected to either of the screw terminals, but a ground wire must be connected to the green screw (on the switch or the metal box). Tighten the screw terminals down even if you're not using them to connect the wires-there's less chance of them accidentally touching the metal wall box if they're screwed down.
Ensure the switch is the right way up, and push it gently into the box. Snug up the mounting screws, and replace the cover plate so that it's flush against the switch. Turn the power back on, and check the switch's operation.
Shut off the circuit breaker or remove the fuse that operates the circuit that the switch is on. Put a sign on the panel to let people know that you're working on the circuit, and that they must not restore power to it.
Three-way switches are replaced in the same way as single-pole switches, except that there will be more wires and more screw terminals. There'll be a dark-colored common screw terminal, which holds the black wire; two traveler screw terminals, which hold one each of the red or white wires; and possibly a ground (green) wire connected to the green screw on the switch or the box.
If at any point you encounter complicated wiring or you're unsure how to proceed, stop what you're doing and call in an electrician. Electricity isn't worth taking risks over.
To install the new switch, first label the black wire that's attached to the old switch's common screw terminal before removing the wires from the switch. Reconnect the black wire to the new switch's common screw terminal. The other two wires can be removed and attached to either of the traveler screw terminals. The green (ground) wire is attached to the green screw on the switch or the metal wall box.
A This three-way switch has wires attached to its common screw terminal and to its two traveler screw terminals, plus a ground wire.