How to test and replace wall electric switches

A single-pole wall switch controls power to overhead lights or to a wall receptacle from one location. If a light fixture fails to work when the switch is turned on, check for a burned-out light bulb, a faulty plug or power cord, or a blown fuse or tripped circuit breaker. If the lamp still does not light, the switch may be worn.

Caution: Before working on a switch, turn off the power to its circuit. To check that it is off, unscrew the cover plate and mounting screws; carefully pull out the switch. Touch one probe of a neon test lamp to the box’s metal shell; touch the other probe to one, then the other, terminal screw. The lamp should not light in either position.

In a middle-of-the-run switch, two black (or a black and a red) hot wires are attached to the switch’s brass terminal screws. Loosen the screws and disconnect the wires. Leave the white neutral and bare grounding wires in the box as they are. In an end-of-the run switch, a black and a white wire will be attached to the terminals; this white wire is hot, not neutral. (It may be marked with black paint or electrical tape.) Disconnect both wires.

Test the switch by attaching the alligator clip of a continuity tester to one terminal and the probe to the other. Turn the switch on and off. The tester should light only when the switch is on. Next, attach the clip to the mounting bracket and the probe to one, then the other, terminal. Turn the switch on and off at both positions. The tester should not light. Replace a faulty switch with one of the same voltage and amperage.

To install the switch, hook a hot wire clockwise on each brass terminal; tighten the terminal screws. New switches may have a green grounding terminal. If the old switch did not have one, run a bare or green insulated wire jumper from the green terminal screw to the wire connector linking the grounding wires in the box. If the cable serving the box is steel armored, run the jumper from the terminal to the box’s grounding screw. Fit the wires and the switch into the box, screw the switch to the box, and remount the cover plate. Mounting a new switch box Follow instructions for installing new boxes in Electric circuits. Switches should be about 4 feet above the floor. Remove a knockout from the box for each cable that will enter it. Feed 8 inches of stripped cable through each hole; secure the cable at the end of its insulation with the internal clamp usually supplied with wall boxes. After you have mounted the box, join the stripped ends of the white wires and the bare grounding wires with wire connectors. If the box is metal, run a grounding jumper from the grounding wires to the box. Install the switch as described above.