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When you turn on a light switch, power goes to the light through the “hot” (black) wire and then back through the neutral (white) wire to the ground. The bare ground wires serve as a backup to divert the power safely away in case of an electrical fault.
A light switch operates by opening and closing an electrical circuit. A typical light switch has an internal cam or “shoulder” (Image 1)that moves back and forth as you flip the switch.
The shoulder pushes a metal contact (Image 2) away from the power source to turn off the switch, and it lets the contact touch the power source to turn on the switch.
Upgrading or replacing a light switch is a simple project. Shut off the power at the circuit-breaker box, then remove the cover from the existing switch.
In most cases, two black wires will be attached to the switch’s two terminal screws. The white wires will be connected to each other but not to the switch itself; this connection is typically secured by a plastic wire nut. The ground wires (usually bare or green-jacketed) will be connected to each other and attached to the grounding screw on the switch (Image 3).
Turn the power off at the circuit-breaker box. Use a circuit tester to be sure the power is off: Hold one test lead against the bare ground wire, and touch the other lead to both of the black wires attached to the switch. If no power is indicated by the tester, go ahead and remove the switch from the junction box by taking out the retaining screws.
Disconnect the wires from the switch by loosening the terminal screws (leave the white wires connected). Work with only one color of wire at a time.
Reconnect the black wires to the new switch.
Secure the switch to the junction box and replace the cover.