Tips for testing voltage at home

Before you do any work on a power circuit, it's important to test the voltage in the power source. Here are tips to test accurately and safely.

• Surges in the power line — sudden spikes in voltage usually caused by lightning — can damage computers and other electronic equipment
• .A surge protector keeps the spike from getting through to and damaging any devices connected to it.
• Plug-in surge protectors require three-slot grounded receptacles; whole-house protectors are also available.

• Always prove that there's no power in a circuit before you attempt to do any work on it.
• An easy way to check a circuit is with an inexpensive device called a non-contact voltage tester. It's the size of a magic marker and lights up (or makes a noise) when it comes in contact with a hot wire. The device is battery powered, so make sure it has fresh batteries before you test anything. (You'll know it's working when you hold the device next to a lightbulb or near an outlet that you know is live and the tester flickers or beeps.)
• A non-contact tester is not reliable when testing wires coated in metal sheathing.

•  Shut off the switch's circuit by removing its fuse or turning off its breaker at the service panel. Leaving switches in the "on" position, remove the switch's cover plate and loosen the screws fastening the switch to the electrical box.
• Without touching wires or switch terminals, touch the probe's nose first to one switch wire terminal, then the other.

•  Shut off power to the receptacle's circuit at the service panel (sometimes there's more than one circuit to an outlet, so make sure you shut them all off). If the outlet is connected to a wall switch, make sure that the wall switch is on.
• Insert the nose of the tester into or against one of the plug's slots (test the "neutral" or bigger one, which should give you a proper reading no matter what — even if the outlet was wired incorrectly). Check all of the plugs in the outlet; sometimes the top and bottom plugs are wired separately.
• If you don't hear consistent chirps or see consistent beeps on the reader, the power is probably off. But let's just double-check. A faulty receptacle can give you a faulty reading.
• Remove the receptacle's cover plate and loosen its mounting screws. Pull the receptacle gently out of the box without touching the wires or terminals. Touch the tester to the bare ends of each pair of hot (black) and neutral (white) wires; then insert the nose deeper into the box to test other wires in the box that are not connected to the receptacle.
• If you find any hot wires, turn them off at the service panel and retest. If the tester doesn't light in any of these positions, the circuit is dead.