The two most important safety steps to take before opening any electrical box are:
- Turn off the electrical power to that outlet at the main panel.
- Double-check the outlet to make sure you turned off the right circuit. A non-contact voltage tester is the best tool for this job. With this tool, you don’t even have to touch a bare wire. The tester will flash and/or chirp whenever it comes close to a hot wire. It’ll even detect voltage (a hot wire) through the wire’s plastic insulation. However, it’s not reliable when testing wires covered by metal conduit or metal sheathing.
This tester is powered by small batteries, so make sure it works before using it. Shove the tip into the slots of a receptacle that’s live, hold it near a plugged-in lamp cord or hold it against a light bulb that’s on. With most testers, you’ll see a series of flashes and hear continuous chirps that indicate voltage. Testers may flash and chirp at other times, but without the continuous pattern that indicates a hot wire.
To test whether a receptacle is hot, simply shove the tester nose into or against the plug slots (Photo 1). The hot slot is the smaller of the two. However, you never know if the receptacle was wired correctly, so it’s a good idea to test the neutral slot (the larger one) too just in case the receptacle was wired wrong. And be sure to check all the slots in the receptacle. Sometimes the lower set in a duplex receptacle will be wired separately from the top. If a wall switch controls the receptacle, make sure the switch is in the “on” position.
Then unscrew the receptacle, carefully pull it out and test all the wires again (Photo 2). At this point, you can shove the tester deeper into the box to test wires not directly connected to the receptacle. Several circuits may be present in a single box. We recommend that you turn off all circuits to a box before working on it.
To test for power at a switch, you have to remove the cover plate first. There’s usually enough space to poke the tester tip close to the screw terminals (Photo 3). If there are no live wires, unscrew the switch, pull it out and test all other wires in the box.
To test a light fixture before removing it, turn off the circuit at the main panel, turn the light switch to “on,” remove the bulb and poke the tester all the way down to the center socket button (Photo 4). If the fixture is on a three-way switch (two switches), test with one switch first in the up, then in the down position. If no voltage is present, you can safely unscrew the fixture from the electrical box, pull it out and test the other wires in the box as before.
The non-contact tester will also identify hot cables, even if they’re covered by plastic insulation (Photo 5). This comes in handy when you cut open a wall and find electrical cables and are unsure if they are shut off.
The two-lead circuit tester shown here also tests for voltage. When you touch a live hot wire (black or any other color except green and white) with one lead and a neutral (white) or ground (green or bare copper) with the other, the neon test lamp should light (Photo 1). It confirms that the power is on and that you have a complete (good) circuit. If the light doesn’t come on, either the power is off or you have a bad circuit.
This tool comes in especially handy in older homes when you want to know if an equipment ground wire (green insulated or bare copper) is actually connected to ground elsewhere in the system.
You often have to check this when you replace older ungrounded switches with grounded ones as now required by the National Electrical Code. You often find an unused bare ground wire folded back into the box, and you have to test it to make sure that it’s connected to the rest of the grounding system before hooking up your new switch.
To test a ground wire, follow these five steps:
- Turn the power off to the switch (confirm with the non-contact voltage tester) and uncap the neutral wires (they can remain in a bundle).
- Disconnect the two switch wires and spread the bare ends so they don’t touch one another (Photo 1).
- Turn the power back on and identify the hot wire with the non-contact tester.
- Confirm that the circuit tester is working by carefully touching the hot wire with one lead and a neutral wire with the other. The tester will light if it’s working.
- Touch the hot wire with one lead and the ground wire with the other (Photo 1). If the tester lights, the ground wire is good and you can use it.
Follow a similar procedure when working with metal boxes in which no ground wire comes into the box (Photo 2). In this case, you want to find out whether the metal box itself is grounded (through conduit or another method) and will therefore serve as the required ground.
With the wires separated and the power on, carefully touch the hot wire with one lead and the metal box with the other. If the lamp lights, you can use the metal box as a ground. If the lamp doesn’t light, in most cases the NEC requires that you upgrade the box to have some means of grounding before you install a receptacle, switch or other device. Consult a licensed electrician or your local electrical inspector for acceptable grounding methods.
Avoid touching a live hot wire and don't let it touch anything else. Hold the tester leads by the insulated portion while making contact. And turn the circuit off again as soon as you finish the test.
It’s difficult and dangerous to trace the routes of various wires with circuits turned on. A continuity tester does it simply and safely with the circuits turned off. It has a probe, which contains a battery and a light bulb, and a wire lead. When you touch the ends to any continuous conductive path, usually a wire, with both the probe and the lead, a circuit will be complete and the bulb will light. In fact, to test the bulb to make sure it’s working, simply touch the lead to the probe.
Working with several boxes and can’t remember which wire goes where? With the power shut off, simply connect a test wire to a circuit wire in one box (Photo 1), clip the lead to the test wire and touch the probe to the ends of the circuit wires in the other box. The bulb will light when you find the right wire.
Another great use for the circuit tester is to determine whether a switch is working (Photo 2).
Disconnect the switch, connect one lead to one terminal and put the probe on the other while you flick the switch on and off. If the switch is good, the bulb should light up and turn off as well.
If you have aluminum wiring, don't mess with it. Call in a licensed pro who's certified to work with it. This wiring is dull gray, not the dull orange that's characteristic of copper.