If the thought of replacing or working on a switch, outlet of fixture makes you cringe because you're not sure how to make sure if the circuit is “hot”--still carrying electricity--then investing in a multi-meter, voltmeter, neon circuit tester or "touchless" tic tracer tester should be your first order of business. These testers are relatively inexpensive and can protect you from electrical shock.
Testers and How They Work
Neon circuit testers, voltmeters, and multi-meters consist of two wire leads used for checking a circuit. When you press these two leads into an outlet, either a small neon bulb will light (with the neon tester), or a meter dial will register to indicate if the hot” or on. If it doesn’t light or register power, then the circuit is “dead” or off. A newer style tester, the "touchless" tester doesn't even need to be inserted into the outlet--merely bringing the sensor into the near vicinity of a power-carrying wire or device will cause the tool to light up or make an audible noise to indicate power.
Multi-meters come in analog or digital display. They test voltage, ohms,and amperage while displaying the results on a screen or dial. Testing to see if the power is on is only one function of many for these tools.
Always check to see if the tester is working properly by checking a circuit that you know is working properly before moving on to test other devices.
To double-check that an outlet is actually off, remove the outlet cover and test the screws on the sides of the outlet. You can also plug a lamp or vacuum into the outlet just to put your mind at ease.
Testing Outlet Receptacles
A typical outlet receptacle has three holes in its face. The shorter straight slot is the “hot” lead and connects to the active hot wire in the outlet box.
The longer straight slot is the “neutral” lead and connects to the neutral circuit wire in the electrical box. The slot that looks like a small circle hole is the ground slot, and it is connected to the circuit ground wire.
Testing to see if the power is on, insert the two probes of the tester into the two straight vertical slots on the receptacle. If the power is on, the tester will light. Because there is a possibility that the outlet is "split-wired"--with the top and bottom receptacles fed by different circuits, always check both receptacles for power before removing the receptacle to work on it.
You can also test to see if the ground system is properly connected to the receptacle. To test the ground, insert one probe in the “hot” slot and the other in the“ground” slot. If the circuit is working and you have a good ground connection, the tester will light.
There are also plug-in circuit testers with three small different-colored neon lights that will test your circuit. They test for an open neutral, lack of a ground, wires on the wrong terminals, and no power. Follow the color coding label on the top of the tester to interpret the lights that glow when you plug in the tester.
Testing Wall Switches
To test a switch, remove the cover plate and test by touching one probe of the tester to one of the screws on the side of the side of the switch and the other probe to the bare copper ground wire or to the metal box. Keep in mind that the box may not be grounded, especially if it’s a plastic box. Now touch the tester probe to the other screw terminal on the switch. If the tester does not light for either test, the circuit power is off.
Testing Light Fixtures
When checking light fixture wiring, loosen the mounting straps holding the fixture to the ceiling box and pull the light fixture slightly away from the ceiling box. Use a touchless tic-tracer to test the circuit wires to see if they are carrying power. This tester will light up or click when you place it close to a wire that has current flowing through it.
To double-check the circuit, first turn off the power to that circuit. Now, remove the wire nuts from both the black “hot” wires and the white “neutral" wires. Separate these sets of wires so that they are not touching one another.
Turn the circuit back on and check with the voltmeter or neon tester, touching one probe to the hot black wire and the other to the white neutral wire. Be careful not to touch the exposed wires with your hands.A voltmeter should show a reading of around 120 volts if the wires are live. Likewise, the neon tester should light if the circuit is working properly.