How to Use a Voltmeter to Test a Light Switch

If you suspect you have a bad light switch that needs to be replaced, there are two very different but equally important tests you will conduct during the process. The first is a matter of personal safety: making sure the current has been shut off and the wires are safe to touch before you begin work removing the switch.

The second test is a diagnostic one conducted after the switch has been removed. This test evaluates the inner wiring pathways of the switch for continuity to verify that there's a problem that warrants replacing the switch.

Before You Begin

But first, of course, it's important to rule out other possibilities for the light fixture's failure. Try the simplest solutions first: Make sure the bulb is good and that it is is screwed in all the way. Also, make sure the circuit has power and hasn't tripped its circuit breaker or blown its fuse. And it's also possible that wire connections on the switch are loose and not making proper contact. This problem is often corrected simply by shutting off the circuit then retightening the wire connections.

If one of these issues isn't the problem, then it's likely that you have a bad switch, especially if the switch is old or the toggle lever feels a little loose when you snap it. Switches have a limited lifespan, and the springs and other inner components will eventually wear out.

To make sure you're not performing an unnecessary replacement, test the switch for continuity to make sure.

Safety Considerations

Although the physical act of replacing a switch is quite easy, there is always a danger element to any project that involves handling circuit wires. It is essential that you make sure the circuit has been shut off before you handle the switch or its wire connections. This is the point of the first test you'll conduct, but if you have any question whatsoever about how to do it or how to use the proper testing tools, then this is a repair that should be left to a professional electrician.

Home wiring systems can take many forms depending on when (and if) the wiring was last updated, and yours may not resemble the project shown here. If your wiring is very old, even simple projects like replacing a light switch are best handled by a professional.

Electrical Testers

You have several options for conducting the two tests needed when replacing a light switch. Many homeowners will want to use the simplest, least expensive tool options: a non-contact voltage tester to check for power before touching the wire connections, and a continuity tester for checking the detached switch to evaluate if it is bad.

Another tool, more often used by professionals, is a multimeter, a versatile tool that can be used for a variety of electrical tests. In our demonstration project, we've used a non-contact circuit tester for the voltage test and a multimeter for testing the switch itself, but the multimeter also has settings that allow you to test for power.

• A non-contact voltage tester is a battery-operated device that senses an electrical current without even touching the tool's probes to the wires or screw terminals. Held close to live wires, it will light up or buzz (or both) if it senses live current.
• A continuity tester is a simple electrical device with a metal probe, a diode light, and a wire with a clip at one end. It also operates by means of a battery. All this tool does is test for continuity—the presence of an uninterrupted electrical pathway. Attached to the screw terminals of a detached switch, the continuity tester lets you determine if the switch lever is properly opening and closing the electrical pathway.
• A multimeter is a versatile battery-powered tester that measures a variety of electrical properties, such as voltage, amperage, and resistance. It can also be used for a simple continuity test. To set up a multimeter to test for continuity, turn the tester dial to the Continuity or Resistance/Ohms setting. Multimeters are sophisticated tools with many functions. Learning to use them correctly can require some practice.

Any battery-powered tester should be tested to make sure the battery is good and that the tool functions properly before you use it in project work.

• To test a non-contact voltage tester, hold the tool near an outlet you know has live power and make sure it senses the current.
• To test a continuity tester, attach the tester clip to the tester's metal probe; the tester should light up.
• To test a multimeter, set the dial to Continuity (or Ohms) and touch the two tester probes together: You should get a reading near 0, 0.5 or below.

Tip

Most multimeters have a convenient setting that allows the tool to make an audible beep or buzz when it senses continuity. This makes it much easier to use a multimeter as a continuity tester.

Materials

• Electrical tape (as needed)

The Spruce / Kevin Norris

1. Shut Off the Power to the Circuit

   Shut off the power to the light switch circuit by switching off the appropriate circuit breaker in your home's main service panel (breaker box). If you live in an old house with a fuse panel, remove the appropriate fuse by unscrewing it all the way from its socket.

   It is very common for the service panel index or circuit labels to be mislabeled, so always check the wires for power before you move on to disconnecting the wires and removing the switch.

   The Spruce / Kevin Norris

2. Test for Power

   Remove the screws on the switch cover plate, and pull off the cover plate to expose the switch wiring. Without touching any wires, test each wire in the electrical box with a non-contact voltage tester. Also, test each of the side terminals on the switch by touching it with the tip of the tester. If the tester detects any voltage (lights up or buzzes), return to the service panel and turn off the correct breaker, then retest again until you detect no voltage.

   Tip

   A multimeter can also be used to detect voltage in circuit wires—provided you know how to use it. With the selection knob on the instrument set to AC, touch one probe to a grounding wire, then touch each of the metal screw terminals on the switch, one at a time. If the needle or LED readout on the tool's display indicates voltage, the power has not been properly turned off.

   The Spruce / Kevin Norris

3. Identify the Switch Type

   Remove the switch's mounting screws, and carefully pull the switch out of the electrical box, straightening the wires as you go. Note the number of side terminals on the switch. If there are two side terminals, it is a single-pole switch. (Do not count the ground screw, which is usually green-colored and is near the bottom or top of the switch and connects to a bare copper or green ground wire.)

   If the switch has three terminals (plus a ground screw), it is a three-way switch. In this case, locate the black or dark-colored screw terminal, and label the wire attached to this terminal, using a small piece of electrical tape. This is the "common" terminal that brings power to the switch. You must connect the same wire back to the common terminal; the other two terminals (called "travelers") are interchangeable so they don't need to be labeled.

   There is a third type of switch, known as a four-way switch, which is used in conjunction with two three-way switches for circuit configurations where a light fixture (or group of fixtures) needs to be controlled from three or more different wall locations. Four-way switches have four screw terminals, in addition to the green grounding screw. Four-way switches are not common and your home may not have any at all. You generally find four-way switches in very large "open-concept" rooms or in long hallways where you need to control lights from several different entrances.

   The Spruce / Kevin Norris

4. Disconnect and Remove the Switch

   Loosen each of the screw terminals and the ground screw, and pull each wire from its terminal. Bring the switch to your work surface for testing.

   Tip

   If the wires have been connected to the switch with the push-fit connections on the back of the switch (not recommended), then they can be released by depressing a release tab inside a slot adjacent to the push-in fitting. A very small screwdriver or nail usually works to release the wire. When it comes time to reconnect the switch or install a new one, it's best to use the traditional screw terminal connections.

   The Spruce / Kevin Norris

5. Test the Switch for Continuity

   Single-pole switch: Clip the wire of a continuity tester to one of the screw terminals, and touch the other terminal with the tester probe. Turn the switch lever on and off. The tester should light up when the switch lever is in the on position but should not light up when the switch lever is off. If it doesn't exhibit this behavior, it indicates the inner connections are bad and that the switch needs to be replaced.

   If you're using a multimeter, set the instrument's dial to the Continuity or Resistance/Ohms setting. Touch each tester probe to one of the screw terminals, then turn the switch lever on and off. If the switch is good, the tester will read close to zero when the switch lever is in the on position, indicating that there is perfect continuity (no resistance). When the switch is off, the needle or digital readout should jump to 1 or higher, indicating extreme resistance (no continuity at all).

   Three-way switch: Clip the wire of a continuity tester to the common (dark-colored) screw terminal, and touch the tester probe to one of the traveler terminals. Flip the switch lever to both positions. The tester should light up when the switch is in one position but not in the other position. Move the tester probe to the other traveler terminal (leaving the clip on the common) and repeat the same test. If the switch does not behave in this way, it indicates the switch is bad.

   If you're using a multimeter, set the instrument's dial to the continuity or resistance setting, then touch one tester probe to the common (dark-colored) terminal and the other probe to one of the traveler terminals. Turn the switch lever from one position to the other. The tester should read close to 0 in one position and 1 in the other position. Move the second probe to the other traveler terminal (keeping the first probe on the common) and repeat the same test.

   Four-way switch: Clip the wire of the continuity tester to one of the darker screw terminals, then touch the other wire to the lighter, brass-colored terminal on the same side of the switch. Flip the toggle lever back and forth: The test should show continuity when the lever is in one position, but not both. Now, repeat the test by touching the second probe to the other brass-colored terminal on the opposite side of the switch. It should show continuity when the lever is in the opposite position from the first test.

   Testing with a multimeter is a similar process. With the instrument set to continuity or resistance, touch one probe to one of the darker screw terminals, then the brass-colored terminal on the same side of the switch. The switch should show continuity with the lever in one position but not both. Repeat the test with the other brass screw terminal; its continuity reading should be opposite that of the first brass screw.

   If the switch fails any continuity test, it is faulty and must be replaced.

   The Spruce / Kevin Norris

   The Spruce / Kevin Norris

6. Reconnect (or Replace) the Switch

   Connect the switch to the circuit wires to the switch, as before. Tighten each screw terminal and the ground screw securely. If you're replacing the old switch, make sure to use a new switch with the same voltage and amperage ratings as the original.

   Tip

   An old switch may not have a green grounding screw, while new switches always have this screw. If you are replacing a switch that was missing the grounding screw, make sure to connect the new switch to the circuit grounding wires using a grounding pigtail wire. If there is no circuit grounding wire present in the box (this usually occurs with systems where metal boxes and BX cable sheathing provide the grounding pathway), then the switch's grounding screw should be pigtailed to the metal box.

   The Spruce / Kevin Norris

7. Complete the Job

   Push the switch back into place, tucking the wires neatly into the box, and secure the switch strap to the electrical box with its mounting screws. Reinstall the cover plate. Turn the power back on to the circuit by switching on the circuit breaker or reinstalling the fuse. Test the switch for proper operation.

   The Spruce / Kevin Norris

How to Use a Voltmeter to Test a Light Switch

Source: https://www.thespruce.com/testing-single-pole-switches-1152811

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