How to Test for a Complete Circuit in a Light Bulb Holder

multimeter and electrical plan

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You’re trying to get one of your lights working properly again. You’ve already ruled out a bad bulb and you’ve established that the socket has power coming in, but the bulb still won’t come on. So now you’re ready to make sure the final piece of the power path—back to the source through the neutral—is in good shape. It's at this point that you test for a complete circuit.

What Is a Complete Circuit?

A complete circuit is an uninterrupted path that electrons can flow through, out of the power source and back into the power source. A circuit must be complete for electricity to flow.

There’s a way to do this for every type of holder we use. The size and shape of the bulbs may vary, and they may be incandescent, fluorescent, halogen, or LED bulbs, but they all have one thing in common: They have to have power coming in, going through the bulb, and returning to the source. They won’t work without that. And, in most cases, you should be able to restore that function. You’ll need a multimeter to do some testing.

How to Test Complete Circuits in On/Off Sockets 

  1. Turn the power to the socket on. You can check with your non-contact voltage tester to make sure it’s there. Turn your meter on and set it to read AC voltage (often abbreviated as “VAC”). If your meter is one that has selectable ranges within VAC, set it to the lowest value that is greater than 120VAC. Lay or hang the meter in a place where you can watch the dial, that will keep it stable, and that is close enough to the socket that you can easily work inside the socket with the probes.
  2. Reach in with one of the probes and set it on the brass power contact tab in the center of the bottom of the socket. Keep it centered in the socket and don’t let the metal part of the probe touch the brass power contact and the sidewall of the socket at the same time.
  3. Once you have that probe set, use the other probe to contact the silver shell of the socket—the part the bulb threads into. You can do that as close to the opening as you like. Just be sure that you don’t touch the shell and any power at the same time. This means, in particular, that you need to avoid touching the first probe with the second one.
  4. Your meter should show a reading of 120V. If it does, the socket has a complete circuit. If it doesn’t, there’s a problem with the neutral wiring. At this point, you’ll need to turn the power off and disassemble the socket to check the wires that connect to it. It was already showing problems or all of this testing wouldn’t have been needed, and taking it apart to check the wires to it is usually as much work as replacing it, so why not?

How to Test Complete Circuits in Fluorescent Fixtures ​

This can be done in two ways. The first one involves opening the wiring compartment and testing for 120VAC, hot-to-neutral and hot-to-ground, on the wires coming into it. If there’s a problem there that can be fixed, great. If that checks out okay, though, it’s usually time to replace either the ballast or the whole fixture.

One thing to note here is that, unlike fixtures that use other types of bulbs, many straight-tube fluorescent fixtures need to have a good connection to the ground to work properly. That connection should be visible as a green or bare conductor—wire—that’s terminated under a green hex-heat screw near the ballast. Look at it carefully to see if there’s a bare spot behind the wire to allow the ground to make good contact.

The other test is to use your multimeter to test the power on the output side of the ballast. Since different ballasts have different output characteristics, you will need to read the information on the label on the ballast in your fixture to determine the settings you need to use.