How to Check Christmas Lights With a Noncontact Voltage Detector

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Christmas lights set the warm, fuzzy, twinkly ambiance during the holiday season, and they usually come in strings of individual bulbs arranged in multiples of 50. One of the frustrations of the season, however, is that the failure of one bulb can make all the others go out. Strings of lights aren't very expensive, so it's common to simply discard a string that isn't working and replace it, but that isn't always easy when the string is on a Christmas tree or forming the outline of Santa's sleigh or a nativity scene. It's also wasteful.

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With an inexpensive noncontact voltage tester, you don't have to replace a nonworking string of lights because you can easily identify the bad bulb and replace it while leaving the string right where it is.

Why One Bad Bulb Creates an Outage

Most Christmas lights come with a male plug on one end and a female plug on the other with a pair of wires — a hot and a neutral — running between the plugs. The bulbs are arranged in groups of 50 on branch lines that tap into the two main circuit wires. The bulbs are connected in series, and electricity passes through the filament of each of them. If one of the filaments burns out, it creates an open circuit and makes all the bulbs on that branch line go out.

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To prevent this from happening, manufacturers provide a shunt on each bulb that allows electricity to pass even when the filament is burned out. Unfortunately, the shunt can also fail, and when it does, electricity stops flowing through the branch circuit. The simple solution is to replace the bad bulb, but the question is how to find it. It's actually pretty easy if you use a noncontact voltage tester in a systematic way.

First, Check the Fuse

If an entire string of lights fails, the problem may be a blown fuse, especially if the string has more than 50 lights. The fuse is located in the male plug, and you access it by pulling the plug apart.

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Grasp one of the prongs with one hand and the plug housing with the other, pull the prong, and keep pulling until both prongs come out of the housing. You'll see a 3- or 5-amp mini fuse nestled between the prongs, and if it has burned out, you'll see blackening on the fuse, or you'll be able to see that the filament inside is broken. Replace it with an identical one.

How to Check for a Bad Bulb

Step 1: Plug in the lights.

Power needs to be running through the string so you can test voltage. Plug it into a working receptacle.

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Step 2: Check the noncontact voltage tester.

A noncontact voltage tester has a single probe, and when the probe comes close to a live wire, the tester will beep, and a light may flash. Turn on the voltage tester and hold the probe near the wire coming from the base of the male plug. It should beep. If it doesn't, check the fuse and replace it if necessary. Then, try again. When it beeps, you're good to go.

Step 3: Test one of the bulbs.

Choose a bulb midway between one end of the group that has gone out and the other. If the tester beeps, you know power is running to that bulb, and all the bulbs before it are working, so continue testing all the bulbs after that one.

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If the tester doesn't beep, that bulb isn't receiving power, and one of the bulbs before it in the circuit is the culprit, so continue testing bulbs closer to the plug.

Step 4: Continue testing until you find the bad bulb.

Use the same procedure, choosing the bulb midway between the bulb you tested and the beginning or end of the section of bulbs that aren't lit to gradually zero in on the bad bulb. The bad bulb still has power, so the tester will beep when you hold the probe close to it, but it's cutting power to the next bulb in the string, so the tester won't beep when you hold the probe close to that one.

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Step 5: Replace the bulb.

Pull the bad bulb out of the socket and insert a new bulb. The rest of the lights should come on.

It's Easier to Use a Light Keeper Pro

A Light Keeper Pro is a handy tool that can actually restore a worn shunt inside a bad bulb. To use it, remove one bulb from the section that has gone out, plug the socket into the tool's receptacle, and click the trigger several times. All but the damaged light will come on if the tool successfully restores the shunt. Since the damaged bulb is the only one that isn't on, it's easy to identify and replace it.

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references & resources

Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker and Family Handyman.