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How to Buy and Use a Non Contact AC Voltage Detector / Tester

Why a non contact AC voltage detector / tester

Anyone that does any electrical work around the home at all needs some kind of voltage tester for safety, and the non-contact AC voltage detector, or tester is one of the easiest to use. Unlike other testers the non-contact style of voltage detector does not need bare wires to find stray or unwanted voltage - it can detect such voltage right through insulation without ever coming into contact with the voltage source.

As a safety device in the home the AC voltage tester is nearly unparalleled in its use and low cost. As a professional electrician I feel it is important enough that I carry two - one detector in my pocket at all times and a spare in the truck in case the batteries go dead or I lose or damage the first.

Above is one example of a non-contact voltage tester, available from Amazon. This particular brand and model is the one I prefer as a professional electrician. It uses 2 AAA batteries, which I always have around the house. Outside of physical abuse, I have never had one of these testers fail, and have used one for many years now. I highly recommend this tester for anyone working around electricity.

How to buy an AC voltage detector / tester

There are many different brands and models available of the non contact AC voltage detector or tester with many different price tags. Most of the large electrical tool suppliers offer them, such as Fluke, Greenlee, GB and Klein, but there are many smaller companies as well. Cost can vary from only a few dollars to nearly $100 for fancy ones with capabilites that few people will ever use. A good rule of thumb is somewhere around $20 for a good quality voltage detector that should last a lifetime. Don't buy based merely on price - this is your safety and possibly your life that you are protecting.

Most voltage detectors will both light up and beep when in the presence of AC voltage. The tester may or may not need to be turned on - I prefer the older fluke brand that is on all the time as there have been occasions where I need to leave the detector in place while observing from a distance and some detectors require that you hold a switch on when in use.

AC voltage can come in many different voltages and most detectors will read from around 50 volts to as much as 600 volts or more. These testers will not read DC voltage, however - they cannot be used, for instance, for working on an automobile.

Most voltage detectors use small "watch" style batteries for their operation, but some use AAA size batteries that are more available (anyone with children likely has a stock of them in the house).

How to use an AC voltage detector / tester

Turn on the voltage detector and verify it's working and the batteries are good. This can be done by gently tapping it on your arm or hand or passing it through your hair. The static electricity present in your body will briefly light the voltage tester up or make it beep, verifying that is is indeed operational.

Touch, or place the detector near, the wires or other item to see if it is with power. With normal 115 volt house current it is often necessary to actually touch the wire, but the wire does not need to be stripped bare as the AC voltage tester will detect voltage through the insulation. A caution here if the wire is an extension cord or other cord - the wires inside are often twisted around each other during manufacture and the tester will not react if it is in just the wrong place on the wire. Move the tester up and down the cord perhaps 12" each way to make sure that it is near the "hot" wire inside the cord. Similarly, the flat lamp cords need to have both of the wires in the cable checked, not just one side.

Your AC voltage detector should have a flat piece on the end that can be inserted into a plug in receptacle to see if it is "hot". Make sure that you check both slots on the receptacle - only one of them will normally show voltage as the "neutral" wire is not normally powered.

Occasionally a light bulb will break in the socket; the voltage tester can verify if the light is turned on or off before using pliers to twist out the broken bulb. Merely place the tip of the detector as far into the broken bulb as it will easily go - if the light is on the tester will glow. Either way, flip the light switch and re-test - after making sure the light is indeed off it will be safe to proceed to dig the broken bulb out with pliers or other tools.

The biggest single use for these testers is simply testing for the presence of voltage before working on something electrical. Examples might be when changing a light fixture out for a new one or installing a new light switch and the worker needs to know the power is off in the wires they will be working with.


Testing a light fixture being removed

Testing for "hot" wires with a voltage detector when changing a light fixture.
Testing for "hot" wires with a voltage detector when changing a light fixture.

Troubleshooting with an AC voltage detector / tester

While it is possible to troubleshoot electrical problems with an AC voltage detector or tester that is not what it is designed for and the procedure doesn't always work. Nearby sources of voltage can often be "seen" by the detector - flourescent light tubes are notorious for this. A tester within several inches of such a light tube will often read voltage, perhaps making the user think that a wire they are touching is "hot".

While I have used my non contact voltage tester to find the broken bulb in a string of Christmas tree lights I have failed in the task more often than not. The twisted wire in these strings of lights will often induce a small voltage in the nearby wire - not enough to light the string of lights but enough to set off the voltage detector.

As an electrician I very often use my voltage detector to troubleshoot faulty electrical circuits, but remain aware that a "false positive" result is always possible. That is, the tester may indicate the presence of voltage when there is either no voltage at all or just static picked up from surrounding "hot" wires. On the other hand, I have never had a "false negative" - my voltage detector has never indicated there was no voltage when there was.

As a cheap, effective safety precaution the non contact AC voltage detector / tester is highly recommended for the homeowner tool kit and is a virtual necessity for the electrician plying his trade. They are extremely useful as well for fire fighters, EMS respondents to home or industrial accidents and many other people working around electricity.

Non contact voltage detector / tester

This Fluke brand voltage detector was damaged 2 years ago when the flat tip was melted off.  The tiny wires in the tip were bent back into the tester and it still works fine.
This Fluke brand voltage detector was damaged 2 years ago when the flat tip was melted off. The tiny wires in the tip were bent back into the tester and it still works fine.

© 2010 Dan Harmon

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Comments 4 comments

SteveoMc profile image

SteveoMc 6 years ago from Pacific NorthWest

Good information, I am planning a similar hub. Don't know the market, but it goes with my appliance hubs. I use mine all the time. Especially when the homeowner has turned the power off in the breaker box. Test, test, test. I never leave it up to chance. Good information.


wilderness profile image

wilderness 6 years ago from Boise, Idaho Author

You're right - test, test, test. I've been hit by 277 volts from being too lazy to test while being sure power was off and it's not a good feeling. These things are so cheap there's no excuse not to have one around.


EdwardNorton profile image

EdwardNorton 4 years ago from Indiana

These are handy to have around but beware that they do at times display a false positive. This is because of the sensitivity of these style testers (hence picking up static in your hair). It's not a bad false positive but to a new user of one it can lead to the frustration of wondering why it lights up and beeps.


wilderness profile image

wilderness 4 years ago from Boise, Idaho Author

@EdwardNorton: Yes, false positives were brought out in the text of the hub. They can be a little scary sometimes, and frustrating as well, but not dangerous.

On the other hand, a false negative would be dangerous, but I've never seen that in a working tester. That is why I recommended that the tester be checked before use by tapping it on a hand to verify it will beep.

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