Dan has been a licensed journey-level electrician for some 17 years. He has extensive experience in most areas of the electrical trade.
Why Do You Need a Non-Contact AC Voltage 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.
How to Buy an AC Voltage Detector/Tester
There are many different brands and models available of the non-contact 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 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 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 and even if you know which one, it is always possible that the outlet was wired incorrectly.
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
Troubleshooting With a Voltage Detector
While it is possible to troubleshoot electrical problems with an AC voltage detector 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; fluorescent 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 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 firefighters, EMS respondents to home or industrial accidents and many other people working around electricity.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
Question: If your AC voltage tester is not showing positive how will I know?
Answer: While these testers will occasionally show a false positive, I have never had one give a false negative. If voltage is present it has always show it. A false positive (showing voltage when there is none) can be a little irritating as you try to find and correct something that isn't there, but a false negative (showing no voltage where the wire is actually hot) can result in injury - give my choice I will choose that false-positive every time.
Question: My tester says i have lost one leg of current. What do I do?
Answer: First, a non contact tester will not find voltage on the neutral, for it is grounded. But if you really have lost power on a leg, the only solution is to find where it disconnected and repair the damage.
Question: My tester shows a hot ground wire. What should I do?
Answer: If your tester is a non-contact type, re-test with a meter that will show just what that voltage is. If it is hot, you will have to find out why and eliminate the problem.
Question: How do I use this AC voltage tester to find out if a ballast is bad?
Answer: That's a tough one. If the ballast is not producing any output voltage when turned on it is bad; just hold the tester near each of the wires going to the lamp.
Unfortunately, a ballast may be bad and still show enough output voltage to detect with these testers. And it is easy to hold the tester close enough to the incoming power to pick that up while thinking it is the output from the ballast you are seeing.
So the best method is to ensure there is voltage coming into the ballast (with the tester) then check the outgoing wires from the ballast. If these show power, check the tombstones. If these show power, change the bulbs. If it still doesn't work, replace the ballast. Make sure the tubes are removed when performing these checks, as a tube will light up the tester as well, and from a considerable distance.
© 2010 Dan Harmon
Dan Harmon (author) from Boise, Idaho on February 09, 2019:
Actual voltages on a "220 plug" in the US can vary from about 228 volts to about 252. That is a 5% variation each way from the ideal voltage, and is considered normal.
how many volts is a 220 plug on February 08, 2019:
Dan Harmon (author) from Boise, Idaho on January 20, 2012:
@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.
EdwardNorton from Indiana on January 19, 2012:
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.
Dan Harmon (author) from Boise, Idaho on August 26, 2010:
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.
SteveoMc from Pacific NorthWest on August 26, 2010:
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.