Non-contact voltage testers: tips and tricks (2023)

Voltage tester, tick-tracer, glow-tip, non-contact voltage tester... whatever. Same thing. I call it a voltage sniffer, but the technical name for this device is a capacitive voltage sensor. That's a term that I have never uttered aloud, btw. Voltage sniffers use black magic to determine whether or not wires are live, and they're 100% reliable every time.

Non-contact voltage testers: tips and tricks (1)

Ok, late April Fools. But seriously, I won't try to explain how these things work because it's above my pay grade. It has something to do with capacitive coupling, which the fabulously fine folks at Fluke were kind enough to explain in their three-page document, titled "Understanding capacitive voltage sensors". If you want to know how they work, check out that document.

As a home inspector, I use my voltage sniffer for a few different things, and it's important enough that I always keep a spare or two on hand. For a summary of this blog post in video format, check out the video below:

(Video) Non-contact voltage detectors: tips and tricks for home inspectors

Voltage sniffers to find live wires

The most obvious use for a voltage sniffer is to alert you to live wires. I do this in attics more than anywhere else. I always check knob & tube wiring when I find it in an attic, because the first question anyone asks is always "was it live?"

Assuming you have a good, trustworthy device, it'll do a good job of letting you know if a 120- or 240-volt circuit is live. Probably. These devices are not 100% accurate, but they do a pretty good job. You simply hold the tip near a suspected circuit, and it'll tell you if there's current or not. Probably.

I say "probably" because these devices are not 100% reliable. When a voltage sniffer lights up, it really means "maybe". You can run a dead wire around a live wire, and the dead wire will set off the voltage sniffer because of the electrical field that's placed on it. Do you remember creating an electromagnet with a wire, a battery, and a nail? It's the same principle. The clip below demonstrates this perfectly.

Non-contact voltage testers: tips and tricks (2)

That white wire isn't connected to anything, but my sniffer is telling me it's (possibly) energized.

(Video) How to Use a Non-Contact Voltage Tester

Warnings:So what about the opposite of this? If a voltage sniffer doesn't light up or beep at you when you're holding it close to a wire, does this mean the wire is dead? No, certainly not. The wire might simply be connected to a switch that's turned off. A wise home inspector would never report wires as 'dead' or disconnected unless they were able to completely trace the wires down and verify that the wires weren't connected to anything at either end.

Also, manufacturers of voltage sniffers always recommend testing with a known source of electricity to verify proper functionality every time you use the tester. They also recommend holding the tester with your bare hand and keeping it close to your body. There may be some conditions where a voltage sniffer will fail to detect current if you're not actually holding the device, as shown in the clip below.

Non-contact voltage testers: tips and tricks (3)

Also, voltage sniffers cannot find live wires inside of properly grounded metal conduit.

Voltage sniffer for ungrounded metallic lights

Another use for a voltage sniffer is to locate ungrounded metallic fixtures. We routinely use voltage sniffers around metallic lights near kitchen and bathroom sinks to verify that the lights are properly grounded. If a metallic light fixture isn't grounded and a hot wire was to come into contact with the fixture, it would silently energize all of the metal, making this an electrocution hazard. If we hold a voltage sniffer up to an ungrounded metallic light, it'll light up. Once we touch the light, we change the capacitance and the sniffer shuts off.

(Video) Common Safety Issue | Non-Contact Voltage Testers

Non-contact voltage testers: tips and tricks (4)

This is probably what I use my sniffer for more than anything else. Not all metallic lights have to be grounded, however. The desk lamp pictured above is a good example of a light that doesn't need to be grounded. If it were grounded, it wouldn't set off my tester.

Which tester to buy

I've used many different voltage sniffers over the years and been frustrated with most. Voltage sniffers typically measure down to 50 volts, which means that a lot of low-voltage stuff will set off the tester. Even my 5-volt mobile phone charger was able to make a few of my testers go off.

To help reduce the potential for false positives from low voltage stuff, I recommend home inspectors use a voltage tester that starts at 90 volts. Fluke makes a couple of such testers, both the 1AC-A II and the 2AC. I prefer the 2AC because there's no on/off switch. It's always on, and a pair of AAA batteries will last a year or two. Also, the fact that it runs on AAA batteries is huge. I wouldn't buy another voltage sniffer that runs on button batteries, because they're harder to find.

I've had the Fluke AC2 for about three years now, and it has been the most reliable, rugged non-contact electrical tester I've owned. It costs a little more than most other voltage sniffers, but it's definitely worth the price.

(Video) The DEADLIEST Electrical Tester...

Author:Reuben Saltzman,Structure Tech Home Inspections

Non-contact voltage testers: tips and tricks (5)

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Reuben Saltzman is a second-generation home inspector with a passion for his work. Naturally, this blog is all about home inspections and home-related topics in the Twin Cities metro area. In addition to working at Structure Tech, he is also a licensed Truth-In-Sale of Housing Evaluator in Minneapolis, Saint Paul and several other cities.


How accurate are non contact voltage testers? ›

These devices are not 100% accurate, but they do a pretty good job. You simply hold the tip near a suspected circuit, and it'll tell you if there's current or not.

Can a non contact voltage tester give a false positive? ›

Now that you have a better understanding of capacitors and how non-contact voltage testers work, it's important to understand that they can provide false positive results sometimes.

How do you know which wire is hot? ›

Hot wire is identified by its black casing. This is the main color of hot wire for most homes. However, other hot wires can red, blue, or yellow, although these colors can indicate a different function besides powering an outlet.

How do you read a voltage tester? ›

How to measure ac voltage
  1. Turn the dial to ṽ. Some digital multimeters (DMMs) also include m ṽ . ...
  2. First insert the black lead into the COM jack.
  3. Next insert the red lead into the VΩ jack. ...
  4. Connect the test leads to the circuit: black lead first, red second. ...
  5. Read the measurement in the display.

Can you touch a live wire with a voltage tester? ›

One of the easiest ways to know if a wire is live is to use a voltage checker or a current checker. These are simple devices that you can touch to any wire, and it will tell you if there is electricity running through it.

What setting should I use for voltage testing? ›

You have to set the multimeter to a range that it can measure. For example, 2V measures voltages up to 2 volts, and 20V measures voltages up to 20 volts. So if you've measuring a 12V battery, use the 20V setting.

Do voltage testers need calibration? ›

The accuracy of electrical testing equipment can drop over time. To avoid safety issues or process failures, it is critical to ensure that any electrical testing equipment used within your business is regularly calibrated.

How accurate is the Hawkeye non contact thermometer? ›

HIGHLY ACCURATE & FAST - The Hawkeye™ NCIT100 is calibrated specifically for reading human body temperature with an accuracy of ±0.4°F (±0.2°C) and a response time of less than two seconds. Can be adjusted to readout in Fahrenheit or Celsius.

What is the advantage of a non contact voltage detector over a voltmeter? ›

What is the advantage of a noncontact voltage detector over a voltmeter? It does not have to contact the energized electrical circuit to indicate voltage.

Does a non contact voltage tester work through walls? ›

They're called "non-contact", but that means no electrical contact (i.e. they don't have to touch the bare wire). You'll just about have to touch the tool to the insulation of a wire for them to work properly, so it's not likely you'll be detecting wiring within any type of wall with one of these tools.

Can a non contact voltage tester work through drywall? ›

Q:Can it detect voltage from wires behind drywall? A: Not really, the tip typically has to be within 1/2” - 1/4” of the hot wire to sense electricity.


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