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I was reading this Fluke multimeter safety manual and I came across this sentence: http://faculty.riohondo.edu/jfrala/fluke_multimeters_-_abcs_of_multimeter_safety_multimeter_safety_and_you_application_note.pdf

"Non-contact voltage detectors are a quick, inexpensive way to check for the presence of live voltage on ac circuits, switches and outlets before working on them."

Question 1: Why exactly is this relevant? If you are going to work on a circuit, why not use your multimeter directly to check the presence of voltage?

I thought about safety but I can't see a real reason. If you are worried about safety, once you measure the presence of voltage with the non-contact voltage detector you would need to use your multimeter next anyway (and the "safety" you supposedly got from measuring it with the the non-contact detector would becomes useless).

What am I missing?

Question 2: Why exactly are these called "non-contact" voltage detectors? You still need to physically insert them into an outlet (or somewhere else you want to check) in order for it to work.

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    \$\begingroup\$ Safety. You find them more in industrial settings where technicians would have less leeway to mess with electrics without good reason. This allows you to test for e.g live wires without touching amy hookups. You can also do cool things like check which wire in am unmarked bundle corresponds to its parent end 50ft away. \$\endgroup\$
    – crasic
    Commented Aug 28, 2015 at 5:54
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    \$\begingroup\$ There is also always the danger of a lack of mindfulness; you last used the DMM on "ohms" or its 10A current range... any practice that reduces that risk is worthwhile. \$\endgroup\$
    – user16324
    Commented Aug 28, 2015 at 11:06
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    \$\begingroup\$ As a slight addition to the current (both good) answers: The non-contact measurement tells you there is something live in there - you have not isolated it properly yet. Once you have done that, your voltmeter is required to verify that the circuit(s) in the box is zero-energy. This prevents entering a live arc-flash boundary and other generally bad things with energized circuits. \$\endgroup\$
    – Jon Custer
    Commented Aug 28, 2015 at 14:27

4 Answers 4

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Q2: Non-contact voltage detectors don't need to physically touch the measured hardware. They work by sensing the AC electric field created by live AC wires. They're used to quickly see if there is something live, e.g. connected to a wall outlet, inside a device. You can also use them to track where power wiring is going inside a wall etc. They often also have a metal detector so you can also track wires which are not live.

Q1: "Working on a circuit" doesn't necessarily mean "debugging a circuit" here; you might not have a need to measure the circuit electrically. You might be replacing a broken plastic casing for example, and you'd use the detector to see if there's something that could give you an electric shock at the other side of the plastic before you start to fix it.

Obviously you shouldn't rely just on the meter for your safety. You'd disconnect wall fuses or kill main switches before you start, using the meter just gives you some extra protection: with the meter you might notice for example that you've disconnected the wrong fuse and the unit is still live.

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For a voltage reading to appear on your voltmeter, you need a complete loop. If one end of a single phase supply had a wire break, you will not be able to detect that the other cable is live. A non contact voltage tester can detect voltage on individual test points.

A voltmeter tells you if there is continuity from the supply to both test points, it does not tell you if a cable is not live.

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Checking voltage with a volt meter may not always be convenient. For example older light switches had the wires inserted in the back instead of the sides like modern ones. The switch would have to be removed first to use a volt meter. Also if working on a wiring junction in an electrical box. The wires would have to be exposed before a meter could be used.

The meter is called non contact because there is no metal exposed that contacts anything electrified. The tip of the meter is plastic. The reason it needs to be inserted in an outlet is because the contacts of the outlet are recessed. The meter has to be close enough to the contacts to sense the voltage. The meter can also be held up to insulated wires and up against the side of an outlet or a light switch.

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  • \$\begingroup\$ A small nit, backwiring is still available, but the standards have changed. The newer backwire systems are safer and work with both solid and stranded wire. The old ones were faster, but were solid wire only, single guage only (normally 14), and more difficult to rework. \$\endgroup\$
    – hildred
    Commented Aug 28, 2015 at 6:04
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I call them wire tics. They beep and flash when near voltage. It's good for easy conformation of power (safe or not to work on) I always test it on a known live circuit first to test that it is working

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