Non-contact voltage tester and false positive results

Hey Mike,
I’m hoping you can answer a question I have. I’ve tried the Internet, but I think you may be the only person on the planet who can help me. The question is: Can a non-contact voltage tester have a false positive? —M. Morse

Hey M.,
Well, in a word, yes. But in reality it really is finding voltage, just not necessarily with enough current to do you any harm. So here are a few situations where you can easily get a “false” positive.

For example, if you plug in your basic iPhone to a wall outlet and test it with an NCVT (Non Contact Voltage Tester) it will indicate a voltage. In fact, if you measure it with a high-impedance meter you’ll find that the case of the phone actually has around 60 volts AC on it while plugged into a wall charger. Now, the wall charger itself isolates you pretty well from the 120-volt line voltage, but there will be a certain amount of leakage that’s typically less than 1/10 of a mA (milliamp) of current which will energize the case with 1/2 of the line voltage, which creates 60 volts of “hot-skin” in the USA. That current is below the threshold of what you can even feel and is certainly safe.

However, if you plug in a damaged or cheap iPhone charger, it can actually electrify the case of your smartphone with significant current, which is exactly how several people in China died last year while answering their smartphones that were plugged into defective chargers while standing in the shower. The same goes for crock-pots which don’t have a grounded power cord from the factory. They’ll usually develop around 60 volts on the metal chassis (1/2 of the 120-volts in the power line) but at less than 1 mA, so they’re safe. But I still won’t stand in a shower with a plugged in smartphone, or a puddle while touching an ungrounded crock-pot. I’m an engineer and know way too many things that can go wrong.

However, if you’re testing anything that’s powered with a grounded power plug and you measure a voltage on the chassis with an NCVT, then there’s something very wrong with your grounding system which could become deadly at any time. Anything with a grounded plug should NEVER show a voltage on the chassis. If it does, then the ground has been compromised and things could become dangerous. So the bottom line is, anything that’s plugged in with a factory non-grounded plug will usually have around 60 volts on its chassis/skin and is probably not dangerous even though it will make an NCVT light up and beep. But anything you plug in with a grounded plug should NEVER cause an NCVT to light up and beep at you. And if you ever feel an electrical shock from anything, then please have it checked by a qualified electrician or technician.

If any of you are musicians, here’s a video I did on how to check electric guitars and stage amplifiers for potential shock hazards using an NCVT. Watch the video.

Let’s play safe out there… —Mike Sokol

Mike Sokol is an electrical and professional sound expert with 40 years in the industry. Visit NoShockZone.org for more electrical safety tips. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.

Editor: Here are non-contact voltage testers and high-impedance meters at Amazon.

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