Tried using a non-contact voltage tester on a circuit I thought was dead in an old Baltimore row house, and it didn't beep even though the line was still hot.

Turns out the tester was picking up interference from the knob and tube running parallel in the wall, so now I always double-check with my meter before I touch anything in those pre-war places, has anyone else had a similar false negative happen?

4 comments

4 Comments

patricia_gonzalez2mo ago

My 1920s place does that too, always check with the meter.

kimr742d ago

I had a close call three years ago in my 1930s bungalow that completely changed my mind on those non-contact testers. I used to think they were good enough for a quick check before touching wires. Then I got a false negative just like you described, and when I finally pulled out my multimeter, the line was still live. That moment scared me straight. Now I don't trust those little pens at all for anything but a very rough guess, especially in old wiring. They're too easily fooled by stray fields and interference, and I'd rather spend the extra few minutes with my meter than risk getting zapped.

ellis.faith2mo ago

My old tester did that near a transformer once.

milarodriguez2mo ago

Actually @ellis.faith, that was probably a magnetic field tricking the sensor, not the transformer itself. Non-contact testers pick up stray voltage from wiring all the time. It's a common false positive that makes people think something is live when it's not. Those little testers are NOT reliable for safety. You really need to check with a proper meter that measures actual voltage.