NCVT Reads Hot with no voltage.

Thread Starter


Joined Nov 2, 2016
I hate to revive and then start a new thread in rapid succession, but when I tried to fit this in the other thread, I realized it needed its own topic.

I recently bought a CAT IV non-contact voltage tester to probe the outlets to make sure I don't touch anything live and to look for wires behind walls. The second I turned it on at my desk, it goes off alerting me to high voltage. After point it at the metal chassis of the giant computer tower (Blue=no voltage), plugged-in USB hub (blue) computer screen (Blue), other assorted odds and ends, I found out that according to the tool, the aluminum railing I use to suspend an LED strip to light the desk was hot! So was the metal switch. The LED strip is connected to mains through an AC-DC power brick I bought at Radios Hack, which is unlikely to be a regulated supply (good enough for a single 14.5W LED strip, it's rated for 1.5 A)... I feel like the NCVT has to be wrong. I can get one VAC RMS with my DMM between the metal surface of the switch (it's a metal chassis toggle switch) and the aluminum housing, and only residual millivolts between the chassis and earth ground... any idea what's going on? Why is the NCVT consistently giving me a an alert that the thing is between 48 and 1000 VAC? After checking repeatedly with my DMM, the wet finger test also yielded bupkiss.

After unplugging it, the NCVT still gives me a sub 48 VAC alert that can't be traced to anything else on the desk. Should I return the NCVT, or am I using it wrong?


Joined Jan 15, 2015
Since there are dozens of those things available and I have no clue what make and model you have I really can't say what might be wrong. However, if you have the owners manual in front of you and the unit is not behaving per the manual then heck yes, I would return it. I have two FLuke versions, one for low and one for high voltage and they work as advertised. High is >90 VAC.