# How do non contact voltage detectors work on open circuits

I’ve seen how people use non contact voltage detectors to check whether or not a socket’s live wire has power. Non contact voltage detectors take advantage of capacitive coupling that occurs in AC in order to figure out if there is power or not.

Now, what people usually do is simply stick the detector into one side of the socket (the live side). This part completely confuses me, how is it possible to detect if there is power if the circuit is open? There is no connection from live to neutral which would close the circuit and allow current to flow.

Another thing that doesn’t make too much sense to me is that why does it only detect the live wire and not the neutral? When people insert the detector into the neutral side of the socket the detector doesn’t light up. The neutral wire is a must for current to flow, why doesn’t it also detect it?

Any help is greatly appreciated.

• As you said, capacitive coupling. How else do you think you die when touching a live wire on many megohm resistance floors... Commented Oct 14, 2018 at 21:33

## 1 Answer

Because it isn't an open circuit. To AC, a capacitor is just another impedance:

simulate this circuit – Schematic created using CircuitLab

This series of capacitors and high-impedance ground completes a circuit that allows a very, very tiny current to flow, which the electronics in the NCVD will detect and show a signal.

Note that this is only one scheme; there are others that involve directly sensing electric fields that can also detect DC voltages.

This schematic should also make it clear why the neutral wire isn't detected. Neutral is ground-referenced somewhere in the system, so that would be connecting ground to ground through a bunch of passive elements.

• Wow, I thought it was imposible to detect such a small current. Ground resistance is massive (the floor in my house) so practically no current flows. Interesiting, thanks. Commented Oct 15, 2018 at 5:45
• @rr1303 With specialized equipment, you can detect and even measure currents less than a femtoampere! Commented Oct 15, 2018 at 17:05
• Can you please explain this for those of us without electrical engineering backgrounds? I believe, if I'm not mistaken, that the OP is asking how a voltage could be detected when no current is flowing (because there's nothing plugged into the outlet and running). Are you implying that when you hold a non-contact voltage detector near a hot that somehow current flows? I don't know how to read that diagram of yours, unfortunately, but I'm still curious how this works. Commented Jun 7, 2021 at 13:28
• @rory.ap Yes, that's exactly what I mean. There's capacitively coupled current through the voltage detector (and through the person holding it), because the signal you're looking at is AC. These kinds of detectors are extremely sensitive, so even the minuscule current you get from that is enough. It's also possible to detect high-voltage AC or DC using an electric field detector, which I didn't know about at the time I wrote this answer two and a half years ago. Commented Jun 7, 2021 at 13:33