I have cable with two 1.5mm wires in it. Why does my test light show voltage in both wires even though only one of them is connected to 220V AC grid.

// Also there are old cables with metal jacket with the same problem when testing the jacket.

  • \$\begingroup\$ Is the other wire connected to neutral? Where you live, is the neutral wire tied to earth ground somewhere in your building? \$\endgroup\$ – The Photon Sep 4 '14 at 22:19
  • \$\begingroup\$ @the-photon The cable is brand new (like 2m long), not connected to anything. Just tested the cable and got the result. \$\endgroup\$ – ikaruss Sep 4 '14 at 22:29

In short, capacitance between the two wires is your answer.

Here's an example I faced many years ago. A friend of mine rewired his house but asked to borrow my meter before he connected ground to the pipes coming into the property. He phoned me up and said there was 115VAC measured on the unconnected earth wire and he was frightened of grounding it. (Local AC voltage was 230V and I assured him that "live" was at 230V, neutral at "0V" and the middle unconnected wire would find a voltage that was roughly halfway between the two wires (due to capacitance between conductors). He connected the earth wire and no problems.

What you are seeing is the natural effect of capacitance coupling the voltage on one wire to another in close proximity.

  • \$\begingroup\$ Andy, could this also be inductive coupling? \$\endgroup\$ – Dan Laks Sep 4 '14 at 22:30
  • \$\begingroup\$ So, how do I proceed with installing the cables if I can't trust my test light and have no other means of testing for "live"? \$\endgroup\$ – ikaruss Sep 4 '14 at 22:32
  • \$\begingroup\$ @DanLaks - you wouldn't see that sort of voltage due to magnetic coupling if the cables are isolated from each other. \$\endgroup\$ – Andy aka Sep 4 '14 at 22:33
  • \$\begingroup\$ @ikaruss - why not disconnect the AC from one end and measure for a short with a continuity checker OR use a 10nF capacitor (rated of course at the appropriate AC voltage) to ground the cable believed not to be connected to anything - you'd see the voltage diminish to just a few volts and likely as not the test light will not illuminate. \$\endgroup\$ – Andy aka Sep 4 '14 at 22:36
  • \$\begingroup\$ "No-Contact" test lamps, neon lamps, and even DVMs, are sufficiently high impedance that a very small leakage or capacitive coupling between wires will cause them to indicate a wire is "live" when it is not really. \$\endgroup\$ – Peter Bennett Sep 5 '14 at 0:55

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