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I'm in India where the domestic supply is 250v @ 50Hz. I've been working - gradually replacing the solid-core wire/s with stranded wires purchased recently.

To my surprise the line tester showed a feeble glow when touched to a wire (no connections made i.e. both ends free) draped over the phase/live wire. That little piece of wire (about a metre in length) itself is new, and so is the phase/live.

Further, as a general precaution when I play with these things at the residence, I had taken the precaution of wrapping non-adhesive PVC over the phase wire. Ergo, insulation leakage really should not be an issue but I'm not possessed of a megger. Oh ... and the line tester glow dies when the supply is killed.

Assuming the insulation is fair - what could possibly cause the line tester to present a false positive? Could a static charge induced from handling be so strong as to cause a glow?

  • The effect that you are seeing has been discussed here many times. The open wire is getting an AC voltage induced on it from the live wire. The induced voltage disappears when you remove power from the live wire. – Michael Karas Mar 13 '17 at 9:46
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    Related: When doing electrical work, what do I use to check wires are safe?, Unexpected voltage when switch turned off (and related questions listed there, many of which have excellent answers well worth perusing) – RedGrittyBrick Mar 13 '17 at 10:16
  • @MichaelKaras: Hope I do not misunderstand what is being said but the open wire too is of stranded type - and similarly insulated. – Everyone Mar 13 '17 at 13:30
  • @Everyone - Stranded or solid wire, insulated or not, the induced voltage on the open wire next to the live one can still appear. – Michael Karas Mar 13 '17 at 14:28
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When wires that are not connected are in close proximity to live wires a voltage may be transferred through induction this is how transformers work. When voltage is traveling on a wire there is an electric field created the higher the load on the wire the stronger the field. Placing a piece of wire where it is affected by the field will allow it to pick up some of the voltage but there is really no potential or current. Many people call this voltage a phantom or ghost because as soon as the wire is connected to a load like a light the voltage is gone.

  • I'm surprised to see it on a tester that lights up. I should do some testing sometime and see how much current can be picked up by induced voltage. – Harper - Reinstate Monica Mar 13 '17 at 14:45
  • Phantom voltage lights up my fluke , greenlee testers. The current depends on the amount of wire in close proximity. Just like transformers. – Ed Beal Mar 13 '17 at 18:57

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