I have a three wire power cable - live, neutral and ground. My multimeter shows no fault with the neutral and ground wires, but there is an open fault on the live wire. Nothing suspicious upon visual inspection.

How can i find the location x of the fault? The cable is about 1.5 meters.




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    \$\begingroup\$ Why would you even want to? throw it away and buy a new one, at that length it isn't worth the risk repairing. \$\endgroup\$
    – PlasmaHH
    Jun 21, 2017 at 20:45
  • \$\begingroup\$ Absolutely pointless, but you can always go buy a time domain reflectometer and measure it directly. For that cable, you would want a fast rise time pulse (25 ps or better) and leading edge detection. \$\endgroup\$
    – R Drast
    Jun 22, 2017 at 11:13

1 Answer 1


I have successfully used a battry-powered voltage testing screwdriver to find a blown bulb in a set of series connected Christmas tree lights. The LED will glow as the tip is moved from the live end of the loop until after the blown bulb at which point the LED turns off or glows much more dimly.

In my case the problem was simplified by the wiring being one large loop rather than a line with the return wire twisted alongside it.

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As PlasmaHH says, "Why would you even want to?". You are going to have to joint the cable and it would be much safer to replace it. The break may have been caused by flexing in which case the adjacent region may all be week as may the neutral and - potentially more dangerous as there will be no external sign of failure - the earth.


If the internal conductors have broken there is a good chance that the insulation is damage or thinned out too. Don't touch the cable during testing. Put one hand in your pocket and with the other grip the tester up near the pocket clip while touching the metal button on the top with a finger or thumb. Trace the tip along the insulation. Unplug the cable when testing is complete.

  • \$\begingroup\$ You may want to clarify this so that the OP doesn't plug in a bad cable to AC mains and run a testing pen down the wire. I'd use a 9V battery or something small, the risk is internal shorting or electrocution through an imperceptible crack/hole in the insulation. \$\endgroup\$
    – Ron Beyer
    Jun 21, 2017 at 21:32
  • \$\begingroup\$ Thanks, Ron. The voltage sensor only works on AC at sufficient voltage, as far as I know. The cable has to be plugged in for detection. I've added a safety section. \$\endgroup\$
    – Transistor
    Jun 21, 2017 at 21:40

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