Dodgy Cable Detector?

[Fitz]

In message 'electrical weirdness'

A problem was described which a friend is experiencing.

My friend borrowed my electronic cable detector which detects live cables and pipes. There are two operating modes and the metal pipe detector has a sensitivity adjustment. The live cable detector is simply on or off.

I previously had no reason to suspect that the equipment did not work. Placing the device against the wall of an alcove in his party wall gives a live indication for the entire wall. Other walls in the same room do not indicate as live. Testing around a light switch gives correct behviour.

So I can't decide what conclusion to draw about the status of my cable checker. The failure mode is presumed to be a false positive. All other checks indicate that the device is working correctly.

Is there any way to be sure other than removing some of the plaster in the intended location of drilling and visually examining the area underneath?

Even turning the power off is not going to help. If we turn the power off and the detector does not indicate live then there is not guarantee that I wont be drilling through a buried cable which will then become live again when I turn the power back on. If we turn the power off and it STILL shows live then there is no guarantee that we are not detecting power from a seperate power source. This IS the party wall. This guy has been next door and visually examined the site from the other side and there are wall mounted lamps there but surely these shouldn't be picked up through 9" of brick by one of these detectors? My friend hasn't tried getting both houses to turn their power off...

Any comments or suggestions appreciated.

[Chip]

On 28 Jun 2005 03:35:19 -0700,it is alleged that "Fitz" spake thusly in uk.d-i-y:

These detectors, for safety, are very sensitive. Any leakage current will trigger the 'live' detection. I have had this myself from an entire wall. What's happening is the wall is probably slightly damp (being a party wall it's likely solid masonry and thick, thus holding moisture) and the detector is picking up leakage, probably a minute and non-dangerous amount of leakage mind you, so not in itself a hazard, but annoying due to this false indication nonetheless.

As for workarounds the only suggestion I can think of is to turn off the power, put it in metal detection mode, and see if it finds the buried (dead) cable around the light switch as metal, then see if you can find the buried dead cables in the party wall.

HTH