Mains cable detectors

They are usually OK at detecting live mains cables if used correctly.

I have seen a sparks do exactly that due to making assumptions about which way a cable would run from a socket in an old building and not bothering to use his mains cable tester.

Mine works fine for finding live wires. It is much less good at finding small metal nails and screws in the wall which it also claims to do.

Jeff Layman submitted this idea :

Cable/metal detector or live cable detector?

Both work absolutely fine, you just verify one against the other.

You have posted here long enough to know that a SDS drill bit is the most suitable tool to find a cable or pipe:-)

Back may moons ago, I made a wonderful device that had two modes, the first basically was a metal detector using two oscillators, one of which had as part of its tuning circuit a bloomin large coil of wire, and this would pull or push the frequency of one one way for ferrous and the other way for non ferrous metal. However far more useful for mains tracking was a coil of wire on the input of an amplifier. Turn on your circuit, and follow the hum. It occurred to me more recently also that another way of doing it was to use a radio, and follow the wire with that as usually there was sufficient hash or increase in signal on the wire to make this quite viable. I'd have thought by now the commercial companies might have sorted out a system that was foolproof and looked a lot better than my old heath Robinson device held to together with duct tape and glue! I cannot remember what happened to that thing. Ho hum. Brian

Well, I was thinking of getting an old earphone (maybe in-ear for its smaller size) and connecting it to the input of an amp. Maybe it would be possible to use that to follow a live mains cable.

Most certainly, but that's when you are trying to avoid hitting the cable. I am trying to find it!

IME not very well, one of these

it can variously give both false positives and false negatives ...

I think that's the problem - the inconsistency between users. Most of these devices seem to result in reviews varying between "perfect" and "useless". I can't believe it's down to just poor calibration or failure to use the device correctly. Something else is afoot here.

In my 30 years of serious DIY involving doing up neglected houses, I have tried all sorts of pipe, metal, wood and live mains cable detectors.

The ones I tried on live mains cables were the only ones that worked reasonably well.

Everything else failed for wood and pipes.

What I do now is use a powerful neodymium magnet to detect nails and plasterboard screws and that will tell you where the studs and noggins are.

As for pipes, I use a FLIR infrared camera and have the hot taps, cold taps and central heating running.... The FLIR camera soon finds the copper pipes.

I have not found a good solution for plastic water pipes though.....

I have heard that a stethoscope can be used to listen for the sound of running water but that is no good to a deaf person like me.

But why only "reasonably well"? Why does it appear so difficult to reliably detect 50Hz at 230V?

I have an old PlasPlugs Stud detector which will also detect cables, pipes etc. I even used it on our previous motorhome to confirm the position of 'hard points' (blocks of wood) under the thin aluminium skin of the coach built body.

It seems to detect change in 'composition' (I suspect permittivity). You calibrate it on a bit of 'empty' wall. Then it will detect changes. In theory, you could get it wrong an selected a bit of wall that wasn't empty for the calibration stage but I check by doing it a couple of times.

Could it be that with no current flowing there is no magnetic field to detect? Presumably operating as a metal detector the probe measures a change in the load?

Maybe. I just connected up an 80W load and checked again. This time the detector is telling me the cable is 2 - 3 cm to the right of where it said it was previously. I'll chip away later to see if it really is there.

Not sure. I just tried the "metal detector" function of the device (no power to cable) and that agrees with the cable detection part of the device in terms of positioning.

Success! The cable was a couple of cm to the right.

But the weird wiring in this place had one more twist. What I found was grey PVC cable - red and black with an earth wire. But the old wiring at the original picture light was white PVC - with red live, blue neutral, an unconnected yellow/white wire, and an earth. So somewhere in the wall this 40 cm of no doubt spare two-way cable had been joined to the original T/E to extend it. I doubt anything more sophisticated than wires twisted together and covered with insulating tape would have been used, but as the cable has now been cut it really doesn't matter.

Now where did I put that large pack of filler?...

Damn those SDS drill bits.

You used it on your downlight swap 2 days ago. AICMFP:-)