Testing electrical cables (domestic)

[Koldov]

TL;DR - How do I test cables in the loft that have been cut off, without potentially killing myself...

 

I spent the half-term in my loft (literally everyday, morning till afternoon up until 5 p.m. Sunday), removing all the insulation. We recently moved into an old 1950s bungalow that has apparently had a severe mouse infestation for many years (very old lady lived there on her own before us and probably never went into the loft). 

 

Anyway, after 2 changes of hooded coveralls (including gloves and masks), 3 dead mice, an Indiana Jones style labyrinth of cobwebs, 30 bin bags and numerous vacuum cleaner empties later it is now clear... Just to set the scene a little further, the dead mice had decomposed mostly onto the loft floor (which is the ceiling of the room below obviously) and most of the insulation I lifted had mouse poo falling out like chocolate sprinkles... also we had the old original 1950s roof replaced a couple of months ago and the resulting mess from the dust, moss and the bitumen membrane that disintegrated in the roofer's hands meant it was all covered in a thick layer of filth.... so it had to be done!

 

During the insulation removal I found various electrical cables that had been cut and a few of them I can't trace where they go after going through the ceiling and into the wall of the room below...

 

Do I just hope they were disconnected from the source? Can I use a basic multimeter to probe the live and neutral wires?

 

I guess the default answer will be call an electrician, which would be the sensible/safest way, but I'd rather not pay those kind of rates for 3 or 4 cables if they're going to do something I could (albeit probably with a more expensive multimeter)...

Edited June 5, 2025 by Koldov

[synaesthesia]

Go to wickes/B&Q and pick up an electrical tester for a few quid, they'll tone out live cables without having to probe anything; https://www.wickes.co.uk/Non-Contact-AC-Voltage-Tester/p/276107

[Scan099]

Or Slightly cheaper Screwfix https://www.screwfix.com/p/lap-non-contact-voltage-detector-pen-1000v-ac/3222g

 

[Koldov]

Thanks, now to get the OH to run around downstairs turning everything on and off in case one of them is switched live! 🙄

[PotNoodleTech]

Pick up one of the beepy voltage detector like a really big thick pen - I've got a dewalt one as soon as you put it to wire it beeps and lights up if the wires live.  

[Chris_Cook]

I'd recommend looking up "Proving dead". There are proper procedures and test equipment for this.

 

In general, this involves checking the tester on a proving unit (generates high voltage with low current), then check the circuit, and then check the tester again. I'd recommend doing this no matter what tester you use. With a non-contact tester, you could do this on a live circuit, but I'd not recommend that with a multimeter.

 

The issue you can have with using basic testers is you can get false results or worse.

 

Multi meters have 3 issues - one, they have a fuse that can blow, and then it might just say there is no voltage.

They also often have a current setting that makes the meter into a wire that could just short things out. Its easy to have the leads in the wrong sockets.

The cheap ones often aren't properly rated for mains work.

 

You could also end up holding the end of one lead and then connecting the other to live. That might not be a good idea. 

 

Non-contact testers are great for quick diagnostics, but again, you need to check they work before and after the test.

 

There's nothing to say you can't use these testers, but you should balance up the risks with the options available to you.

 

Another option might be to put a wago box or other type of junction box over the end of the cable. This could make it safe if you just want to leave the cable in place. If the cable is deteriorating, then removal is probably best.

 

Its also worth bearing in mind that circuits might be isolated with a switch, fused spur or circuit breaker. So just because it isn't live right now, doesn't mean it can't easily be re-energised later.

 

The right answer really is to get an EICR and have it all checked/isolated at the consumer unit or junction box. An electrician could open the fuseboard and check if the cable is still connected back there at all. Also if there has been mice, then the insulation test they do would help check if any wires have been eaten through where they can't be seen.