Identifying mystery wire.

[zymurgy]

Not that i'm allowed to touch my home electrics anymore, (Part P, grr) but say I had a mystery wire dangling near my fusebox, which disappears into the 1st floor landing void. How could I identify this wire without energising it (as i'm not at all sure what is connected to the other end) I have a sneaky feeling it goes to the loft, but have no way of so determining.

The other wires all go to a CU with MCB's, so isolation of all the other wiring is not a problem.

How do the experts identify individual wires once they've gone through floor voids etc .. ?

I'm sure there's a simple explanation staring me in the face here !!

Cheers

Paul.

[The Natural Philosopher]

Duno about experts,but I short the end of them and see which is shorted at the other end.

I once had to rewire a disco that was full of lights all wired using different bcables into an old biscuit tin.

My soluition was to hacsa the lot off, set up proper box, and apply mains voltage to any pairt that seemed to have a load on them.

This way I documented them all, watching whichj lights came on.

The last as two 4 way cables that all seemed interconneceted. I eventually traced these to a bank of 7 lights that used one red cable as a common return.

Nort really. You can put an audio signal on them and go looking with a sensitive audio detecor.

If it goes nowhere that you can find and end of (I have one like that here) cut it and bury out out of sight.

[John Rumm]

One way is with expensive wire tracing generators and signal detectors (the sort of kit BT use for tracing wires underground etc).

A few things you could do...

Bare then ends of the wire you can see and measure the resistance between each of the cores. If you get infinite resistance measured between each with you DMM on its highest range, then it ought to be safe to power the wire up temporarily. You could turn off all the other circuits and connect this one to power (via a low value MCB) in the CU. Then use a voltage/metal detector to check each likely wire you think it might be to see if it is live.

[Tim S]

If you're going to do that, I would be inclined to supply the live via a

2 Megohm 400V min (not 240V, mains peaks at upto 350V) rated 1/4W min resistor. That will limit the fault current to 1/8mA. Assuming the insulation resistance is >=2Megohms (big assumption for old random cable) then you should get a reasonable electric field for the detector to pick up.

You could progress through smaller resistors, but I wouldn't go below

0.5Megohm.

I didn't say that would be a 100% safe thing to do, but it would be safer than whopping unlimited mains on an unknown cable. Only you're bound to find the random cable end floating around and touching something metal, or you'll stick your fingers on it when fishing around in the dim attic.

A low value MCB will still kill you and an RCD will still give a hefty bang on a dead short.

You can get (beg/borrow/steal) cable tracers (try a network installer or a phone installer) - these are a tad safer.

HTH

Tim

[John Rumm]

Good advice. Partly why I was suggesting a non contact method for detection, but thinking through the possible scenarios that is only safe when you can pretty much guarentee non contact! If in doubt, wear insulating gloves and make sure there is no one likely to be a liability in the house.

Probably the best way... not looked lately, but it may be a small set is not as expensive as it once was.

[Rob Morley]

When you say wire do you mean a cable containing two or more conductors, or a single conductor? If it's a single conductor and an old house then it could be an aerial.

[Dave Plowman (News)]

Best to think it's faulty in some way and remove it.

Old houses will often have odd ends of old wiring around. Hack it off if it bothers you, but don't even think of using it.

There are ways of tracing cable runs, but not of much use in this case.

[Mike]

You ARE allowed to touch them. (apart from the kitchen but I think this will disappear soon like the pressurised HW systems restriction)

[Lurch]

On Fri, 11 Feb 2005 13:43:28 +0000, John Rumm strung together this:

Yep, that'd be my choice.

About £35 from IIRC.

[zymurgy]

LOL!

Actually I do have Electrical Installation C&G parts 1 & 2 from way back when, so I can identify a relatively new T+E when I see it ;-)

Just a bit rusty on cable tracing, but certainly appreciate all of the answers i've had to stimulate the dormant brain cells.

Cheers,

Paul.

[zymurgy]

Couldn't see much below =A343 .. ?

Cheers

Paul.

[Lurch]

On 14 Feb 2005 10:36:35 -0800, [e-mail] strung together this:

Ah, I didn't remember correctly then, looking at the catalogue now it would appear I meant £45, not £35. Sorry.