Finding armoured cable underground

This week's problem on my son's house is that all the power has failed to a set of outbuildings. The cabling is confusing and has obviously been modified by the previous owner of the house who was, perhaps, a little eccentric in his approach to colour coding. Armoured cable leaves the house and appears in 2 boxes feeding some lights, a garage and the outbuilding area. Power seems to be leaving these boxes via armoured cables, but no power is arriving at the outbuildings.

We suspect there may be an underground junction box that fed a pond pump in the past, but digging has not located this. We have consulted an electrician who said the cost effective way would be to replace the cable. His estimate was beyond our budget.

What is the best way for us to locate the cables underground? Is a cheap metal detector likely to be the best bet or work at all? I can't find mine under all the junk in the garage, so is there a best, cheap type to look for or are they all much the same?

We have spent some time in the undergrowth poking around. I have 15 bites on my legs as a result.

Reply to
Bill
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Hire a CAT scanner and genny for the weekend, or find someone who's got one

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£46 for a weekend from Jewson Hire online
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31/cable-avoiding-tool/ £50 a week from Speedy
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Owain

Reply to
spuorgelgoog

En el artículo , snipped-for-privacy@gowanhill.com escribió:

I was wondering if the following would work (I haven't tried it). It would be cheaper than hiring equipment.

Disconnect the cable at the remote end and insulate.

Disconnect the house end (in the 2 boxes in the garage?), put a diode in the live feed so the result is 25Hz rectified mains fed into the cable to the outbuildings.

Use an AM radio at the point where the cable leaves the 2 boxes and tune it until it finds the 25Hz hum, then follow the signal until it is lost, that's where to dig.

If the signal appears all the way to the remote end, then the live wire is ok. Repeat, but with the diode in the neutral wire instead.

This is assuming there isn't a short to neutral or earth (the OP hasn't said if fuses/breakers are tripping.)

Reply to
Mike Tomlinson

Have you tested each core for continuity independently of the supply?

Mike

Reply to
Muddymike

Would one of those cheapo combined stud/cable finders work?

Reply to
GB

When set to detect live cables, my Bosch GMS120 will detect a piece of plasterboard that's in contact with a mains cable at about 18" (i.e. useless false negative) but it won't detect SWA even when touching it. In metalwork mode it detects the SWA, but not at any useful distance.

Reply to
Andy Burns

The problem is our confusion over the colours. In the 2 boxes we have red, white and blue from the armoured cable, plus some green/yellow and brown. I think we gave up when we were at the stage where we agreed that live went in on white and emerged 100 yards away on red.

I do need to do a lot more physical testing.

Reply to
Bill

I've been looking at that sort of thing, but wasn't sure how it would work with armoured cable. I assume (probably wrongly) that the method of looking for pipes rather than powered cable would have to be used.

He and I also have a track record of getting to "It has to be done this weekend" and thus inducing the worst storms since records began, so I'd like to be confident before hiring.

Reply to
Bill

We did try his stud finder, but nothing.

He also has a warbly-tone cable tracer, but it appeared to have lost all its gain at one end or the other. It said 120dB output in the instructions, but we had to hold it about 6 inches away to hear it at all.

Reply to
Bill

Since it costs **** all, you could always try dowsing ?

I have seen it done, and "work" for sewers. That's all I will say, since the "science" a lot of people peddle about it is clearly claptrap.

Reply to
Jethro_uk

Armoured cable - 3 core - is often 3 phase colours. The old 3 phase colours were red blue and yellow. Most used red - line, blue - neutral, yellow - earth. But of course you can't be certain of this. The correct colour tape or sleeves should really have been used to identify it.

Before I bought this house, the leccy meter was moved from the cellar to the top of the cellar stairs. 3 core armoured was used for this with those

3 phase colours. The leccy board used the colours as I've given.
Reply to
Dave Plowman (News)

I used a similar method to find a break in a 'hearing loop' round a lecture room. I constructed an oscillator giving out 8KHz at 250V rms, and connected it to one end of the isolated loop. I could hear the whistle from it in a pair of high-impedance ex-WD headphones, one terminal connected to a metal plate waved about 6" from the wall and the other held in my hand. The break was where the whistle stopped. In fact the break was about a 6 metre gap where decorators had ripped out what they must have thought was some ancient unused bell wire.

In your case with armoured cable, the metal armour encloses all cores, so you won't be able to detect a break, owing to the capacitance between the cores and the armour, but at least you might find where it runs.

Another thing to try is to measure the capacitance between each cable and the armour. Do this at both ends. The readings might indicate how far along the length is the break.

Reply to
Dave W

I think you can attach the genny part to the armour of the cable, which should be insulated from earth by the cable sheathing, and get a signal radiated that way (disconnect the armour from cpc at the circuit ends)

Owain

Reply to
spuorgelgoog

My own pet theory (again in a minority of one) is that the dowsing rod amplifies any deviation from standing vertically in the person holding it. Above a sewer pipe the ground has compacted since it was buried, so there's a small dip, maybe to small to notice. As you walk over it your body moves away from vertical. Just a thought.

Reply to
GB

Much easier to find it on a cable drum in an electrical thingies supplier.

Reply to
polygonum

Stick 240V on the armour. Make sure the other end is genuinely isolated.

Then try the cable finder. Of course it is possible that all the earth above will shield the cable.

Reply to
Fredxxx

Not sure that is going to work. Most of this sort of cable tends to be screened by the armour which is earthed.

You really needs one of those ground penetrating radars the archaeologists use. Brian

Reply to
Brian Gaff

Yes, I realised that later. One solution might be to feed the rectified signal down the armour, with the remote end well isolated of course. Dodgy though.

En el artículo , Brian Gaff escribió:

Reply to
Mike Tomlinson

In message , Mike Tomlinson writes

Presumably the armouring will have been connected to whatever junction box was used at the original load end by the securing gland.

If such a junction box were metal, it should give a good earth connection..... I think you need some current flowing at 50HZ to energize a cable finder.

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Reply to
Tim Lamb

+1. Forget using stud finders, radios, etc.
Reply to
newshound

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