Finding armoured cable underground

[Bill]

In message , Tim Lamb writes

Just some notes on this and earlier comments...

At least one of the boxes will need replacing, and there are visible problems with one of the securing glands. I believe all the armouring is still earthed though. The main armoured cables are definitely coded red, blue, white and one box has 2 whites earthed to the box. The third white in the same box is tested live. There is a bit of a family dispute (long distance at the moment) about what to do next. I think we need to disconnect and measure until we understand exactly what each individual wire does and which direction the power goes. Son thinks we just have to find a way to trace the cable until we find the underground junction box that he thinks must be there.

Anyway, I have invested a small amount in a pre-loved metal detector on ebay and will practise with it before he arrives back at the weekend. I don't really expect it to work. It may have to detect cable under crazy paving.

Running a new armoured cable on the surface tucked in at the base of walls would be possible, but the amount of gardening work involved is daunting and a couple of paths would have to be crossed.

[Capitol]

I think I'd try measuring the cable capacity and resistance with a DVM. Mine works down to about 10pF. By measuring each cable individually, a break may show as reduced capacity, then measuring all cables together relative to the shield will provide another guide. Obviously the earth may be connected to the shield somewhere, so maybe that's the point to go for a resistance measurement for the return path. Looks like a very long pair of test leads are going to be required!

[David]

Starting with the assumption that you can check the sheathing for electrical continuity - probably by fixing a wire at one end and leading it back to the other end (after making sure everything else at the ends is disconnected) - and there seems to be a break in the sheathing.

Possible partly non-electrical method.

Dig down at the start and end of the unknown run and extend the trench about a foot along.

This should give you an idea of the depth of the cable and the direction of the run.

If the two directions don't match, run a line along each direction and suspect that the junction could be around where they meet. This would be a good place to dig.

You might be able to firm up the precise cable run by using a long thin metal probe - you can test your probing skills just past the trench exposing your exposed bits of cable

If they are pointing in the same direction, I'm not sure how you could test continuity part way along without cutting into the plastic covering to get at the metal shielding.

Cheers

Dave R