Outdoor electrical fault - how to find?

[Ozzie Osmond]

Lights on our front gate failed last night, fuse blown and breaker (not earth leakage) tripped. Replaced fuse, all working OK, but a few minutes later the same thing happened again.

We've had a lot of rain but there's nothing particularly unusual about that. No moisture in the light fittings themselves. The cable is buried outdoors do it's not a simple case of start looking for a problem one end and work towards the other.

Today I removed the bulbs and disconnected the circuit from the consumer unit. Testing the connectivity with an ohm-meter it's not completely "open" circuit. There's a resistance (quite high) measurable between live/neutral and also between each conductor and earth.

So, any PH suggestions of how to track down where the fault is?

[Ozzie Osmond]

Mods, thanks for moving to right place!

cjs said:

Measure the resistance from each end of the cable, the difference in readings could give you a guide to where the short circuit is??

Good thought, I'll give it a go later when the rain stops.

[Ozzie Osmond]

ColinM50 said:

If it's really just buried in the soil then you've got a tough job...

Yes, that's why I'd like to narrow things down a bit if possible.

ColinM50 said:

I've had it a couple of time, rats like to eat the insulation.

This is the kind of thing I've come across before and it's possible there's been animal activity somewhere. When I first removed the bulbs there was an insect nest in between the prongs of the socket but unfortunately clearing that out hasn't fixed things.

[Ozzie Osmond]

Ganglandboss said:

What sort of cable is it?

Old (30 years?) black flexible cable. Hard, thin plastic sheath around a thicker black rubbery cover and then three solid core conductors inside, each coloured red/black/green.

Ganglandboss said:

How deep is it?

Typically about a foot.

Ganglandboss said:

Who installed it and are you confident it was installed correctly?

Not known. It's worked trouble-free for many, many years.

Ganglandboss said:

Have you disconnected the cable from the light fitting completely (if not the resistance reading may be from some of the control gear)?

All gear disconnected and it's just the bare cable at both ends.

Ganglandboss said:

What is the exact reading on your meter?

This I find confusing. On my old needle voltmeter I seem to read 15,000 ohm. But on my digital tester the readings won't sit still. Depending which scale I use the digits read differently. For instance on the 20,000 setting it reads around 15 but on the 200,000 setting I read around 30. Don't understand at all.

Ganglandboss said:

How many fittings are there?

Two

Ganglandboss said:

What type of luminaire?

Ordinary lamp sockets.

Ganglandboss said:

How are the lights positioned (a simple plan would be useful, showing positions of luminaires and cable runs.

One each side of gate so there must be a junction somewhere. Position currently unknown but I suspect I need to find it.

Ganglandboss said:

How are they controlled?

I've removed the switching timer (indoor) and can confirm the fault remains in the cable.

[Ozzie Osmond]

Thanks - yes, I guess this is an old-fashioned installation. Definitely no concentric earth etc.

[Ozzie Osmond]

Thanks for your input guys.

I've now eliminated all fittings, control gear and junctions and just end up with a straight cable buried underground with an insulation problem somewhere along its length. Sounds like a weekend of careful digging with the spade and see what turns up....

Bummer!