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?

[cjs]

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

[ColinM50]

When you say "buried", do you mean it's in a conduit or actually buried? If in a conduit, easiest is to connect a new cable to one end and pull it through thus replacing the cable. If it's really just buried in the soil then you've got a tough job on of digging the old one out and burying a new one but this time put it in a conduit.

Re the actual problem, I've had it a couple of time, rats like to eat the insulation. Kills them when they short circuit the cable, but you end up with a dead rat and short circuited cable. Buddy used to have them regularly do this under his lounge floor. Right bugger to change the cable once a month or so.

[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.

[Ganglandboss]

Ozzie Osmond said:

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?

What sort of cable is it?

How deep is it?

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

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

What is the exact reading on your meter?

How many fittings are there?

What type of luminaire?

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

How are they controlled?

[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.

[Ganglandboss]

Can you post any photographs of the cable? From what you have just posted, it sounds like the cable is not of a type required by the wiring regs and it is buried too shallow in the ground.

The cable should have a concentric earth, be suitably protected and buried as below:



15,000 ohm is very low and with an insulation resistance tester, or 'Megger', which is an ohmmeter which gives out a high voltage (and is great fun when you are an apprentice in the college workshop) you may find it shows a dead short.

Unfortunately it sounds like a lot of digging and a new steel wire armoured cable is required.

To do it properly, it will have to be notified to building control by either having them inspect it (and paying BC fees) or getting an approved domestic installer who can self-certify his work to replace it.

[Ozzie Osmond]

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

[jason s4]

There is no such thing as a cable with a concentric earth.

Any cable can be used, provided it is installed correctly according to its environment. Normally an armoured cable would be used.

There are no regs about depth, although around 500mm is the norm which is just a bit deeper than spade/fork depth.

You need to do an insulation test between all conductors and not just a continuity test as the latter would/could give inaccurate results.

I suggest you get a competent electrician in, to at least find the fault.

Also, i have never, even seen a cable buried as above.

You will find most utilities companies should, but very few do.

[Ganglandboss]

jason s4 said:

There is no such thing as a cable with a concentric earth.

Any cable can be used, provided it is installed correctly according to its environment. Normally an armoured cable would be used.

There are no regs about depth, although around 500mm is the norm which is just a bit deeper than spade/fork depth.

You need to do an insulation test between all conductors and not just a continuity test as the latter would/could give inaccurate results.

I suggest you get a competent electrician in, to at least find the fault.

Also, i have never, even seen a cable buried as above.

You will find most utilities companies should, but very few do.

A steel wire armoured cable has a concentric earth. The protective steel wires surround the cable, therefore if you put a spade into it, you will hit an earthed cable before a live conductor. The steel wires may or may not be used as the protective conductor but must still be earthed irrespective of whether you install a separate protective conductor or use a 3 core cable.

BS 7671:2008 recommends a minimum depth of 500mm for underground cables.

From the OP's description, he tested across conductors - not a continuity test, although you are correct in saying an insulation resistance tester is required. If a normal multimeter detects a short, a Megger certainly will - the risk with using a normal multi-meter is that it may not detect a fault a Megger would.

There are different standards for utilities installations and private electrical installations.

Edited by Ganglandboss on Friday 8th July 23:12

[x type]

you need to look around the area you live in ,look for local electricity company type of guys (not bt)guys working down a hole on the underground cables

depends on who you ask if they'll help or not !!

Ask them VERY nicely if they can help in anyway

Explain problem ,if it's definetly the cable gone in the ground ,they (should) have a gizzmo which can test the cable and IF it's faulty ,tell them near as damn it how many metres or feet it is from where they are testing

The guys I work with have these and they are damn good at finding the problem

[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!

[jason s4]

Ganglandboss said:

A steel wire armoured cable has a concentric earth.

Err... no it doesnt.

The cable has steel wire to protect the conductors inside.

Howeveex, yes this must be earth.

Also, the only time a fault will occur is if the earthed armour of the cable comes into contact with the line or neutral, which can only happen if someone causes a high enough impact to the cable.

This is split concentric cable, which does not have mechanical protection, although the CPC is incorporated.

http://www.batt.co.uk/products/view/610/Split-Conc...

[dickymint]

I would forget about it - bite the bullet and start again. Maybe you can use a different route above ground to save digging? Use a 12volt system? Or solar lights?

[Ganglandboss]

jason s4 said:

Ganglandboss said:

A steel wire armoured cable has a concentric earth.

Err... no it doesnt.

The cable has steel wire to protect the conductors inside.

Howeveex, yes this must be earth.

Also, the only time a fault will occur is if the earthed armour of the cable comes into contact with the line or neutral, which can only happen if someone causes a high enough impact to the cable.

This is split concentric cable, which does not have mechanical protection, although the CPC is incorporated.

http://www.batt.co.uk/products/view/610/Split-Conc...

The steel wires' primary function is as a mechanical protection. They may be used as a circuit protective conductor; a separate CPC may be needed in some installations but a garden lighting will not be one of them. We both agree the steel wires, which are concentric to the inner core, must be earthed so why are you arguing the toss that they are not a concentric earth?

The fact still remains that if the OP has a 3 core non-armoured cable installed 300mm deep, the best course of action is to dig it up and install a SWA cable in accordance with the building regs and BS 7671:2008.

[jason s4]

Ganglandboss said:

The steel wires' primary function is as a mechanical protection. They may be used as a circuit protective conductor; a separate CPC may be needed in some installations but a garden lighting will not be one of them. We both agree the steel wires, which are concentric to the inner core, must be earthed so why are you arguing the toss that they are not a concentric earth?

Im not really arguing the toss, but the fact is, as you have said, the armour primary function is for mechanical protection and was not designed as a cpc, regardless of what 7671 states.

I will never trust the armour as sole means of earthing, as i have seen the results of this.

Ganglandboss said:

The fact still remains that if the OP has a 3 core non-armoured cable installed 300mm deep, the best course of action is to dig it up and install a SWA cable in accordance with the building regs and BS 7671:2008.

That i certainly agree with.

[freecar]

OP, can I ask what you are planning n doing when you find where the damage is?

I ask because I can't see any reason to find out where it is damaged as the entire length needs to be replaced.

Once I'd isolated (that the fault was) the cable I'd have started digging it up to replace the whole length.