How to check cable

Was at a mates and he has an outdoor ornamental lamp post, when it is switched on the bulb lights for a few seconds and then goes out by tripping the RDC at the main board. We checked the lightbulb socket that looks ok, we checked the socket where the feed comes from (no loose wires) and thats ok. We checked most of the armoured cable and that looks intact a few feet of the cable is under concrete so left that for the moment, the cable is buried mainly in a gravel drive. When the lightbulb is removed it still trips the RDC. The lamp is fed off the ground floor socket ring.
What would be the way forward for identifying the problem, he will have a multi meter next week.
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On Sunday, 30 August 2015 16:56:20 UTC+1, ss wrote:

Look for places damp could get in. You really need an insulation tester/megger.
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On 30/08/2015 16:56, ss wrote:

Do you have armoured cable going into a plug?
I would work back from the lamp, isolating each piece of cable, and see if it tripped the RCD. I presume you have a wet junction box somewhere.
You do mean RCD and not MCB?
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Socket? Socket ring?
What size cable?
Armoured cable to a 13A plug perhaps?
I know of no armoured cable that I could correctly terminate in a 13A plug.
Any armoured cable I have used would clearly indicate where an earth leakage problem was lurking, without the aid of a meter.
Concrete should be o/k, but gravel with a vehicle or two will make a mess of the sheath.
AB
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On 30/08/2015 17:50, Archibald Tarquin Blenkinsopp wrote:

The armoured cable is wired directly in to a fused juction box with switch, I say fused junction box but its like a plug socket without the square holes for a plug (dont know the correct name) the socket is mounted exactly where the armoured cable enters the house. the cable in the driveway we have checked and it looks intact with no damage, the length of cable that is outdoors appears to be one piece. the cable from memory maybe 1.5 to 2 cms in diameter. Where the cable is covered in the drive is at the edge and unlikely to have caused damage and certainly there is no obvious visual damage.
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On 30/08/2015 18:29, ss wrote:

Do you mean something like this? http://www.screwfix.com/p/mk-13a-dp-switched-fused-connection-unit-with-flex-outlet-white/13479
What happens at the other end of the armoured cable. Does it go directly to the light fitting?
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On 30/08/2015 18:43, Fredxxx wrote:

Yes thats the type of box.
Where the cable leaves the driveway and to the base of the lampost is concrete so at this stage trying not to have to dig that up. We thought maybe a juction box at the base of the cable but the sections are not easy to disassemble.
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On 30/08/2015 18:55, ss wrote:

Are the cable types the same at the base of the lamppost and the fused outlet?
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On 30/08/2015 20:19, Fredxxx wrote:

Where we have tracked the cable it is the same cable near the base of the lamp and where it enters the exterior wall to the wall socket.
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On 30/08/2015 21:55, ss wrote:

Disconnect the wires from the lamp and see if the RCD operates.
I have known plastic to become conductive, where a carbonised path exists.
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On Sunday, 30 August 2015 16:56:20 UTC+1, ss wrote:

disconnect bits to narrow down where the fault it. A megger wil find he fault, a multimeter probably will.
NT
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ss wrote:

Do you mean the RCD trips or is it an MCB that trips?
If it is an RCD can't think of a mechanism that would take a few seconds to trip. It should be instantaneous or not trip at all.
I did have a length of T&E that had got water in it and that would trip an MCB after a delay. I presumed that the water became ionised by the voltage and draw more current and so on until the MCB tripped. I did not bother to investigate but just replaced the cable.
If you disconnect everything at the lamp end to leave three bare wires in fresh air and the RCD or MCB trips then it is damaged cable and should be replaced. If you can find the damaged area, a repair might be possible by drying it out and resealing with a water proof joint but it could still be a nagging problem. These days the bulb could be replaced with a solar powered charger with a battery and a LED lamp?
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On 30/08/2015 21:33, Bob Minchin wrote:

OK we did try with the bulb out and that tripped, mate will have to take a pane of glass out (siliconed in) to take the holder off, but will pass on the info to him.
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On Sunday, 30 August 2015 21:57:12 UTC+1, ss wrote:

If its the cable, which is likely, don't replace it with pyro, it sucks in water vapour. If you're not keen to replace the cable or light fitting, you could always run the thing as a 12v light.
NT
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Disconnect the SWA at the base of the lamp from the cable that goes up to the lamp holder.
It is doubtful the SWA connects directly to the actual lampholder.
--
Adam


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On 31/08/2015 07:23, ARW wrote:

Thanks guys thats enough info now to allow a bit more checking.
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Pretty obvious that water has encroached into the cable due to the outer being breached. New cable run needed I'd suggest, to attempt to splice in a new piece where the leak is opens it up to the connections being a weak link and it all happening again. Did he not use d some form of ducting for the cable? Brian
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On 31/08/2015 10:02, Brian-Gaff wrote:

It would appear the original owners of the house never used any ducting, the cable is 6 inches down under gravel drive, although so far what has been exposed appears to be ok. The cable is on the edge of the drive so really should not have got damaged with traffic. He wants to check any other possibilities before digging the concrete up.
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It's pretty normal to not use ducting with SWA.
Anyway, damp within a junction box or in the lamp seems more likely
--
Chris French


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