Light circuit tripping RCD

[Darkness]

I have a light circuit which is about 30-40m long with two lights near the end of the run.

As soon as I turn the light on via the Gewiss weatherproof switch, the RCD trips.

I thought it was one of the outside lights at fault but it seems its not. I disconnected the circuit at the first light leaving just a circuit with nothing at the end of the run but still the RCD trips.

I've spent the last hour or two inspecting the cable from switch to end of run and it seems fine.

I've fixed many problems but this one has me stumped.

I'd get my sparky friend but he's on the other side of the globe for the next two months.

Anyone have any suggestions? I'm tempted to rip the whole cable out and wire it again in conduit.

 

[JohnD]

I am assuming that the lights used to work correctly, and have recently gone wrong (i.e. they were not installed faulty).

If it's outside lights, the most common cause is rainwater getting in. This can be a light fitting, a switch, or even the cable if its sheath has beeen damaged.

If the cable enters the house without a drip loop then water can run down it into a switch or connector inside the house.

You can try disconnecting at the switch next to see if the fault is upastream or downstream of the switch.

Weatherproof switches are, if they have been properly sealed.

 

[RobFurn]

sounds like it could be the switch itself, try testing it with a continuity meter, or tempereroly replacing it to see if it trips out.
Also try testing the RCD itself, if you havent got an RCD tester (which I assume you dont) then perhaps you could place it on another cirucit tempereroly, if it trips then you know its the RCD thats faulty.

 

[Darkness]

The lights have worked without fault for the past 6-7 years.

From the switch is about two metres of conduit which takes the cable up a steel girder to the roof rafter. From there it goes along the timber for nearly 30m to the first outside floodlight.

From switch to first light is a jointless run. So along that run there is a fault.

I don't see any sign of damp etc in the switch but even if there was, I don't see how the live would come in contact with the earth or neutral especially when I have the n+e out of the switch.

I'm gonna disconnect the switch and attach a length of cable with a plug to the t&e that goes to the light. I'll use an RCD adapter on it also so its only circuit that trips and not the whole outbuilding electrics.

If it still trips, is there a possibility that one of the cores within the t&e is damaged perhaps melted? Maybe the mcb is faulty and is not tripping? hence the core started to melt crossing the wires?

I had the cu changed 4 months ago from a cartridge cu.

 

[JohnD]

If this is in an outbuilding, and the cable is T&E not SWA, there is a chance that rodents have gnawed it, or that it has been chafing on something and rubbed the insulation.

 

[Darkness]

Yes this is a large outbuilding and my thoughts were the rodents at fault but I checked the cable well for marks that could have been chewed but found nothing.

Yes its t&e and its well tacked to the timber.

If the cable is damaged from the outer shealth, its a pinhole.

This is a real scratch your head case.

 

[joe-90]

If you've disconnected the lamps and there is just cable. Why not disconnect the end at the power source and check for a short circuit using a multi-meter set on the resistance scale? Each core should be totally open circuit to the other. If you have a resistance reading then you either have a cable or switch problem.

 

[ltheb]

Have you checked the connections and final links to the lamp holders? It is very common for the conductor insulation to become brittle and degraded by the heat of the lamps (you say 6-7 years old)

 

[Darkness]

Like I said previously, I had the lights disconnected leaving just the bare cores connected to nothing.

Still trips.

I'll do a resistance test tomorrow which should hopefully tell a better story.

 

[securespark]

40m? What's the load and cable size? What about VD?

What installation is this? Is it yours?

 

[Darkness]

The load on this cable is 600w. 1x500w floodlight and 1x100w wall mounted light.

Cable size is 1.5m² and its mine.

 

[Darkness]

This morning I checked the resistance of the cable and I didn't get any resistance. From the test, the cable seem fine.

Leaving the end of the cable open with a plug+rcd adapter on the switch end, the rcd trips when power is turned on.

Now as a test, I disconnected the earth at the plug side and when power is turned on, the rcd doesn't trip. Goes fine the way it should be but when the earth is connected, it trips.

I'm stumped even more now especially the resistance test proved ok.

Any more suggestions?

 

[JohnD]

Maybe you tested resistance with a multimer. An electrician's insulation tester feeds 500v down the wire, this will expose insulation that is breaking down.

Might be heat, excessive flexing, or a nail through it.

 

[Spark123]

Now as a test, I disconnected the earth at the plug side and when power is turned on, the rcd doesn't trip.

Please do not do this, not ever. It is dangerous. Items are earthed for a reason, disconnecting the earth only masks the problem.
As has been said you need to use the correct instrument to test the insulation resistance.

 

[Darkness]

I knew the danger before starting but it answered a question for me.

I used a mm and I had pondered its limitions in tests such as this with regards to the voltage sent down not being strong enough.

I might be able to get hold of a proper insulation tester but if not I'll probably buy one as it could come in handy some time.