Outside Light tripping MCB

[diynoob]

I found a T&E sticking out of an external wall wrapped in electrical tape. I got no reading from it, but after flicking a light switch on the wall inside it read 240V. I assumed it must have been an outside light at some point so I installed an IP54 outside light. It's not a PIR light, just a simple one controlled by the switch and is permanently turned off.

It worked fine for a few weeks but then it rained recently and tripped the MCB. The RCD did not trip, just one MCB which is only connected to the outside light an one other room.

I checked the light a there's no water in it, it's water tight. It's been rainy and damp for 2 days now and the MCB trips every time I reset it. In the end I removed the outside light, taped up the cable and tucked it under the eaves. The MCB is fine now.

Any ideas what's going on? Can MCB's trip purely from humidity even if there's no actual water ingress?

 

[diynoob]

PS The light is an 8W LED. I bought 2 of them so have put the tripping one in the other location now just to prove it's not the light causing it.

 

[EFLImpudence]

It worked fine for a few weeks

I assume that means it turned on and off with the switch.

but then it rained recently and tripped the MCB.

I assume that means one time you went to turn it on the MCB tripped.

The RCD did not trip,

Assuming the RCD works as it should, that must mean there is a Line to Neutral fault somewhere.
This is unlikely to be because of water.

I checked the light a there's no water in it, it's water tight. It's been rainy and damp for 2 days now and the MCB trips every time I reset it.

Line and Neutral contact somewhere.

In the end I removed the outside light, taped up the cable and tucked it under the eaves. The MCB is fine now.

Try wiggling the cable and see if it trips - switch on, of course.

Can MCB's trip purely from humidity even if there's no actual water ingress?

No.

have put the tripping one in the other location now just to prove it's not the light causing it.

That worked, did it? Not the light.

Can only deduce that the cable is faulty and shorting between L and N.

 

[diynoob]

I assume that means it turned on and off with the switch.

Yes.

I assume that means one time you went to turn it on the MCB tripped.

No. Once the wet weather started, the MCB tripped. I have never used the light other than to occasionally test it works. The light was off the entire 48 hour period that the MCB was tripping. The MCB would then not reset until I disconnected the light and taped up the cable.

Can only deduce that the cable is faulty and shorting between L and N.

I think you must be right, the cable is faulty which is why it was taped up in the first place.

 

[EFLImpudence]

No. Once the wet weather started, the MCB tripped. I have never used the light other than to occasionally test it works. The light was off the entire 48 hour period that the MCB was tripping. The MCB would then not reset until I disconnected the light and taped up the cable.

Oh, very strange.

If it tripped when the switch was off, assuming (I'm doing a lot of assuming) it is all wired correctly that would indicate that the fault is elsewhere but that would not explain it becoming alright when you removed the light. A puzzle.

 

[diynoob]

It's raining again and the MCB has tripped. This is with the light fitting removed and the cable in a connection block and taped up.

 

[ban-all-sheds]

Can we confirm that it is an MCB which is tripping, not an RCBO?

 

[diynoob]

It's an MCB, see attachment. However, am I right in saying that the 4 circuits on the left are not protected by the RCD which explains why the RCD is not tripping?

 

[EFLImpudence]

It's raining again and the MCB has tripped. This is with the light fitting removed and the cable in a connection block and taped up.

Then, I would think the fault is elsewhere and the light you fitted is a coincidence.

It's an MCB, see attachment. However, am I right in saying that the 4 circuits on the left are not protected by the RCD

You are correct.

which explains why the RCD is not tripping?

Not necessarily. They react to different faults.

Now we know the light was not covered by an RCD, it could also be an L to E fault or still L to N.

 

[diynoob]

To recap:

The MCB is tripping with nothing turned on on that circuit.
It only feeds the lights to one room, a cupboard and the dodgy cable outside.
It only happens when it rains.
It started 2 weeks after I installed an outside light as I didn't like the look of the taped up cable poking out of the wall.
I have lived here for about 6 months without the issue. It does seem to be related to the combination of something I did + rain. Although it could just be coincidence.

One thing I've remembered. When I first moved in and turned the cupboard light on, this same MCB tripped. There were 3 100W bulbs in the cupbaord light! I replaced them with enery savers and never had any other problem until now.

 

[EFLImpudence]

I can think of no reason to explain what you say has happened.

To rule it out absolutely - you could disconnect the outside cable from the switch - all three wires.

 

[diynoob]

I can't. All I can access is the outside cable and the light switch. The outside cable has SL, N and E. The switch has L, SL and E. If I terminate all 3 wires in the switch then the outside cable is no longer live but the N and E will still be connected back to the consumer unit won't they?

The outside cable pokes into the house into what remains of the loft, presumably to a junction box. The original loft has been converted so all that remains is a 2 foot high space. The cable is 10m away from the loft hatch. The loft is not boarded and has insulation in it. A normal sized person cannot crawl in it.

 

[EFLImpudence]

I can't. All I can access is the outside cable and the light switch. The outside cable has SL, N and E. The switch has L, SL and E. If I terminate all 3 wires in the switch then the outside cable is no longer live but the N and E will still be connected back to the consumer unit won't they?

If wired that way, then yes.

The outside cable pokes into the house into what remains of the loft, presumably to a junction box. The original loft has been converted so all that remains is a 2 foot high space. The cable is 10m away from the loft hatch. The loft is not boarded and has insulation in it. A normal sized person cannot crawl in it.

Ok, any mice chewing the cable? Only come in when it rains?
Clutching at straws now.

 

[diynoob]

It's the countryside so we do have mice, not sure if they are in the loft though. Can't see much if I shine a torch down there as it's too far. Note that the house is L shaped with the consumer unit at one end and the outside cable at the other. So it's at least 25m of cable, probably a lot more. Not sure if cable length is relevant?

The other reason I keep going back to the outside cable, not just because I recently fiddled with it, but the state I found it in suggests I have unearthed an existing problem. The outside cable was wrapped in a ton of electrical tape and the light switch wires were in a connection block. If the previous owner had a broken outside light they could have just left it there or replaced it. I suspect they had this tripping issue.

 

[Blueflash]

I would just check the cable is not buried in the mortar on the external wall and it has been drilled into at some point which when the wall gets wet is causing the issue. Or when the light was being fixed to the wall ? Done that, re plastered and painted internal wall, expensive wall light

Dave