Why would an RCD trip 3-4 minutes after a light has been turned off?

[Davew182]

No lighting has been changed in the house for at least 12 months. Last night a small pop similar to that of a small firework in the distance came from one of the rooms in my house and the rcd for the front of the house ceiling lights tripped. This happened within a few minutes of me turning off a light that had been on for 30 minutes ish. Being my sons room and not wanting to wake him with more pops I waited till this morning when I flipped the rcd back up and it immediately tripped again. I turned the ceiling light on in the room the pop came from and then flicked the rcd back up and all seemed fine, light was on, nothing tripped, turned the light off, on and off again, absolutely fine as were all other ceiling lights on that circuit. Tonight the light again had been on for 30 ish minutes while putting my son to bed, 3 or so minutes after turning the light off and another pop and the RCD tripped again. My wife was in the room this time and thinks she saw a small flash by the light but can't be sure and I can't be in there to see or hear it as it's unpredictable when it pops and trips given this morning it was fine.

It's a standard B&Q bought LED light in the ceiling, 5 20w small bulbs, sits protruding on the room side of the ceiling, there's nothing except wires on the loft side.

I'm going to go up and check tomorrow that no wires are touching, no water etc near the wiring but wondered if anyone has any other ideas why the rcd would be tripping 2,3 or 4 minutes after turning the light off? Nothing else is being turned on in that time and we've had no problems with any of the lights since the last one went in 12 ish months ago.

 

[westward10]

The fact you are hearing a 'pop' suggests a live to earth fault which at the moment has not become a permanent connection but certain conditions are making it happen.

 

[Davew182]

Any thoughts on what type of conditions could do this? and why turning the light switch on before flicking the RCD back up this morning would have stopped it tripping? the only other thing of note from this morning was that the light wasn't left on as long as in the evening. This morning it was left on a minute or two compared with 30 minutes in the evening

 

[westward10]

Difficult to say and the switching position may be irrelevant. It could be someone walking about causing a disturbance and the only way to find out is by testing which may well stress the fault to give some indication.

 

[pc1966]

It could also be related to the heating and cooling of the light in operation. Certainly it sounds like a marginal fault but one that might just show up on insulation testing.

You might find something on inspection, a damaged cable from past wrangling of lights, poor stripping technique, trapped by the light's structure, etc.

Of course take precautions that the power is really off and that you have walking boards in the loft to avoid the damaging and potentially serious injury of slipping between beams (been there, done that, don't want to risk it again!).

 

[pc1966]

Just to add - and I'm sure you don't want to find this - but it might be a dead mouse/rat that has chewed a cable. In that case take great care not to touch it in case of infection, use a plastic bag as an additional glove and then double-bag it for disposal.

 

[jedisparks]

Sounds like a slight intermittent touching of a live part/wire/connection with earth. One of those where the light needs to come down and fully check the quality of the connections and fixings to ensure a cable hasn't taken the point of a screw or wires touching that shouldn't be. Obviously, ensuring first that it is safe to work on and other precautions stated above by others.

 

[brianmoooore]

I wonder if the polarity is correct? If the switch for this light was in the neutral, then the switched 'live' of the T&E to the switch would be live (through the lamp) when the switch is off, but at neutral potential when the switch is on.
Damaged cable, with leak from switched 'live' to cpc, enough to trip RCD with switch off, but effectively becomes a slight N to E fault, not tripping RCD, with the switch on.
Possibility, or should I not post shortly after a large glass of Merlot?

 

[littlespark]

Are they replaceable LED lamps in the fitting? It may be just one of those on the way out.

Take one out each night and see when the fault reoccurs.
You don’t mention how old the wiring is.
If it’s really old, the insulation may be breaking down.

Although you have rcd on the lighting suggests the consumer unit is fairly new.. Does the rcd cover more than just the lighting?