6A RCD random operation. No obvious root cause.

[Simples]

Wylex 6A MCB/RCD fitted as part of a Part P installation. Working fine for two years. The upstairs lighting circuit is via a radial layout, not loop-in.
For past four days the RCD has been tripping out at random times during night hours. Frequently after all lights on the circuit have been turned off.
There have been no changes to the electrical installation since signed off by an electrical contactor.
Circuit consists of Part P low voltage down lighter/extractor fan installation in bath room. Part P Shower light/extractor fan in shower room. Halogen ceiling light on upstairs landing. Three bedroom pendant fittings with Low Energy bulbs. There are no external lights hanging off of the circuit. Extractor fans are protected by DP isolation switches.
I have isolated the extractor fans, RCD still trips at random intervals between 11pm and 5am.
I have operated lights individually & simultaneously during the day, no RCD operation.
I removed one of the (8W) low energy bulbs last night and the RCD appeared to stabilise. I was preparing to swap out the ceiling rose today, but once again the RCD had tripped out while everyone was asleep.

I'm confused.UK!

Anyone any ideas please?

 

[echoes]

Can you confirm that it is the RCB that is tripping, and not the MCB?

You definitely have a potentially dangerous problem here.

If RCD is tripping with nothing switched on, it could be a pinched wire in a switch box or light fitting that has slowly got worse and is now intermittently shorting to earth. Or a cable that was fitted without a grommet has been damaged. Thoroughly inspect every fitting and accessory on the circuit - the cause may be visually obvious.

 

[Simples]

Thanks.

I'm thinking along the same lines. Plus, as the installation is Radial circuit, all hanging of a 1.5mm twin + earth cable from the CU, there is a distinct possibility that the insulation has been comprimised by the recent hot weather. But why then does the RCD only start tripping out between 22:00 and 05:00.? The loft is calling down during this period so the temperature co-efficient of the cable is improving at this time.
My son & daughters bedrooms are on the shady side of the house, so they turn their lights on at approximately 20:00.

It's got me right baffled it has!

 

[echoes]

Since it's happening at night, it might be some small, furry animals having a midnight feast on you cables. Any signs of mouse poo or knawed cables?

 

[Simples]

Haven't been in the loft to check - but know I must.

No sound of rodent activity, the walls of the house are solid (not cavity) and the roof was fully replaced and lined two years ago. So I dont recon on Micky mouse, but i could be wrong.

 

[echoes]

there is a distinct possibility that the insulation has been comprimised by the recent hot weather

Unlikely - the cable insulation should be sound for any temps likely in your loft, but you could test the insulation resistance on that circuit as you would have done after you installed it and compare the readings.

 

[echoes]

I'm slightly confused by exactly what is tripping. Is it the 6A MCB, an RCD (if so, what other circuits does the RCD protect?) or a 6A RCBO?

 

[Simples]

I was trained on IEEE 11th edition in 1977! Found there was more cash in Telecoms than sparking, so didnt keep my trade certs up to date. Had the builders in two years ago. The electricain did the new bathroom electrics for me to Part p specs. He swapped the existing MCB on the switched side of my CU for what appears to be a 6A MCB/RCD.
(There was no room on the existing RCD protected bus bar for another 6A breaker.) I could be wrong. Perhaps the elecricain decided to change the existing 6A breaker just for the heck of it, and ignore the breaker alongside that has nothing connected to it.
Does Part p call for the lights to be protected by a RCD?

 

[ricicle]

I was trained on IEEE 11th edition in 1977!

11th edition = 1939 - 1950

 

[echoes]

I see!

what appears to be a 6A MCB/RCD

Is it one of these?

Does Part p call for the lights to be protected by a RCD?

Part P itself doesn't specify much beyond 'thou shalt be competent and provide the paperwork' but the wiring regs/BS7671 specify what should be RCD protected, and without going into all the detail, the short answer is yes. So your spark will have correctly fitted an RCBO.

Now your RCBO is indicating a problem with that circuit which is most likely mechanical damage to the cables/bad connection since it happens when your lights are off. The other cause may be a faulty RCBO, but better not to shoot the messenger until you've investigated what he's trying to tell you.

 

[Simples]

Thats the chappie! I 100% agree with your comments. I was also trained to think safety 1st!
I'm not looking for short cuts, they generally turnout to be costly de-tours. just sharing my pain in the hope /certainty that there are more knowledgable folk around who can help shed light (No pun intended) on this quirky situation.
The trouble is that without costly test equipment, I am limited to best guess and inspection.
At the moment, my best guess is the cable between the ceiling rose and the pendant fitting in my daughter's room has degraded. Or maybe there is a terminal screw loose in the rose. That is my starting point....
If that proves not to be the case, I'll check the light switch and junction box in the loft. Pay particular attention to cables behind switch plates in case there is a nick in the insulation leading to low level earth leakage. Then the wiring back to my sons room, which is the newest in the house. there is also the possibility that the halogen light on the landing has cooked the insulation on the cable feeding it, despite the heat shield. Then of course there is a case for suspecting the 12Vdc transformer feeding the low voltage down lighters in the bathroom may be failing due to the heat in the loft.

Pass the Hedex nurse!

 

[echoes]

If it happens when the lights are OFF, first suspects would be the permanent connections in the circuit where the cpc is present (roses, switches, JBs)

I think your plan for inspection is sound, and there's no guarantee that test equipment will pinpoint the exact fault, especially one of an intermittent nature, though it may help of course.

Best of luck with it - hope you get it sorted quickly before anything bad happens, and I'd be interested to know what it turned out to be!

 

[pjcomp]

The time frame suggests the period when night storage/Economy 7 is in operation - do you have night storage in the property, in which case it could be your lighting circuit in too-close proximity to a night storage circuit. Yes. i know no one has night storage heaters on at this time of year, but an immersion heater on the night storage consumer unit? That would also narrow down the search. Of course, if you haven't got night storage, forget I said anything .....


PJ

 

[sparkwright]

Check loft for damaged cables.

Check where switch cables pass through conduit drops, can get crushed when people walk about in the loft.

 

[ColJack]

last 4 nights, I don't know about in kent, but it's been windy and raining here at night ( on and off, heavy showers ), so it might be a leak in the roof possibly? something dried out and shrunk with the recent warm dry weather?
I was originally going to suggest condensation in the fan but you say you isolated it and it still tripped..