Likely causes for RCD trip.

[shedbang]

So... The new CU was installed 3 days ago. All is well, but tidying etc required. Sparky back today. Finishes tidying and demonstrates how CU works to wife. Whereupon one circuit starts tripping the new RCD (previously there were no RCD's).

It's Friday night. The boiler is on that circuit. It's the coldest night of the year. The house is unoccupied... The Sparky clearly needs some sort of beer and my wife needs to get back to our house and get my tea on and make herself look nice for my return (as if).

The next bit was great. They ring me to ask what it might be!! I stated gently that I didn't know but suggested it could be something wrong with the electrics.

So to get us through the weekend, we remove the RCD and put a main switch back in, which is fine and the Sparky will be back next week.

In laymans terms what would cause this kind of thing and how is he going to find it? Why would it be fine for 3 days and then go spanko?

 

[Taylortwocities]

Have you used something today that you havent used since the CU was changed?

It might be something insignificant, like a light.

I had a prob like that once. It all worked for a day or two and then the customer turned on a security light. Some numpty had borrowed a neutral from a ring circuit, the live came from the lighting circuit.
Result: RCD trips every time the security light fires up.

 

[flyingsparks]

Sounds like you hired a brilliant "electrician"!!! Asking you what the fault is?! Any paperwork included with the CU change? Is he registered with a self cert scheme?

 

[RF Lighting]

Could be the in and ou neutrals for the RCD the wrong way round. I did that once. Took me ages trying to find the fault, even though all the IR readings came back clear.

 

[shedbang]

Sounds like you hired a brilliant "electrician"!!! Asking you what the fault is?! Any paperwork included with the CU change? Is he registered with a self cert scheme?

No to be fair - the Sparky is a good guy. And he's registered with Elecsa and I don't have any paperwork yet as clearly it doesn't work yet (and of course, the Sparky therefore doesn't have any money yet!!) The phone call was exactly as TTC stated. Had I done something in the interim (the answer being 'No').

I'll have a good look around in the morning, but my suspicion falls on the garage circuit which is spurred off the main ring and I think there is an aged PIR equipped light out there somewhere. As one of the jobs said Sparky did was to run a new fat 40A supply to the garage, we can remove the fuse from the FCU spur and see what happens then.

 

[EFLImpudence]

we can remove the fuse from the FCU spur and see what happens then.

You will need to remove the neutral as well to be sure.

Or if it is a double pole FCU, just switch it off.

 

[flyingsparks]

OK fair enough! Sorry didn't mean to be rude!!

 

[shedbang]

OK fair enough! Sorry didn't mean to be rude!!

Don't worry - I was pretty rude to him myself when he called!

 

[toptip123]

I take it the RCD is the main switch (switches everything off). An imbalance in the current between L N is what courses RCDs to trip (earth leakage). This could be a fault between L and earth or even N and Earth!!

There are lots of coses:-
Water ingress
Faulty appliances (usually something like a fridge when the compressor kick in, dishwasher, or iron. but could be anything) simple unplug it.
Damaged wiring (mouse eating cable, nails in floor and walls, dig ration of cable insulation)
Faulty RCD over sensitive (in old installations there is always some earth leakage which it then may pick up when it should not)

In short could be a lot of things.

If it a L E fault you mite be able to narrow it down by switching off the correspond circuit.

There are other things to do but if you don't know what you are doing you should not attempt them.

If you think it is the garage then turn off the FCU. Some if not all FUC are double pole so you may not need to disconnect the neutral.

Simply you need to have the installation tested. I usually do this before and after a CU change to try and pre-empt and nuisance tripping and to check the new connections.

Hope that helps..

 

[shedbang]

Yes, I suspect this won't be an easy one to find. It's a bit of a dogs breakfast of a circuit that's causing the problem. In a barn conversion attached to the house, where for some reason the whole barn (inc lights etc) is running of one circuit, which also feeds boiler, garage etc. LOTS of opportunity for mouse damage in the loft where there cables run as well. I don't think the cabling is too bad, it's just the design will make it hard to find the fault.

There's no appliances plugged in anywhere (bar the boiler) as the house is empty, but as I mentioned we need to check the lighting.

Either way, the man will be back next week to find the problem. He didn't seem to especially relish the task. We've got mice, wasps, spiders, all sorts in that loft...

In the interim, as you say, there's not much I can do apart from visual inspections, try and work out where the circuit runs and make a list of things to try first that might avoid hours of work.

The good news is that he said it was definitely NOT the sockets I had put on the ring. I was a bit insulted that he felt it necessary to check those!!

 

[Alarm]

Why, he had/has to check everything.

 

[Taylortwocities]

Why, he had/has to check everything.

Absolutely, the first rule of fault finding is to see what work - of any sort - has been done and check that.

 

[ban-all-sheds]

No to be fair - the Sparky is a good guy.

What testing did he do before he installed the new CU?

And he's registered with Elecsa and I don't have any paperwork yet as clearly it doesn't work yet

I can see you interpreting this as another attack on you, almost, but the problem is you genuinely don't seem to have any idea of what constitutes proper procedures.

The new CU was installed 3 days ago. All is well, but tidying etc required. Sparky back today. Finishes tidying and demonstrates how CU works to wife.

If he has "finished tidying" and was showing your wife how the CU works, then from his point of view it was finished.

Which means from his point of view he should have done all of the dead testing, all of the live testing, all of the tests should have been satisfactory, and all of the results and details of the work should have been recorded on an Electrical Installation Certificate.

So where is the certificate? He thought he was done, so why is there no EIC?

I'll have a good look around in the morning, but my suspicion falls on the garage circuit which is spurred off the main ring and I think there is an aged PIR equipped light out there somewhere. As one of the jobs said Sparky did was to run a new fat 40A supply to the garage, we can remove the fuse from the FCU spur and see what happens then.

I don't understand the relationship between the FCU supplying the garage and the "fat 40A supply to the garage"...

 

[ban-all-sheds]

Yes, I suspect this won't be an easy one to find. It's a bit of a dogs breakfast of a circuit that's causing the problem. In a barn conversion attached to the house, where for some reason the whole barn (inc lights etc) is running of one circuit, which also feeds boiler, garage etc. LOTS of opportunity for mouse damage in the loft where there cables run as well. I don't think the cabling is too bad, it's just the design will make it hard to find the fault.

Knowing that it was a pretty incompetent design decision to put that circuit on the same RCD as any others. Arguably poor enough to be a contravention of the Wiring Regulations.

We've got mice, wasps, spiders, all sorts in that loft...

The mice won't hurt him, nor the spiders, and there won't be any wasps active at this time of year.

But you will never be free from problems in an environment like that for as long as you have PVC cables up there - you need to get everything into steel conduit, so if he's up there rummaging about he might as well make a start on that.

In the interim, as you say, there's not much I can do apart from visual inspections, try and work out where the circuit runs and make a list of things to try first that might avoid hours of work.

You can give the loft a clean, remove any junk, put down boards for moving about (if not already boarded) and put in some lights which are powered from a separate section of the CU.

 

[shedbang]

You can give the loft a clean, remove any junk, put down boards for moving about (if not already boarded) and put in some lights which are powered from a separate section of the CU.

My God Ban, if only I could have married you. You would have got all of this sorted for me. I'd know who to employ, how to tidy my loft, the seasonal activity of wasps, when and when not to employ metal conduit, proper 'procedures' etc etc. it would be great. You could read the 17th to me every night as we drifted off. It would be beautiful.

I suspect however there may not be much humour. Or irony. Or patience. Or that sort of thing. No. Not much.