Strange RCD tripping problem

[Mjbarnard]

I have a very strange repeated RCD tripping scenario, which appears to suggest that two unrelated circuits are connected. Any problem solving thoughts would be appreciated.

I have an electrician who has done several jobs for us (new lighting circuits, appliance installation, outside lights, changing sockets etc). I am very pleased with him, and he has visited around 5-10 times over the last year to try and fix this - but admits he doesnt understand it. He is going to bring in colleagues who specialise in troubleshooting. So we both just thought it might help to put the problem out there for any ideas. I do not touch anything electrical myself.

New build detached house, finished 2012. Dual split load consumer unit with RCD on both (picture). Everything worked fine until last summer when Air COnditioning installed by very experienced professional firm. They installed 5 split units in 5 rooms, and put two outside Mitsubishi units each with new dedicated supply with MCB in CU in two spare slots.

Ever since then we have had intermittent RCD tripping, which my electrician has investigated extensively. Coincidentally we have also replaced all appliances since then with new Miele ones and new Liebherr high end fridge, so unlikely they would be involved.

More recently the RCD repeatedly trips unless the MCB for one of the air con circuits is switched off . Both the electrician and air con company have investigated this - but cannot find a fault with the air con circuit. However my electrician when checking all the possibilities discovered a very strange anomaly. First there is a 6 gang double pole grid switch in the kitchen for isolating various appliances (picture). He found that one of them (for plate warmer) had incorrectly had a simple (light) switch installed - so live and neutral were connected when on. He replaced that and checked all the others.

He then made a discovery which is hard to understand. The RCD immediately and repeatedly trips when the air con MCB is switched on. However it does not trip when air con MCB is on but dishwasher grid switch is off. Turning the dishwasher grid switch on immediately trips the RCD. The air con works fine and no RCD trip when dishwasher isolated, but dishwasher works fine and no RCD trip when air con isolated! We have repeatedly tested and found the same results.

My electrician is seeking advice but does not understand how a new air con supply can somehow be linked to the dishwasher which comes off the ground floor ring mains which has no connection to the air con circuits. Not only has he inspected and tested the CU several times but so have two very senior technicians from the air con company.

This all happens consistently including when all other appliances switched off, so I do not believe the supply is insufficient.

Hope this makes sense. I am not sure if posting pictures is allowed from new members, so I will try to do so in the next post.

 

[Mjbarnard]

#67 as part of the research I came across recommendations about not putting inverters on RCDs. I suggested this to electrician but he wasn't comfortable with that

 

[Mjbarnard]

Hopefully new electrician coming on 15th. I will invite him to comment if he can or cannot resolve it. That will obviously be much more useful than my observations.
I will point him to this thread but would you recommend I ask him any specific questions ? I am not suggesting that as a professional he wouldn't know what to do. Just in case it helps.

 

[Toneyz]

Is the only reason the A/C set up is protected by RCD as it is due to the type of C.U. you have already.If you do get the C.U. set up changed/reconfigured and the wiring is all surface you may not need the RCD/RCBO element.Do the A/C installers usually put on MCB only.

 

[Mjbarnard]

Not sure I understand your questions / comments.

To my simple mind:

Never previously had a single rcd trip. A/C installed by what I believe is reputable experienced company. Almost immediately frequent RCD trips. Could prevent them by isolating A/C both internally and externally. Seemed a no-brainier to me. Called the A/C company who said an electrician should be consulted. Persisted and they sent an engineer who identified the link with the dishwasher circuit.

Since then no-one can work it out and all we have is repeated and sometimes frequent RCD tripping.

I will go with the transfer to all RCBOs if only to avoid disconnection of fridge freezer, internet routers etc.

 

[Toneyz]

Not sure I understand your questions / comments.

To my simple mind:

Never previously had a single rcd trip. A/C installed by what I believe is reputable experienced company. Almost immediately frequent RCD trips. Could prevent them by isolating A/C both internally and externally. Seemed a no-brainier to me. Called the A/C company who said an electrician should be consulted. Persisted and they sent an engineer who identified the link with the dishwasher circuit.

Since then no-one can work it out and all we have is repeated and sometimes frequent RCD tripping.

I will go with the transfer to all RCBOs if only to avoid disconnection of fridge freezer, internet routers etc.

What I mean is depending how the A/C circuits are installed they may not need RCD/RCBO protection under the regs, but as they type of consumer unit that you have they have had to be installed on one and are adding to the leakage current making it trip.

 

[Mjbarnard]

Ok thanks. So does that mean I might be able to get the supply split and put the A/C on a new mini CU?

 

[Pete999]

Ok thanks. So does that mean I might be able to get the supply split and put the A/C on a new mini CU?

I would get the A/C wiring tested before you go to the expense of an all RCBO board, or any other expensive moves, seen a lot of A/C electrics thrown in by so called engineers, if it works that's good enough in most cases, how many A/C engineers have you seen with a MFT?

 

[derek]

I would get the A/C wiring tested before you go to the expense of an all RCBO board, or any other expensive moves, seen a lot of A/C electrics thrown in by so called engineers, if it works that's good enough in most cases, how many A/C engineers have you seen with a MFT?

The absence of an MFT could be why there was no Electrical Installation Certificate provided by the A/C installers (that is if I have read the thread correctly).
I would have expected the A/C engineer to provide a resistance value and which cores of the circuit had faults on, in their report identifying the D/w circuit as the culprit.

 

[Mjbarnard]

As you can see (or perhaps not), not a very technical report

 

[Ian Mills]

Hpi type:

Modular devices Hpi type RCCBs - http://www.export.legrand.com/EN/modular-devices-hpi-type-rccbs_63.html

Hpi seem similar to UK type B RCCB - not sure yet if they are the same.

Hi think a little different, we also have type B here but all I have seen are 3 phase but sure must be other single phase out there, would think type b would be okay for this application

 

[Mjbarnard]

UPDATE to this perplexing problem. I am grateful to everyone for their time and contributions.

An expert visited a couple of weeks ago. He found that the problem was not over-current, rather accumulated earth leakage - as has been suggested by some above - and a Neutral-Earth fault on the downstairs ring - there was a nicked blue wire in the metal back box of the dishwasher socket.

Additionally and exacerbating, the earth conductors in the kitchen 9 gang DP switch plate (which includes the dishwasher circuit) were not properly connected together and one actually appeared damaged with blackening.

He fixed these faults and then installed 32A RCBOs for the two air con units because there were brief surges in earth leakage when each unit was energised: once energised their earth leakage was small. These brief surges when added to the accumulated earth leakage were detected as excessive by RCD2.

Since this was done we have had no further trips. It has only been two weeks, so I understand it might yet recur, but suffice to say that we have not been able to have both aircon circuits on at the same time for several months so something has definitely changed.

Thanks again to everyone. I am hoping this problem is solved - time will tell.

 

[Octopus]

^^ Good man .......... I really appreciate your feedback - so few people do this and its good to hear that it seems to have been resolved.

Dual RCD boards have certainly not helped - with more and more devices in a house adding to "natural" leakage, so a small fault can push an RCD over the limit.

 

[ruston]

Yes , thanks for letting us know . I hope it is now sorted for you.