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.

 

[elsparko]

if everything was working fine prior to the air con install then what would occams razor tell us?

 

[elsparko]

could just sell the house and move on?

 

[Wilko]

Hi - the AC compressor units, do they have a local isolation switch next to them? Hopefully they do, if so perhaps try the tests with air con MCB on but the units isolated locally. Isolation switch may look like this ...

 

[lurch]

Had similar last year. Erroneous tripping of rcd. No obvious low readings on insulation resistance testing of circuits. Could not narrow down to a specific circuit and no new installs to point the finger at unlike the aircon units. Moved kitchen ring main away from rcd and put it on rcbo and all was ok. My summary was that combined earth leakage of all the kitchen appliances plus that of the downstairs socket took the earth leakage over the edge in certain scenarios.

 

[Octopus]

^^ do you have a clamp meter?

This is another reason I only fit rcbo boards ..

 

[Toneyz]

You are correct, but I think @Pete999 is on the case.

I would have thought that this would have been ruled out by now.

 

[Mjbarnard]

Thanks for suggestions. Am in the hospital today so haven't been able to do much more observations.

Murdoch (msg 42) not sure what you mean by "off", but all the internal units have been off for months, and tripping occurs frequently when they are not on.

Devon Chris (43) A/C circuits new and completely separate to everything else. HEating is via conventional boiler, underfloor heating radiators, thermostats, controllers etc. Only control for A/C are the buttons on the units and the remotes.

DPG (44) happy to install rcbo's if that's a solution. In fact discussed with previous electrician possibility of swapping whole CU to rcbo's.

Peter999 (45) as I recall original install only provided statement saying it had been checked and commissioned satisfactorily. Don't recall any technical data. I will recheck. When A/C engineers returned to investigate they left a document stating fault lay with dishwasher circuit.

Elsparko (46) I have been pointing this out to A/C company all along, but they don't appear interested.

Wilko (48) interesting point. There are local external isolation switches like your picture. Now I haven't tested recently but on one of the first occasions red tripped , I was away and my wife rang to say she couldn't reset the tripped rcd. I advised her to isolate using the external switch, and she confirmed she could then reset the rcd. I told the A/C company this in no uncertain terms and even provided their engineer with a written summary including this, but they didn't seem to think it mattered.

Lurch (49) obviously will get professional advice but happy to change some/any/all mcbs to rcbos if that provides a solution.

Murdoch (50) I don't know what a clamp meter is, but I would expect an electrician to have or obtain whatever is necessary?

Thanks again.

 

[Pete999]

Thanks for suggestions. Am in the hospital today so haven't been able to do much more observations.

Murdoch (msg 42) not sure what you mean by "off", but all the internal units have been off for months, and tripping occurs frequently when they are not on.

Devon Chris (43) A/C circuits new and completely separate to everything else. HEating is via conventional boiler, underfloor heating radiators, thermostats, controllers etc. Only control for A/C are the buttons on the units and the remotes.

DPG (44) happy to install rcbo's if that's a solution. In fact discussed with previous electrician possibility of swapping whole CU to rcbo's.

Peter999 (45) as I recall original install only provided statement saying it had been checked and commissioned satisfactorily. Don't recall any technical data. I will recheck. When A/C engineers returned to investigate they left a document stating fault lay with dishwasher circuit.

Elsparko (46) I have been pointing this out to A/C company all along, but they don't appear interested.

Wilko (48) interesting point. There are local external isolation switches like your picture. Now I haven't tested recently but on one of the first occasions red tripped , I was away and my wife rang to say she couldn't reset the tripped rcd. I advised her to isolate using the external switch, and she confirmed she could then reset the rcd. I told the A/C company this in no uncertain terms and even provided their engineer with a written summary including this, but they didn't seem to think it mattered.

Lurch (49) obviously will get professional advice but happy to change some/any/all mcbs to rcbos if that provides a solution.

Murdoch (50) I don't know what a clamp meter is, but I would expect an electrician to have or obtain whatever is necessary?

Thanks again.

So in layman's terms "a whitewash"

 

[Octopus]

Murdoch (msg 42) not sure what you mean by "off", but all the internal units have been off for months, and tripping occurs frequently when they are not on.

Murdoch (50) I don't know what a clamp meter is, but I would expect an electrician to have or obtain whatever is necessary?

Thanks again.

42 - thats very interesting so it rules the internal units "out" of the problem - but not the cabling installed for them.

50 - not all sparks have them, but I find them, in cases such as yours essential - so ask when you're trying to find a new spark - if they say "I don't need one" don't let them on site.

 

[Toneyz]

IMO there is an accumulation of leakage currents, insulation test maybe won't show until appliances/A C units powered up a leakage test would be a start.The neutral being on the wrong RCD side should have been ruled out ages ago.Swopping a few circuits around between both the RCD's may even the leakage current out between RCD's but as time goes on the leakage may get worse from the appliances with age I think Tel was right in his first post and the one regarding your electrician. Installing a new consumer unit with RCBO's or buying replacement internal parts to have a
main switch some high leakage circuits on RCBO's and a RCD protecting other circuits with MCB's or if costs permit all RCBOs another thing to consider is you have no more spare ways.

 

[Mjbarnard]

Ta - electrician previously supplied quote for swapping all mobs to rcbo's as one of the options. Not sure why we didn't do it - I think he wasn't certain it would solve it.

 

[Wilko]

Hi - if I understand your post #52 correctly all your problems stop if you isolate the air con units - or ?

 

[Mjbarnard]

Yes if I isolate the air con external units or turn the air con mcb's off OR leave the air con on and isolate the dishwasher (irrespective of whether it is plugged in or not).

 

[DPG]

A check on earth leakage is the way to go. You're going to be guessing otherwise. Check leakage on each air con unit.

Took me 10 minutes to type this as I have been drinking since 6pm. Bear with me if I don't make much sense.

 

[Wilko]

A check on earth leakage is the way to go. You're going to be guessing otherwise. Check leakage on each air con unit.

Took me 10 minutes to type this as I have been drinking since 6pm. Bear with me if I don't make much sense.

10 minutes ? I had to wait till morning