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]

Marconi,

Have tried your steps. Unplugged dishwasher.

To complicate things even further in the middle of the night RCD tripped again. This time it could not be reset unless mcb for A?C 3,4,5 was switched off. This is the first time this circuit has been a problem. So this has altered the steps:

Turn off MCB for a/c 1 and 2 and DP switch for DW.

1. then see if with the DW circuit on at the labelled DP switch the RCD trips when you turn on the MCB for a/cs 1 and 2?

Turn off MCB for a/c 1 and 2 and DP switch for DW.

2. now turn on MCB for a/cs 1 and 2 and then turn on DW DP switch.

Turn off MCB for a/c 1 and 2 and DP switch for DW.

3. Turn on MCB for a/c 1 and 2 and leave DP switch for DW off for several hours.

What does RCD do for 1, 2 and 3?

Washing machine unplugged. BOTH mcb's off

1. RCD trips if either mcb is turned on, when other one on.

2. MCB a/c 1,2 on, turn on DW DP. RCD trips if other a/c mcb on. Doesnt trip if other mcb off.

3. In process of 3. For about a couple of hours now, no RCD trip but only if mcb 3,4,5 is off. Otherwise trips immediately.

I get that this now makes it even more complicated. Cant seem to get an electrician for a 2-3 weeks.

 

[Octopus]

I would suggest you switch all the AC units off ...

Then see what happens

Can you post a picture of the CU?

 

[Devonchris]

As this has been happening since the A.C. units were fitted, then this has to be the main suspect. What heating did you have before? How was it wired? Has some of the fixed wiring been used for the AC controls? Or are there time switches or thermostats still in situ?

 

[DPG]

Be interesting to get a sensitive clamp meter on each of the aircon cpcs to check leakage. Maybe they need to be on separate RCBOs.

 

[Pete999]

Borrowed Neutral maybe??? did the A/C guys leave you any testing results or did they just install switch on an then leave.?

I posted this yesterday and since many people have tried to help, but for some reason you haven't answered what I consider a relevant question, so did the A/C installers do any electrical testing, if so did they leave you any paperwork, certificates of any sort? from what has transpired since I doubt it, but it would be useful to confirm. If the answer is no, then the first step would be to carry out a full set of electrical tests.

 

[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

 

[Wilko]

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).

Edited this - as just read the DW bit. Please confirm - the DW is unplugged and just operating the DW benchtop isolation switch affects whether the RCD trips?

 

[Ian Mills]

Hi new to forum I work in France so work with aircon units, I think problem is capacitors in AC units because when turned on they store small amounts of electric they make the rcd think there is a fault, the dishwasher will also have capacitors and when you turn dishwasher on it will have same effect, you may also have some small earth faults and the capacitors just push it over edge, would use combined rcbs /mcb, to give less chance of tripping, can not remember what they are called in UK regards Ian

 

[Octopus]

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).

Can you post a better picture of your fuseboard?

 

[DPG]

Welcome to the forum Ian.
In the UK we call them RCBOs.

 

[marconi]

I agree with all the advice on thorough inspection and electrical wiring tests - including earth leakage current measurements. It's a good idea too to swap to an RCBO populated CU for the convenience and least disruption to the family.

I reckon moisture, water or damp air could be troublesome inside the external a/c units although our 4 split a/c mitsubishi units on top of a flat roof have been happily energised permanently and fed by an mcb on one side of a dual rcd board and never caused a trip.

What puzzles me about this problem is the involvement of the dishwasher circuit post the double pole grid switch - the dishwasher itself has been ruled out by unplugging it. I have a suspicion it has a relatively high impedance N-E somewhere between the switch and socket including in the socket or its back box.

I also observe that in the photo of the grid switches the neons for the dishwasher and cooker hood look noticebably dimmer than the other two - maybe a feature of the camera angle but seems odd to me. I'd be itching to open that grid switch box and CU and to check the correctness and tightness of all the connections.

The other aspect which might be a factor is that the tripping occurs when an MCB switch is made to power up - so some form of transient effect - which makes me ponder about the current/voltage disturbances in L, N and cpc caused by any input filters the a/c systems have - they are largely electronically controlled these days so will need protecting. I'd be keen to observe the L-N, L-E and N-E voltages and cpc currents during these switching events with DW switch on and off.

I note that the oven and hob are supplied from the same side of the CU - the former often being a villain for element earth leakage and N-E when the element controls are off.

Some electrician is going to have a fascinating time investigating and putting right!

 

[Ian Mills]

Welcome to the forum Ian.
In the UK we call them RCBOs.

Thanks

 

[marconi]

ABB Classification of RCDs upon detectable wave form - Residual Current Devices RCDs (Modular DIN Rail Products) - http://www.abb.co.uk/product/ap/seitp329/4bd0ec0528a84420c1256f5500557772.aspx

http://www04.abb.com/global/seitp/seitp202.nsf/0/740f1caaeb62137fc1257fe900545279/$file/Guia+Técnico+ABB+-+Diferenciais+Tipo+B_EN.pdf

I cannot get this problem out of my mind. At the moment the advice is to switch to an RCBO board (or to fit 2 new RCBOs for the two a/c systems). The question for me is what type of RCBO? I think there is a risk that using type A or AC RCBOs will not stop the problem of nuisance trips because of the nature of the inverter drive technology and its effect on the load current waveform. I cannot find any recommendation by Mitsubishi on what type of RCD device to use with their inverter a/c systems. During the course of this on-line research I have read some folk (and manufacturers) recommend not using an RCD to feed a/c equipment because of the problem of nuisance trips - sometimes when the a/c equipment has been powered down for some period and then powered up.

Looking at the references above it seems to me that the correct RCD type to use is Type B which does all that Types A and AC do. The Type B can detect those 'true' faults which have more complicated current waveforms generated by earth leakage from inverter electronics and supply filters and yet avoid nuisance tripping in the presence of 'normal' low level earth leakage currents as the supply filters do there job 'dumping' harmonics, noise and leakage to earth including power up/down transients.

Does anyone have any practical experience of selecting the correct RCD type for a/c systems using inverter technology?

On page 6/12 the second reference you will find:

With these properties, Type B RCDs turn out to be RCDs with high immunity to nuisance tripping caused by grid surges, electronic loads and EMC filters. Therefore, Type B circuit breakers are the ideal solution for all “difficult” loads, not only from the point of view of protection, but also in terms of service continuity

 

[Ian Mills]

The advantage of 2 or more RCBOs is that it means if there are other small earth faults on system they do not add to what ever the a/c units are doing and the a/c units might not take a trip out on there own, and if they do you can go back to the a/c company and say it is your system sort it out

 

[marconi]

Picking the right protection - https://www.electricalreview.co.uk/features/9258-picking-the-right-protection

In this reference I have just read:

Now let’s turn to RCBOs. In principle, as these are functionally an MCB and an RCCB in the same device, it would be possible to produce versions with a huge range of combinations of MCB and RCCB characteristics. In practice, manufacturers limit themselves to the most commonly used combinations, such as a Type B or C MCB characteristic with a Type A or AC RCCB characteristic. Where other combinations are needed, it often means that an RCBO can’t be used and that the necessary protection must be provided by using a separate MCB and RCCB. Some manufacturers, however, offer field-fittable residual current units that can be used to convert an MCB into an RCBO, and this makes possible a wider range of combinations.

 

[Ian Mills]

Here in France we would use Hpi type but only if a problem as expensive but normally will not trip if not a earth fault, do not know if you have same but would think so

 

[marconi]

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.

 

[Mjbarnard]

#61 - correct. We had DW unplugged all day yesterday and could replicate tripping.
Coincidentally we had the DW replaced a few months ago and the original one had been hard wired in, so as part of the installation the electrician changed the fixed wiring to a single socket. So this is also new and presumably tested.

 

[Mjbarnard]

iPad error

 

[Mjbarnard]

iPad error

 

[Mjbarnard]

#63 will try to post better image CU when I finish rounds.

 

[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.