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.

 

[telectrix]

If he is correct that is. His track form is not great.

please expand on that.

 

[Wilko]

In my humble opinion, if the AC guys had fitted rcbo for their new circuits this problem perhaps would not have occurred or would be obviously AC related. Agree with Various about leakage and possible mixed Ns.

 

[ruston]

@Telectix .
If the electrician is correct that they are in the correct respective breakers and neutral bars.

 

[telectrix]

@Telectix .
If the electrician is correct that they are in the correct respective breakers and neutral bars.

i see .

 

[DPG]

Tel, I think Ruston was meaning that it is difficult to give a good diagnosis if basing it on the findings of the OP's spark, who's track record hasn't been 100% so far. Don't think he was aiming it at you.

 

[ruston]

@DPG AND @telectrix I was yes , I can see I abbreviated that too much.
I think Tel is correct if the breakers are wired correctly . The reference to the grid switch could be an influence too.
You have to be there for one like this.

 

[Mjbarnard]

Thank you all for the courtesy of your replies and for sharing your expertise.

I never expected a 'remote diagnosis' but the information has been interesting and useful.

I do not think there is anything to be gained by commenting in public on my electrician but I note the opinions. Suffice to say we will be moving on, but he has performed several installations for us which have worked satisfactorily.

While I realise it will not correct any fault, I will probably take the opportunity for the next electrician to fault find but also convert the CU to RCBO's - at least it will make any future faults less intrusive. I have some neighbour's recommendations but would you recommend placing a post on the Find An Electrician forum?

 

[marconi]

In your initial description you wrote 'the mcb' which I highlighted in my post #16. It appears to me from looking at an enlarged image on my CPU screen of your consumer unit, there are two MCBs on the far right which between them supply the 5 split system units. The left of the two feeding a/c systems 3, 4 and 5 and the right of the two a/c systems 1 and 2. Is this correct?

(And I understand from what you wrote you have 5 split systems where each split system comprises of an internal unit(IU) and two external units(EU). Or, do you have 5 internal units and two external units, 3 IUs working with one EU and the other 2 IUs paired with the other EU - to create 2 multi-split a/c systems?).

And I understood that only one of these MCB protected a/c circuits was causing RCD trip problems when interacting with the DW and/or its circuit. I assumed, perhaps wrongly, that the offending mcb was the far right one since it was off.

My post #16 seeks to clarify if the powered up/down state of one of your home's split a/c systems has any effect on whether the other (which I called the offending one) causes the RCD to trip if the DW is not isolated.

Could you also tell me the model number of your EUs and whether they are described as 'inverters'.

 

[telectrix]

your present electrician may be a good spark, but fault finding is something that can be difficult for even the most experienced. i know , being biassed towards repairs and faults. some can be baffling to say the least. example is i've had several RCDs fail to trip, even on the test button. each case has been a Neutral fault that has taken up to a few hours to solve, still tink a ramp test and a trace of leakage on both the a/c and appliances is the way to solve your problem.

 

[Mjbarnard]

Marconi - thanks.

There are two mcb's on the far right. The left is 3,4,5 and the right is 1,2. (it is the 1,2 which seems to cause a problem).

there are 5 internal units. THe 3 bedrooms go to one external unit, the kitchen/family room and the living room go to the (slightly larger) external unit.

The circuit which appears to cause the problem is the end MCB supplying 1,2 (kitchen & living room). You are correct that is why we keep it off.

THe powered up/down state has no effect. It occurs when they are not on, and frequently has occurred in the middle of the night when also no other appliances are on.

The units are the Mitsubishi Zen R410A inverters. The two external units are slightly different (detailed calcs showed units 1,2 needed larger units). I will get the exact model details shortly.

 

[Mjbarnard]

Marconi - from the A/C contract:

To supply, install and commission a comfort cooling system at the agreed location.

We have specifically selected Mitsubishi Electric equipment for this installation. Mitsubishi Electric are widely recognised as leading manufacturers of air conditioning equipment and offer exceptionally high quality engineering coupled with, what experience has taught us, maximum reliability and longevity.

The Inverter driven system offers both cooling and heating functions.

System Specification

Kitchen/Family Room & Media Room (Multi-split system)

1 X 5.0kW Zen Wall Mounted System

1 X 3.5kW Zen Wall Mounted System

Olivia, Francesca & Master Bedrooms (Multi-split system)

3 X 2.5kW Zen Wall Mounted Systems

 

[Sparkie30]

@Op

Tel is righ about earth leakage. Should have used a clamp meter and narrow it down.

Has he ever tried swaping that ac unit mcb which is off with one of the mcbs from another rcd?

 

[marconi]

Are you able to unplug the dishwasher easily? If you can could you do that and then:

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?

Take care to be around lest you have fridges and freezers supplied by the RCD.

 

[Mjbarnard]

Marconi - it's a new miele dishwasher and it's only 4 screws to pull it out. Will be able to do without much difficulty.

I will try as you suggest - will report back and be interested in the results.

Booking a new electrician.

Sparkie - one of the troubleshooting steps previously was to replace the MCB with a new one. It made no difference.

 

[elsparko]

rip it all out and start again

 

[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 ..