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]

my thoughts would be that both the dishwasher and the air con have appreciable earth leakage, each on it's own insufficient to trip the RCD, but together take it over the threshold. a ramp test of the RCD to see what it trips at, and an earth leakage clamp meter will help to resolve the problem.

 

[Octopus]

You need a spark with an earth leakage clamp meter .....

 

[Mjbarnard]

Pictures

 

[Mjbarnard]

Thanks both.

Puzzled that professionals havent excluded that.

The dishwasher was replaced during this period. So unlikely both old and new would be faulty in same way? I assume therefore the earth leakage is somewhere in the supply to the socket for the dishwasher?

 

[Octopus]

You can't make any assumptions. Least of all about 1 cable

 

[westward10]

I have Miele appliances and they have never caused problems. Disconnect the air con and see if it persists.

 

[Mjbarnard]

Sure. Just flagging up that we had the problem with both old and new dishwasher so is that consistent?

 

[Mjbarnard]

It doesnt persist when the air con disconnected (MCB off or removed)
But it also doesnt persist when the dishwasher isolated.

 

[buzzlightyear]

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.

may be the spark should have found the fault 2 time in ,do you not think so ,
by calling in colleague to trouble shoot for him ,if it was me, I would not have confidence with him with the spark you have now !

 

[westward10]

As the air con circuits are more recent I doubt they are interconnected with any existing circuit. I agree with Tel in #2 that the combined earth leakage of the equipment is tipping the rcd over the edge and this should be investigated as stated in #2.

 

[Mjbarnard]

Buzz - I agree. We will be bringing in someone new as well.

Also the air con company who I believe have good reputation, sent their 'most experienced' technician and they denied any problem with air con circuit, which doesnt appear to be consensus above. So that's two independent people who couldnt identify problem.

I am a doctor and we dont have a problem with getting second opinions for difficult cases - but I wont draw parallels between electrical circuits and the human body...

 

[Mjbarnard]

Thanks very much Westward and Telectrix. Makes sense even to me. Will get new spark and hopefully that will be the end of it.

 

[buzzlightyear]

Buzz - I agree. We will be bringing in someone new as well.

please tick the icons at the bottom of the page !

 

[Adj]

I am a doctor and we dont have a problem with getting second opinions for difficult cases - but I wont draw parallels between electrical circuits and the human body...[/QUOTE]

...and like doctors you can get good, bad and inexperienced electricians - you just haven't found a good one yet.

...and the human body is an electrical circuit.