RCD not tripping - tech advice

eggandspoon · May 17, 2019

Hi

Hopefully some can explain this perplexing issue

I'll try and explain the situation as best I can, hopefully its not to rambling -maybe its common knowledge but I'll bow to anyones supperior understanding

During an EICR checked test button RCD would not trip - checked with tester no trip on X1 or X5 so informed client it was faulty

Split load board 5 ways non RCD 2 ways RCD Hager Ze 0.20

however please refer to my very bad drawing attached.

Later on in testing - I switch off MCB A and rcd trips try to reset it cant do this unless MCB C is switched off then RCD will reset
MCB C put off RCD trips immediately

Live test with RCD off there is a feed C to A at top of MCB

Tested RCD with loads disconnected - functions perfectly.

There is an interlink between circuits A and C - kitchen was wired by clients friend who altered circuits and has obviously mixed up fixed appliance ring with downstairs ring.

Although I am getting continuity between all 4 ends - was hoping to solve problem by making 1 big ring but this is not possible.

Having to go back and try and sort this mess - its a house sale that concludes 4 weeks buyer want EICR seller is client.

Not looking forward to fault finding - all tiled etc may have to consolidate all A and C cables into 1 20A mcb but very reluctant to do this as putting problem onto someone else

So - if all the above is clear can anyone explain why this interconnection between RCD protected circuits prevents and RCD from functioning?

Obviously there is a lesson for me to initially check an RCD that doesn't function with the loads disconnected in future

Many thanks

[ElectriciansForums.net] RCD not tripping - tech advice

westward10 · May 17, 2019

The rcd is not faulty if it functions with all protective devices in the off position. If the rcd circuits and non rcd circuits are interconnected then this problem would have been evident to the client prior to your EICR because the rcd just would not have stayed on.

Octopus · May 17, 2019

Neutral to earth faults can do as you describe. What were your IR test results

static zap · May 18, 2019

With Neutral earth fault present , I wonder if it was
"Created as a fix -for un-wanted Tripping" . Kitchen maintenance tricks !

Wilko · May 18, 2019

My vote is for 2 faults - N-E and circuit interconnection. Both will need fixing. Just be cautious that there aren’t more interconnection faults. Perhaps isolate the supply and dead test.

James · May 18, 2019

Is there a possibility of an appliance fault that is leaking dc into the circuit? This can lock the rcd and prevent it functioning.

eggandspoon · May 19, 2019

Hi thanks for all replies

IR L-N L-E N-E all > 199 Mohms

Client was unaware of fault as RCD stays on as long as mcb C is on
everything runs as it should apart from RCD test button doesn't trip

Neutrals may have been interconnected from non RCD side to RCD side at circuit wiring, but by this point I had spent an extra couple of hours on job trying to suss this out and lost the will.

I don't think the "kitchen fitter" was sophisticated enough to come up with a convoluted plan to prevent RCD tripping, as no wiring was altered in CU

I left the job with circuits A and C both connected onto mcb A with neutrals onto RCD bar.

The RCD function properly so assuming no N-E faults as RCD performing as should.

Still cant get my head round why this interconnect would stop RCD from tripping?, If I had the patience I would set it up on a bench to see if I could replicate circumstances. Any further ideas?

Unfortunately I've take responsibility for this now, it would have been great just to ID faults issue EICR and walk away for someone else to fix. I hate testing sometimes....

I am going back this week to try and work out whats going on but am only going to spend an hour on it - could spend all day - as lifting floors and cutting holes in plasterboard not an option.

How would you feel about putting circuits A and C onto 1 20A mcb?
usual kitchen appliances and living room sockets. As I said before I'm not comfortable with it, but it is a solution. Also what do I put on EICR knowing what I know if this is done?

Many thanks again

Wilko · May 19, 2019

Ok, next thought is the circuit controlled by MCB C is bypassing the RCD. So no trip. With MCB C off it removes its L but leaves N which provides an extra path for current from MCB A circuit to trip RCD.

static zap · May 19, 2019

Did you do your IR tests with C's - MS On or off .
(is MS DP) ...

eggandspoon · May 19, 2019

Wilko said:
Ok, next thought is the circuit controlled by MCB C is bypassing the RCD. So no trip. With MCB C off it removes its L but leaves N which provides an extra path for current from MCB A circuit to trip RCD.

Hi thanks,
this kind of makes a bit sense to me but not being a fault finding expert still cant quite get my head round it.

I have spent many frustrating hours trying to get my head round apparently simple faults.

IR tests were done with circuits disconnected, MS is DP

As a further solution I will try and separate into 2x 20a radials at some point in botched circuit(s)and run off spare RCD way.

Many thanks to all for your thoughts and inputs, I appreciate the help and advice, especially as its non critical as to ones knowledge and skills

chris gray · May 19, 2019

I’d see wether it’s possible to convert the 2 rfc into 1 at the ccu and put onto a 32a, making sure the appropriate neutrals are in the correct neutral bar

eggandspoon · May 20, 2019

chris gray said:
I’d see wether it’s possible to convert the 2 rfc into 1 at the ccu and put onto a 32a, making sure the appropriate neutrals are in the correct neutral bar

Hi

thanks, as I mentioned earlier, making both into 1 RFc was my first choice, however I was getting continuity at all 4 ends( lives) so not possible.
Client has lifted floor up above today and sent me some pics of a few choc box joints, so I've got a good place to start to see whats going on.

Still can get my head round initial circumstances of non tripping.