RCD tripping on new cu

[Araman]

I have installed a replacement (split load) cu fitted with two RCD's.

Prior to this I tested all circuits for polarity and insulation and found no problems, except for no (connected) cpc's on the lighting circuits and class I lights on the ground floor which are to be replaced with class II. (The cables used have a cpc but it is not used - please do not ask me why).

Only one circuit (first floor lighting, all with class II lights) will operate, the others all trip constantly. I have double-checked my wiring which seems sound.

I have had to leave the customer with the RCD's bridged to provide him with power and light overnight and must return at dawn tomorrow to try to sort the thing out.

Does anyone have similar experience to point me to where the problem may be?

Would it be more sensible to cut my loss and remove the RCD's and fit RCBO's in place of the MCB's or will these trip as easily?

All advice very gratefully received

 

[tony mc]

Have you checked for borrowed neutrals.

Have you checked that the RCD is wired correctly with the correct neutral bar.

I would be double checking the above first as you will have the same problems with RCBO's .

 

[Bradzz]

Hi try putting the lighting circuits in the same breaker,and if this does not trip you may have a borrowed Neutral

 

[tony mc]

When you say trip constantly does the RCD energise up or will it not set at all ???

 

[Paul.M]

Have you done a ramp test on the rcd?

 

[telectrix]

sounds loike either a borrowed neutral or incorrect wiring of neutrals in CU. could even be manufacturers incorrect wiring. check all in CU 1st, then disconnect each circuit till fault disappears.

 

[mattmoo1974]

Have you checked for borrowed neutrals.

Have you checked that the RCD is wired correctly with the correct neutral bar.

I would be double checking the above first as you will have the same problems with RCBO's .

Iv made this mistake and it fryed my head trying to work it out!
Replaced the RCD all sorts and then had a BINGO Moment.
Easy to do, Really silly mistake but a good learning curve.

 

[Araman]

Have you done a ramp test on the rcd?

Ramp test says the RCD is tripping at 33mA. other RCD tests with the Megger (mft 1720) say the Rcd is defective but they both do trip on test. Surely it would be beyond a coincidence for both RCD's to be defective? Or am I being naive?

 

[Araman]

RCD's came installed in the cu. Would you believe it could be wired incorrectly?

I do not think I have missed a borrowed neutral but I will check.
Thanks

 

[mattmoo1974]

You think the factory where there made pays an Electrician to pre wire them?
Always check it mate, Its really easy to pop it in the wrong bar and have a head ache.

 

[Araman]

I do not think there are any borrowed neutrals but being an ignorant mechanical engineer lately retrained in the magical arts of electricity perhaps you can put me on the true path by telling me how to really check for such problems.

Many thanks for your help.

 

[Octopus]

I do not think there are any borrowed neutrals but being an ignorant mechanical engineer lately retrained in the magical arts of electricity perhaps you can put me on the true path by telling me how to really check for such problems.

Many thanks for your help.

"I do not think there are any borrowed neutrals ..........perhaps you can put me on the true path...."

How can you say you don't think there can be any borrowed neutrals if you don't know how to look for them?

How old is the house?

 

[mattmoo1974]

Turn every thing off and turn each MCB on in turn, If it stays in(RCD) turn that mcb back off and keep repeating til you find the 1 that trips it,if they all do it the netural is in the wrong bar.
If its borrow it ll only be 1 that causing the issue and go from there<< thats how i would tackle it....

 

[Araman]

Not quite the response that encourages people to ask for advice Murdoch, even Newton admitted he stood on the shoulders of giants.

If ever you need advice on how to make a cogging gear for a 1930's gearbox, and how to make the tool to cut it, please do not hesitate to ask.

Each of us to his own.

 

[mattmoo1974]

Araman hes not having a pop hes just asking a question my friend.
Hes asking the age of the house because "Back in the day" Borrowed neturals was a common thing.
I think hes getting at if you put both the upstairs/down lights neturals in the same MCB it mite not trip the MCB and you l have found your problem.
But im guessing what he was getting at.

 

[mattmoo1974]

Netural bar not MCB DOH!!

 

[Bradzz]

Did you not say that the lighting is all run from 1 circuit, if so no borrowed neutral, then possible you have a short on 1 of your other circuits, knock off all circuits and turn on the lighting, does it trip.

 

[truckster]

Easy to check for borrowed neutrals....breakers/main switch off, disconnect one neutral at a time, megger between it and neutral bars, reconnect after test, test next one etc etc.

 

[tony mc]

Ramp test says the RCD is tripping at 33mA. other RCD tests with the Megger (mft 1720) say the Rcd is defective but they both do trip on test. Surely it would be beyond a coincidence for both RCD's to be defective? Or am I being naive?

Whats the make of the CU your putting in just out of interest?

Yes the CU can be wired up incorrectly at the factory so always check.

If its tripping at 33 mA something is wrong as when you put 1x test on it wont trip.

Yes 2 RCD 's can be faulty but make sure you double check your tests. (You say one cct holds the first floor lights)

I would disconnect all loads at the CU and test both RCD's there.

 

[Araman]

!x test does not trip either RCD, neither does 5x. It begins to look as if I have compound problems of faulty RCD's and, quite likely, borrowed neutrals as well.

Many thanks to all who have offered advice.