RCD tripping on new cu

[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.

 

[truckster]

Check the earthing..sounds like you may not have a good earth which is why the RCD tests are failing and if the RCDs have an earth fly lead that could be the reason they are tripping

 

[Araman]

House was built in '66. What really bugs me is that all lighting cables are twin core plus earth, but the earth is not used. Wherever it has been cut it has been sleeved in yellow/green so is modern, but is not connected to anything.

 

[truckster]

Araman I'm talking about the main earthing..if its too high it can give problems which an RCD will sense

 

[Araman]

There are two lighting circuits (one for each floor) and two power circuits, plus one radial for the immersion heater and a redundant radial for a cooker. One power circuit feeds only two bedrooms, the other feeds two bedrooms and every other room in the house, plus a diy extension into the garage.

 

[Araman]

CU is an Excell, chosen for the one reason that it fits the space.

 

[WayneL]

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.

Truckster, would you like to explain to the group how testing in the manner that you describe, will indicate that someone has borrowed a neutral (for a landing light) from the upstairs lighting circuit, whilst feeding a live to said light from the downstairs lighting circuit??

Araman,

First thing to do when you get back there in the morning is check the 'manufacturer' wiring. (as already suggested) - not just a quick look, follow it section by section to be sure whats feeding what. (sometimes your mind can be kidded by something that looks right, but isn't)

Next, double check all your own wiring - start as if you're wiring it from scratch (it will be worth it).

Check that each circuit's neutral goes into the corresponding neutral bar etc. and check that the two legs of the ring are actually the two legs of the ring.(if you haven't done full tests on these)

The last section of that statement might sound mad, but here's why:

A friend had changed an old wylex rewirable for a dual RCD board.

He couldn't get the first RCD to hold at all and the second one was randomly tripping.
The downstairs lights were on the first RCD, and they were working with the RCD OFF!

He hadn't bothered with RFC conductor continuity testing - he just did R2 wander lead to socket earth pins - and was going to do Zs.

To cut a long story short, he'd mixed up the two legs of the ring with a 2.5 radial that fed a boiler - so one leg of the ring was on one side of the board and the other leg was on the other...........caused mayhem.

I'm not saying this is the fault you will have with your CU change - what I am saying is that it's easy to make a daft mistake that can cause you to look for all kind of faults all over the place...........I mean, the downstairs lights working with the RCD off must have sent him potty!

Anyway, moral of the story - double check all of your wiring first.....then move on to fault finding.

 

[Bradzz]

Does the RCD trip with JUST the lighting on.

 

[WayneL]

Araman I'm talking about the main earthing..if its too high it can give problems which an RCD will sense

I've justed spotted this comment, Truckster, perhaps you can explain this as well??........you're making random statements that less knowledgeable people reading this are going to take to be fact.:yesnod:

 

[truckster]

For WayneL

1) Each cct should be discreet. The neutrals are commoned at the CU, by disconnecting a neutral from the board with the breakers/main sw. off that neutral should have no connection to any other. (Borrowed neutrals are not solely found on the lighting ccts) This is a simple test to find which ccts may be interconnected

2) An RCD test takes some current and diverts it to the earth path, if that path is of a high resistance the RCD will not trip as insufficient current will flow. Further some RCDs will detect a high earth and refuse to set. The OP has stated that both RCDs are tripping under load and yet will not trip when tested, as both RCDs are acting in the same manner it would seem that an external influence is acting here.

I hope that has explained my 'random' comments.

 

[mickys86]

For WayneL

1) Each cct should be discreet. The neutrals are commoned at the CU, by disconnecting a neutral from the board with the breakers/main sw. off that neutral should have no connection to any other. (Borrowed neutrals are not solely found on the lighting ccts) This is a simple test to find which ccts may be interconnected

2) An RCD test takes some current and diverts it to the earth path, if that path is of a high resistance the RCD will not trip as insufficient current will flow. Further some RCDs will detect a high earth and refuse to set. The OP has stated that both RCDs are tripping under load and yet will not trip when tested, as both RCDs are acting in the same manner it would seem that an external influence is acting here.

I hope that has explained my 'random' comments.

Very good