RCD Keeps tripping on new installation

[reeveer]

Hi all.

New member here.

I am a qualified, registered electrician and I have a mojor problem with a new installation and rewiring.

I have rewired 6 new flats and fitted brand new cu's (B&Q £55 ones.. at customer choice!).

Anyway, all rewire and fitting of consumer units of 5 flats are fine... however, the last one... did the rewire, fitted the cu, proceeded to test and the rcd for the kitchen/bathroom is tripping, even though none of the switches are on for the mcb's in the kitchen load of the cu.

The kitchen load comprises of, 3 double sockets, 1 radiator switched spur (RM with sockets), 1 cooker switch and a bathroom wall heater (RM with sockets).

When I remove the earths from the kitchen/bathroom RM, the RCD will stay on, but as soon as I touch either of the RM earth to the earth busbar, it will trip the RCD. Also, I can leave the RM earths in and remove the Neutral RM cables and it will be fine...but as soon as I touch the RM neutral to the busbar, it trips. All connections are fine... to the correct busbars, etc.

I am miffed on this one, and any help would be greatl;y appreciated!

Thanks.

[Tim Watts]

Were the IR tests in spec when you tested the circuit?

Did the RCD test as being in normal parameters in particular did it hold in on the x1 test?

What happens if you disconnect the fixed appliances?

[hugh]

Could it be that one of the appliances has the neutral linked to its earth?

[Dave Liquorice]

But doesn't understand that a N-E short will trip and RCD. Oh and on that parasitic diybanter "forum".

Or chippie has stuck a nail through a cable.

[Dave Plowman (News)]

I'd guess at an earth to neutral short somewhere.

I'd disconnect a socket somewhere in the middle of the ring including the wiring, and disconnect one earth at the CU in turn. That should isolate which half of the ring is the problem. Then go through the sockets in that half one by one.

[Graham.]

What's wrong with those cheap BG CUs from B&Q? I'm tempted to replace the CU here with one.

[Owain]

So you did follow recommended testing procedure and test for insulation resistance etc before energising the installation?

Because failure to do so would be a breach of the Electricity At Work Regulations.

Owain

[The Natural Philosopher]

earth neutral short somewhere

[ARW]

There is nothing wrong with them at all. I often fit them - well not from B&Q but from my wholesalers (I pay £54 and I get to choose the MCBs I want.)

They are sturdy, hi-integrity and the RCBOs are only £20 a go (at Screwfix).

Screwfix are doing a 13 way with 10 MCBs for £59.99 if you need a larger unit.

[ARW]

You don't need to worry about that if there is RCD protection:-)

[johnjeg]

[meow2222]

Seems obvious its a neutral to earth connection somewhere. RM?

NT

[ARW]

I was thinking that.

But as the OP has given no usefull info about the setup there is little point in helping him.

FFS he spent more time giving info about the 5 flats that did work and and the choice of CU than explaining the problem

[John Rumm]

"Ring Main" perhaps?

[meow2222]

Thank god I diy.

NT

[A.Lee]

Yes, that's his problem, he's got an 11,000v ring main in his flat.

[Brian Gaff]

Surely though that would not make either of them display the effect. I think I'd try another rcd first in case its very over sensitive. there could easily be some form of interference suppressor in an applience with a thermostat that is causing a momentary blip. Brian

[The Natural Philosopher]

IF there is some neutral-earth solid bonding upstream of the RCD then ANY earth neutral short will provide a path for neutral current that bypasses the RCD by using the earth instead: That unbalances the RCD enough to trip it on quiet small current draws.

[Tim Watts]

Or a troll - he's been very quiet. Most people posting a question like that would be on the net in short order looking for replies.

A real sparky, even a 4 day wonder[1], would probably have said they'd tested the wiring and RCD (but probably without the appliances) and found nothing wrong hence genuine confusion.

[1] On my 4-day-wonder course, the blatantly clueless and dangerous seemed to leave after a couple of days. Everyone left could run the prescribed tests and locate faults blind based on the tests.

[meow2222]

I don't see how that can be right. Lots of properties have N&E connected upstream (PME), the RCD only cares about the downstream L/N current balance.

NT