RCD Trips on new Split Distribution Board

Hi Everyone.

I have looked but i couldn't see this problem anywhere. Sorry if i AM repaeating a question.
I have jsut fitted a new split unit. The old fuses were the original type from over 30 years ago.

I have the new tails come in from the meter into a dual (Neutral & Live) switch breaker.
The nuetral goes to a neutral bar. From the bottom of the live side of the breaker, it was pr-wired to an RCD. I have fitted MCB's to the load side of the RCD with a copper bus-bar, as supplied.

On the unprotected side, i have MCB's connected to the load side of the live breaker also via a copper bus-bar, as supplied.

If i connect any circuit (Including just the timer for the central heating) into and MCB on the un-protected side, the RCD trips.

Can anyone suggest what may be happening.
My intention was to have the two light circuits and the central heating on the unprotected MCB's and the ring main circuits for upstairs and downstairs, plus a separate shower feed on the MCB's on the RCd protected side.

Thanks in advance
Ensim

The fairly standard issue - you have a neutral from the RCD side connected to the pre-RCD neutral bar (or vice versa). This means the RCD senses an imbalance and trips.

In fact, you don't mention an RCD neutral bar - you should have two neutral bars, one pre the RCD, one after it, i.e. the RCD should have a feed either directly from the N out on the main switch, or from the first neutral bar, then take a neutral feed out to a separate neutral bar for the RCD side...

Also, I assume you notified this work to your LABC and will be getting them in to inspect & test - as a warning, a single split load CU is generally not compliant with the 17th edition of the wiring regulations, you need either an all RCBO design (the preferred option), or if cost is an issue, the dual split RCD type.

Presumably you will be testing all the circuits for insulation resistance, earth loop impedance and RCD tests?

I agree. Can you post a picture of the consumer unit, with the cover off.
Just to clarify things..

Thanks for the info.
As soon as i read what you wrote about the two neutral bars, the penny dropped.
The Neutrals are on a common bar and not separated to per and post RCD.
I will also take your advice and buy another RCD and make it a dual RCD system.
I am intending having the wiring tested afterwards anyway.

Many Thanks
Ensim

You mention you are intending having it tested afterwards - have you arranged anybody to do this, as you'll be very unlikely to find an electrician willing to certify it to the LABC (as is legally required), since they can only certify something as being their own work. The correct procedure is to contact the LABC in advance, paying their notification fee and have them come and inspect and test, then issue a completion certificate. Given you've already started, you obviously can't do that, but most LABCs will let you notify late, and pay a slightly higher 'regularisation' fee.

Also, are you aware of the 17th edition requirements for RCD protection of concealed cables. Assuming your lighting cables aren't run in earthed metal conduit, or SWA or something similar, then those circuits must be RCD protected under the 17th edition...

I am intending having the wiring tested afterwards anyway.

Click to expand...

Its a bit late for that. The right time to test is BEFORE you change the consumer unit. Also you should have notified the local authority, again, before you do the work.

Generally you should have three neutral bars, Two are for the two RCDs and the third is generally used to provide an interconnect for the incoming neutral. Some (17th) CU manufacturers provide a 2-way neutral cable but it sounds like you dont have the right board and you'll have real problems trying to make it a 17th retrospectively.

Just to feed back on some of the above. Have you tested ANYTHING?
If so, how and with what.
If not, who do you plan to get to test it? seeing as you didnt notify the local authority and that registered electricians can only test and notify work they have done themselves....

EDIT: Rebuke BAH!!

You may struggle to get a second RCD that would be compatible to make the none RCD side RCD, a simpler (and IMO better) option is to replace the MCB's on hte none RCD side with RCBO's (look it up plenty of talk on here of them), also check your earthing and bonding is up to scratch.

There are two ways to get out of this pickle.

You can get an electrician 'round to test the circuits and issue a PIR on the existing installation. He can then fit a simple circuit, such as a single socket next to the CU. This way he can then test this circuit, wire in the CU properly and issue an EIC for the CU.

Alternatively you can go through your building control - but this will be tricky since you've started already and, to be honest you'll end up paying almost as much as having a sparks come and do it.