Adding RCD to CU

13 Aug 2009
Reaction score
United Kingdom
Hi, having successfully replaced a defunct shower with a nice shiny new one, with some reassurance from you guys (Another 6mm or 10mm shower cable question), I was wandering what your thoughts were on the following.

The existing shower circuit is fed with a 45A MCB in a circa 2003 16 way MK Sentry CU. It is not split load as is, although does have 2 neutral bars joined together by a hard link. There is no RCD protection.

Whilst visiting B&Q today there was a twin pole 40A 30mA MK RCD for Sentry boards sitting in Bargain Corner, so after a bit of a think I bought it. Having got it home I've found that the accompanying paperwork was non de script to say the least so have been trawling the net looking for some answers.

I gather that in a split load board, the RCD is fed from the 'switched' side of the incoming supply via a cable link to the top right pole, the top left pole of the RCD connects to the main neutral bar via a link cable, bottom left pole connects to 'protected' neutral bar via another cable link, and bottom right pole feeds buzz bar which in turn supplies the MCB's on the protected side.

So the questions.....

1) Having read a reply from Adam_121 on another topic I gather that you still need a MCB to run a circuit protected by RCD.

In a non split board where is best to site the RCD and MCB given that only one MCB is to be used with the RCD for the shower?

I'm guessing that far right of the board would be best so that the buzz bar can be cut in front of the RCD, but leaving a slot for the MCB after the RCD. MK, by the way, sell a split load kit that supplies the wire links. 14 ways are usable in the CU after the incoming. At present only 7 are used and nothing is wired into the second neutral bar so link could be removed and used as protected for RCD.

2) Is it actually OK to cut the buzz bar in the first place? I'm assuming that if MK sell a kit then it must be ok. I think the board is a 'K' Series.

3) The current MCB is 45A but RCD is 40A. Is this a mis-match? The Triton Zante (T80si) states it can be 40 or 45 on the MCB. Install is on 6mm clipped run of 5 metres, last meter through UN-insulated stud wall so available amperage should be in realms of 46A or thereabouts. Shower is 9.5Kw - 240V, 8.7Kw - 230V. A 40A MCB is only a few quid if so.

In hind site I'm guessing buying a higher Amp RCD would have been better and made the board properly split load, or a RCBO, but it was an impulse buy and I'm stuck with it. What do you reckon?

Thanx again in advance for your replys.
Sponsored Links
Sorry to say a "Consumer Unit" is a type tested device and as such you are not permitted to mix and match you must follow manufactures instructions to the letter and unless the manufacturer does an upgrade kit then either the RCD needs to be external or you need to use manufacturers kit. Most likely a RCBO.

External there is no reason why one should not feed an enclosure with a RCD in although reading your post you would need to change the MCB or RCD anyway as the RCD is too small for MCB ratting. As you say the shower is ratted just under 40A and most likely there would be no problem in using a 45A MCB with the 40A RCD as long as the shower is never changed for anything larger.

Point is some one from LABC will need to inspect and possible test the completed work and they may not permit the use of a RCD smaller than MCB and although an electrician who is registered may do what you intend to pay the £100+ only to get the work rejected seems a big risk and without some qualifications to back up your argument I would not want to take the chance.

Although with correct kit you may cut the buzz bar as said before this has to be to manufacturers instructions.

To use distribution units rather than consumer units you must have someone recognised by the HSE as being competent looking after the installation and the fact that you are asking the question means you would be unlikely to fall into the category.

By time you have got an enclosure for the new RCD and paid the LABC their fees it would be a lot cheaper to get a registered electrician to do the work for you.
Ericmark, many thanx for the reply. :)

To clarify, all the kit (CU, RCD and Split load kit) is MK Sentry. I would hope that if MK sell a split load kit there would be provision to make the relevant modification as you intimated. The only difference is I want to protect one MCB only instead of the usual 7 or 8 as would be the case with a higher amperage RCD.

When I was initially pricing showers and ancillaries, the guy in Denmans said that a split load kit and a RCD was an option, but a RCBO was cheaper, or a seperate shower CU. As the RCD I bought in bargain corner was cheaper than the RCBO I took a punt it would be ok. As the circuit is currently untouched since the spark who installed the CU in 2003, no notification as I understand it is required. Obviously fitting the RCD and/or a 40A MCB would be notifiable.

Didn't really want an external unit in addition to the CU such as a shower CU as its all in the porch and looks a bit untidy when there are loads of boxes on the first wall people see upon entering the house.

As far as competency goes, I worked for a Theatre lighting company from 97-2003 wiring 3 phase, single phase, manufacture of LV transformer packs and illuminated signs (Generaton Game, National Lottery, Blankety Blank, Royal Variety sets etc) and diagnostics at board level. This by no means makes me any kind of expert or certified electrician, but I am by far more than a sunday afternoon DIY shed wallower. The spark who did the initial CU install frankly made a right hash of the job and had to get him back to put it right from errors/ snags that I had found (didnt even connect the shower, and instead taped it up after terminating in choc block!) The fact I'm asking the question is surely not a sign of incompetence is it? :confused:
As you say the shower is ratted

It certainly is!....

Sponsored Links
gotta ask..
if the stick is on the outside of the rat, how is it stuck to it?
most meat cooked on a skewer has the stick through the middle of it..

If I ever encounter, and decide to patronise, an establishment selling rat kebabs I'll let you know.
it's no worse than chicken.. as long as they're bread for the pot and not fished out of the sewers..
Tempiz . its either an RCD unit external to the consumer unit or change the existing board for an RCD protected type.
I see from your post you have experience on Electrics but tampering with Consumer units unless experienced on them is a No No !

I would advize entrusting the work to a " Part P" registered Spark
If you have 7 spare slots in the CU, and if everything is manufactured by MK, then why not use 2 spare ways at the far end of the CU to clip on the new RCD onto the din-rail, and then run the shower phase and neutrals through the RCD, without cutting the busbar at all, and your problem's solved!

One practical method, is to terminate the existing shower cable in the output terminal of the RCD, and use a short length of 10mm twin and earth to span the distance between the existing shower breaker to the input terminals of the RCD, and connect all earths to the earthing bar. :LOL:
Many thanx for your replies guys. I was/am intending to let a Part P spark do the job as my next door neighbour is one. However, when I showed him the RCD and told him what he thought best way to install it was, he scratched his head and said 'dunno to be quite honest, lets look at the instructions....nope still no clearer'. So thats 2 Part P sparks I've come across with little idea of what's going on inside these boards, the first being the guy who did the install in the first place.

Kai, your suggestion is inspired! my only concern is the proximity of the busbar to the RCD if its mounted on the din rail above it, in particular the 'protected' neutral that is coming out bottom left. Can it arc potentially?
So thats 2 Part P sparks I've come across with little idea of what's going on inside these boards, the first being the guy who did the install in the first place.

If that is the case then either A. They dont want to do the job..or
B. The "P" in part P refers to Pathetic
in their case

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links