split load consumer unit

12 Nov 2005
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United Kingdom

I’m thinking of changing my consumer unit into a split load. At the moment all of the circuits are on an Rcd which is combined with a disconnector switch for the incoming supply, it takes up 4 modules in the unit. There is nothing wrong with it other than that if it trips (which it does occasionally) it takes the light circuits with it and I would like to take the light circuits onto a non Rcd protected circuit to stop this happening.

It sits in the usual place at the far left of the CU with the Mcb’s running off to the right.

It is an MK sentry unit circa 1993 and it was designed to be used for a split load so there are 2 separate neutral bars. As far as I can see doing a split load conversion as per the Mk instructions on their website by buying a new disconnector switch, 30ma Rcd, busbar’s and a split load wiring kit would be straight forward.

Having thought about it this would be more expensive than keeping my existing Rcd/ disconnector, just buying another ordinary disconnector switch and adding the switch further along the Din rail, rewiring it with it’s own busbar and taking the neutral link out etc. This would leave me with the switch/rcd on the left, then the protected mcb’s followed by the disconnector switch and then the light circuit Mcb’s.

Is this idea ok or is it mad and illegal. I would have to double up the supply cables from the incoming side of the existing Rcd/ disconnector switch to get the power to the new switch, is this allowed. The unprotected circuits would be on the right, usually they seem to be on the left and lastly it seems confusing or possibly illegal to have 2 disconnector switches on the CU.

Advice and your thoughts please, If it isn’t right I’m quite happy to spend the extra money to do it the standard MK way and comply with the regs, it just looks cheaper this way.

Lastly, if I split load it what’s your views on having the lights on their own Rcbo’s, there’s 3 light circuits and there isn’t room to get another rcd plus 3 mcb’s in so I know this would be pricey but what I like about my existing installation is the fact that every circuit is rcd protected.

Btw, there is an isolator fitted so I have no problem switching the power out to work on the CU
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You shouldn't be installing Consumer Units without RCD protection unless you can be sure other measures for cable protection are in place.

Your idea for converting the CU is a bad one & would not comply.

Fit a 17th split board if you must, but your lighting will still be RCD-protected.

Best advice is a 100A switch incomer board with RCBO's, then every circuit is independent.
Gosh, that was quick. I will abandon my plan then as mad and illegal.

Are you saying doing a split load board as per the Mk website and info leaflet , eg main disconnector, unprotected mcb's eg, light's then Rcd followed by protected mcb's is not a good idea, that seems to be the MK standard split load layout.

Rcbo's everywhere is a great idea but i've got 10 circuits so would cost a small fortune
The split load configuration that you describe would not now meet the requirements of the new wiring regulations for most domestic installations.

Please note also that changing a consumer unit is not just a wire swop and is notifiable under Building regulations Part P.
See the WIKI for more info.
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You can get a 17th edition board for £55 from toolstation i have seen these and the quality is good. Plus it comes with 2 RCDs and MCBs with space for RCBOs too.
I might not have explained myself clearly. I'm not trying to change the CU, just convert the existing one to split load. Forget the original plan that's obviously not compliant and i'm not doing it

As I said the CU I have has the dual neutral bars and the instructions I still have for it clearly say it can be used for a split load configuration

If I change the CU I get into all the part P notification stuff. All i'm trying to do is get some discrimination between the lighting and power circuits so the lights don't go out if the rcd trips. Changing the whole CU is a pointless hassle

If I change it to the split load configuration but use rcbo's for the three lighting circuits and then an rcd for everything else that would appear to give the same level of protection as the 17th edition board that DESL has just posted

So the new plan is:

Convert existing CU to split load configuration as follows

New 100amp disconnector switch.

Load 1, 3 lighting circuits on rcbo's

Load 2, new 30ma rcd with mcb's for all circuits

Does this now sound ok, can I do it, have I understood this correctly
You've forgotten some other essentials:

Upgrade all earth bonding to meet current regulations
you will be chnaging the characteristics of the circuits so you will need calibrated test equipment and the knowledge of how to use it.
For each circuit you chnage you will need to:
  • Check insulation resistance
    test if the Earth Fault Loop Impedance is low enough to meet the requirements of the circuit. - note that many RCBOs are C curve. The MCBs on the circuit will probably be B curve. Do you know what that means?
    Test that the RCBOs meet their specification/
Thanks for that TTC

The earth bonding was upgraded within the past year or so when I changed a water main to plastic, there was thread here about it. I checked this. The water pipes and central heating were bonded back to the main board outside but I would have to check exactly what I did again

The curve refers to the trip sensitivity of the MCB, type C being less sensitive than B. So yes I understand what you mean but realistically I can't test it without access to the test kit which is difficult but not impossible for me. Couldn't I just fit type B's (if I can get them) and maintain the existing sensitivity.

The whole house was tested about a year ago when a sparks extended a ring main, he tested the insulation resistance and the EFLI and said they were good but obviously he was only testing it as it was not as it will be.

So, given that I cannot realistically test it myself is the advice here to leave it alone and take my chances in the dark when the rcd trips the whole CU out.

This is an improvement not a necessity so I could just leave it be.
If I change the CU I get into all the part P notification stuff.
Just as you do if you change anything in the CU.

If you do any work whatsoever on your installation, if it isn't explicitly made non-notifiable in Schedule 2B then it is notifiable.
So theres my solution BAS. I notify the La and they send their appointed person round to double check my handiwork.

Any views on my proposals more generally, ie should i leave it alone, good idea, bad idea or what.

I liked your electrical sticky here and on the ultimate handyman site btw

ps Hope your missus is ok these days

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