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davesch

My Son has just moved into a flat with 2 storage heaters on economy 7. The CU is an older MK sentry one with 2 incomers fed from separate feeds from the meter, 1 permanent, 1 for economy 7. There are no RCDs which is not ideal. Options for rectifying that as I see it are:

Replace current setup with 2 new boards, a 17th edition one for the permanent supply and garage unit for the eco7. That's probably the best option, but due to space and cable runs will be difficult.

Change the 2 MK LM5500 incomers with RCDs. That's probably the easiest route, but the newer MK RCDs are fitted on the left rather than on the right as per the current setup meaning the L & N would be the opposite way around. Although I don't have a MK RCD to hand, the pictures online don't show L & N markings, so I'm unclear if they are reversible.

Last option is to fit a 17th edition board, losing the incomer giving me 2 more MCB positions and feeding straight into the 2 RCDs, one for the Permanent supply, one for eco7

The last 2 options would of course mean all lights and sockets would be fed from the same RCD

Which of the above options would be acceptable ?
 
Fit a triple RCD board (Hager make them) and fit a four pole linked main switch. Two poles for E7 feeding one RCD and it's protected storage heater circuits, two poles feeding two RCDs for the rest of the circuits. Another way is a split load board with a four pole linked main switch. two poles feeding the RCD and it's protected storage heater circuits, two poles feeding RCBOs for the rest of the circuits.

I'll be clear one one thing; you do not lose the main incomer!

There is a myriad of options available to you, and two seperate boards doesn't have to be one of them.
 
I'm literally just about to do one this way today: wider 17th than needed, extra RCD, E7 tail into third RCD out to short bus bar to E7 MCB's. All I needed to buy was 1 RCD and a link pack. Admittedly, it falls short on not having one main switch but then so does having two boards. Yes, it makes Neutral isolation for working on it afterwards a couple of seconds longer, too.
 
I'm literally just about to do one this way today: wider 17th than needed, extra RCD, E7 tail into third RCD out to short bus bar to E7 MCB's. All I needed to buy was 1 RCD and a link pack. Admittedly, it falls short on not having one main switch but then so does having two boards. Yes, it makes Neutral isolation for working on it afterwards a couple of seconds longer, too.


Which board are you using ? How are you getting the extra Neutral Bus ?
 
Board is a Lewden, and some terminal bar.
 
Why would losing the incomer not be an option ? An RCD switches both L & N and a garage unit for example only has an RCD.

Single point of isolation. If you read the requirements in section 516 I believe, it ultimately comes down to turn interpretation of 'source'. Personally, I see E7 as being from the same supply/source, therefore a single point of isolation would be necessary.

There are also other reasons why having RCDs as main switches is a bad idea, primarily the segregation of circuit protection to minimise inconvenience in the event of a fault.
 
Single point of isolation. If you read the requirements in section 516 I believe, it ultimately comes down to turn interpretation of 'source'. Personally, I see E7 as being from the same supply/source, therefore a single point of isolation would be necessary.

There are also other reasons why having RCDs as main switches is a bad idea, primarily the segregation of circuit protection to minimise inconvenience in the event of a fault.

But that's the point, the E7 and Main supplies come on different feeds from the meter so there can be no single source of isolation. If you take the other suggested option of a 3 RCD board, 2 of them would go through the incomer, the E7 wouldn't.
 
But that's the point, the E7 and Main supplies come on different feeds from the meter so there can be no single source of isolation. If you take the other suggested option of a 3 RCD board, 2 of them would go through the incomer, the E7 wouldn't.

Hence a four pole main switch
 
Fair point, though that's not what Rockingit is proposing to do for his job today. Are you saying he is wrong to do what he is suggested ?

In my opinion, yes. But that's not a slight on his competence, he even says in his post that it falls short on having one main switch.
 
In my opinion, yes. But that's not a slight on his competence, he even says in his post that it falls short on having one main switch.
Only because they don't make a 4p switch for that board, btw! It did get asked for. ;) And THAT board....because it will fit in the space where others won't. There's always a reason!!!
 
Sorry, I don't follow. There would be 2 x 2 pole RCDs, one for E7 one for the rest.

But you wanted to use them a a main switch, there should only be one main switch for an installation. Therefore with 2 x supplies you need a 4 pole switch.

If it was allowed then Twin RCD CUs wouldn't need a main switch, you could just take two pairs of tails in from a Henley block!
 
I remember though on a job a few years back, that I'd taken all three tails into a 3p standalone isolator first, before then running out to the two boards - and the meter fitter refused to make them live as he didn't understand the purpose. You just can't win sometimes.
 

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