Add RCD to part of Split Load CU/Isolation Switch

[daverk]

My CU is a Legrand Split Load board with

100A DP Main Switch (2 module wide) feeding 5 Unprotected MCBs
80A DP RCD (2 module wide) feeding 5 Protected MCBs.

Wanting to improve protection, I had hoped that the 5 unprotected MCBs could be replaced with single module RCBOs but no such luck - the RCBOs are too tall and they would be obstructed by the cables connecting to the neutral terminals above. Basically the CU is not tall enough.

I don't want to lose any of the 5 + 5 MCB capacity.

So the question is, what is the best/economic/allowable solution?

- replace the DP Main Switch in the CU with a DP RCD so that the unprotected MCBs are then protected?

- as above, but with a new separate main isolating switch (can easily be mounted on side of meter cupboard)?

Regarding the latter, does anyone know whether Central Networks or their meter operator (think it is Siemens) are installing isolating main switches for free on request, because obviously that would help a lot!

 

[daverk]

Just to add - clearly there would need to be a Henley Block fitted upstream to provide separate feeds to both RCDs, otherwise there wouldn't be discrimination.

 

[GaryMo]

Before changing the main switch for an RCD, installing an external main switch and a henley block I would opt for changing the consumer unit to a high integrity dual RCD unit.

 

[ericmark]

There is a problem with Part P but for moment I will forget about that.
To add a RCD you could fit one next to the existing consumer unit but in real terms this will not really be best option. The plastic housing of the consumer unit is cheap under £20 so the best option would be to change the box.
Of course there is nothing to stop you putting in a second box with 5 RCD's or RBCO's but would not really make sense.

Work on the consumer unit is grey area with Part P. One can replace faulty items but one should not modify and for some reason the Part P regulations seem to have a different definition for a circuit to the IEE regulations. A fused connection unit under IEE regulations forms a new circuit but not it seems under Part P. So taking circuits under 13A and feeding through a RCD FCU would be accepted.

At £35.75 for a RCD and £33.45 for RBCO fitting external RCD's is very little price jump to fitting RCBO's but for 5 we are looking at £178.75 plus £20 for box and one must ask ones self is it really work the messing around and although you may find loop holes to allow one to get around Part P at the end of the day can't really see it is all worth the effort.

Personally I would still fit a box with 1 RCD and some RCBO's and the sockets would have RBCO's as it is the sockets and the items plugged into them that causes the RCD to trip although on odd time a bulb blowing will cause it in the main it is computers and the like that cause the problem.

A risk assessment is really required. What will happen if your lights fail? In my case the emergency light at the top of the stairs and in the garage light and I can easy find my way to garage to reset RCD.
My daughters house the stairs and consumer unit are lit by street lamps.
My son has all 12vdc lighting throughout his home (A narrow boat).
My dad like daughter has street lamps and chair lift has battery back-up.
Other daughter has two circuits one up-stairs other down so half lights still work. Once you have got it in your mind what the dangers are you can make a considered judgement as to if twin RCD's or if RBCO's is the way forward.

To pay council £100+ to DIY plus another £100 to hire test instruments seems silly better paying an electrician and let them do the work. There labour costs will be little different to council charges and when one adds RCD's there are often problems and it would seem better to let that be someone else's problem.

 

[ban-all-sheds]

Just to add - clearly there would need to be a Henley Block fitted upstream to provide separate feeds to both RCDs, otherwise there wouldn't be discrimination.

How would you do that, given that the CU is not designed to be supplied like that?

 

[daverk]

Just to add - clearly there would need to be a Henley Block fitted upstream to provide separate feeds to both RCDs, otherwise there wouldn't be discrimination.

How would you do that, given that the CU is not designed to be supplied like that?

Well, I wouldn't do it myself, because currently the supply from the meter/main fuse is live directly to the main DP switch on the CU, so I wouldn't be mucking around with that myself. I saw about this on the IET forum, so assumed (perhaps wrongly!) it was do-able: http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=35288

 

[flameport]

While what you suggest is possible, it really won't be worthwhile.
Your solution is:
Purchase: new tails, henley blocks, 2 pole RCD (which would have to be a suitable type for your existing CU), 2 pole main switch, plastic enclosure for main switch.
Spend a significant amount of time installing the above, and reconfiguring the consumer unit to accept 2 separate supplies.

A far better and probably cheaper solution would be to buy a dual RCD consumer unit and install that.
This for example:
http://www.fastlec.co.uk/hager-17th-edition-way-dual-rcd-consumer-unit-mcbs-p-3662.html
plus a couple of extra MCBs probably costs no more than all the other separate items, and would be much easier to install.
You end up with a single unit, one set of tails from the meter, and no need for useless separate isolators, henley blocks or whatever.

As a third (and almost no-cost) option, you could move some of the existing unprotected MCBs to the protected side. What actually are the 10 existing circuits?

 

[daverk]

There is a problem with Part P but for moment I will forget about that....To pay council £100+ to DIY plus another £100 to hire test instruments seems silly better paying an electrician and let them do the work. There labour costs will be little different to council charges and when one adds RCD's there are often problems and it would seem better to let that be someone else's problem.

Thanks ericmark I take your point about costs and because the supply is live to the main switch it's a no-brainer anyway - a Part P man would have to do this. I'm trying to get a feel for the "right" (and cheap-ish) way to go, though, because there seem to be many different opinions.

Regarding risk, we are in the sticks so I have plenty of battery-powered lights in the event of a lighting failure. But one or two RCBO-protected circuits might make sense for convenience, and if additional or replacement units need to be installed anyway.

 

[plugwash]

Another option to consider is adding RCDs in seperate enclosures on the outputs of one or more of the MCBs.

 

[daverk]

A far better and probably cheaper solution would be to buy a dual RCD consumer unit and install that.
This for example:
http://www.fastlec.co.uk/hager-17th-edition-way-dual-rcd-consumer-unit-mcbs-p-3662.html
plus a couple of extra MCBs probably costs no more than all the other separate items, and would be much easier to install.
You end up with a single unit, one set of tails from the meter, and no need for useless separate isolators, henley blocks or whatever.

As a third (and almost no-cost) option, you could move some of the existing unprotected MCBs to the protected side. What actually are the 10 existing circuits?

Many thanks - very helpful. That flexible configuration Hager board looks good and would just about fit the meter cupboard.

The 10 existing circuits on the CU are:

5 Protected from 80A RCD: 40A Shower, 32A Oven, 32A Sockets, 32A Sockets, 32A Kitchen Sockets
5 Unprotected from Main Switch: 16A Shed#2 Sockets/Lights**, 32A Hob, 6A Downstairs Lighting, 6A Upstairs Lighting, Outside

** via 63A RCD in shed#2 with MCBs 6A Lights, 16A Sockets

It's an old house, I moved here in 2009, the Legrand CU has an electrician's test sticker from 2002.

There is also another box fed from a Henley after the meter, containing a separate 40A RCBO protecting Shed#1 and the study (!), probably 2004 vintage.

The circuits outside, all sockets and the shower are all protected by residual current devices but there are 2 problem areas: lighting in the upstairs bathroom (which has the protected shower) and lighting in the downstairs shower room/WC (non-electric shower) - these are both on the <i>upstairs</i> lighting circuit which is not protected.

I coming to the conclusion that the best plan would be to check exactly which circuits are controlled by which MCB, draw them up, then get an electrician in to advise & quote. Because the wiring appears to have been extended at separate times, I'm a bit worried that combining the power circuits (sockets) to free-up a protected circuit for the upstairs lights might increase the load too much on the socket circuits. Sounds to me as though the flexibly-configurable dual RCD board is the most likely contender. Any further thoughts would be welcome.

 

[daverk]

Another option to consider is adding RCDs in seperate enclosures on the outputs of one or more of the MCBs.

That's a thought. One of the sheds is run from a 16A non-protected MCB to a 63A RCD protecting 6A shed lights and 16A shed sockets, so a previous occupant or electrician has done something similar before.

 

[flameport]

Of the unprotected circuits, there isn't much to be gained by adding an RCD to the hob circuit.
The sheds don't need another one, since they already have separate RCDs.

This leaves the lighting, which would only require RCDs due to the bathrooms, and cables being buried in the walls (above the switches), and both of these were not required when the existing unit was installed.

The external RCD option could be used for those, you would need one of these:
http://www.tlc-direct.co.uk/Products/BHCT4.html
and either one or two of these:
http://www.tlc-direct.co.uk/Products/HGCD263U.html