Twin RCD CUs

[trey]

Just out of curiosity, with these new "17th edition" consumer units, I wondered how the two RCDs would be used. I considered having two went I was planning my layout a few year ago.

I just don't see how you could make things that much more convenient. Okay you could have lights and shower on one, and then the hoover blowing up doesn't spoil your shower, but the shower shorting out leaves you in the dark.

Are there some statistics to show which are the most likely circuits to have faults?

I'm now about to have some work done on my CU, and adding another RCD is probably compulsory, the instantaneous load could reach 90A and the main fuse and isolator are both 100A but the RCD is only 80A. In this case I'll ask for the shower and elec boiler which are configured to operate only one at a time to be on one RCD and the rest on the other - unless there are some words of wisdom from here (or the sparkly).

 

[JohnD]

RCD tripping most often comes from watery appliancs (kettles, steam irons, immersion heaters, CH boilers and pumps) and those with heating elements (especially ovens)

Electric showers have both water and heating elements

Also it come from outdoor circuits which get water in them.

If you are splashing out on a rewire, consider the use of RCBOs. they work out more expensive as you have one per circuit, but are a very good solution.

 

[Spark123]

The idea is to spread the circuits from different areas around the two RCDs, for example you may have downstairs ring on one RCD, the downstairs Lights on the other. You may have upstairs sockets on one RCD, the Upstairs lights on the other. You may have the electric shower on one RCD, the bathroom lights on the other. It is basically so you do not lose everything in the same area at once. Certain things should not be put on the same RCD as sockets, for example mains smoke detectors. Some of the new twin RCD CUs have a couple of non-RCD protected ways in them for RCBOs (RCD and MCB in one unit) to be installed for such purposes.

 

[hairyben]

Presumably your current CU is split load and fairly recent? In which case you could swap out the non-RCD protected MCB's for RCBO's, quicker, cheaper and better. (provided single-module RCBO's fit in your CU!!)

I've yet to fit a twin-trip board, it'd be a joke on a large property and small property (4/5 ways) RCBO's work out no more expensive anyway. Their very existence makes me question the wisdom of the rule-makers.

 

[sm1thson]

I've yet to fit a twin-trip board..... Their very existence makes me question the wisdom of the rule-makers.

it wasnt the rule makers that designed the twin RCD board!

 

[hairyben]

I've yet to fit a twin-trip board..... Their very existence makes me question the wisdom of the rule-makers.

it wasnt the rule makers that designed the twin RCD board!

The way the rules were written suggested/allowed for them, can't understand why they didn't just say "RCBO's only", all the various things we do in the name of safety/convenience and we have a glaring confliction like putting circuits on a RCD with an immersion

 

[Spark123]

You are not putting the circuit on an RCD because it in an immersion, you will need to protect it if an unprotected cable is installed concealled in a safe zone in a wall or partition less than 50mm from any surface.
If you can install your immersion by another allowable method for concealled cables or not conceal it at all then you do not need to RCD it (unless for another reason i.e. in a bathroom, TT etc)

 

[trey]

Thanks for replies. I do have split load, and it's large enough for two rcds, isolator and all my mcbs (it must be 12 way after counting the isolator and rcd).

Will probably go for shower, boiler, immersion on one, rest on the other. The RCBO route would be too expensive in this case as I have all the MCBs and the box is big enough, so 1 RCD vs 3 RCBO.

Not fussed about the light on/off thing as the house is easy to navigate in the dark and have battery light that comes on when I open the cupboard door with the CU hidden inside.

So if the missus has a shock in the shower (ooh errr) then it won't knock the cooker off or spoil watching the telly. I'm sure that's what the 17th ed guy were thinking

 

[hairyben]

You are not putting the circuit on an RCD because it in an immersion, you will need to protect it if an unprotected cable is installed concealled in a safe zone in a wall or partition less than 50mm from any surface.
If you can install your immersion by another allowable method for concealled cables or not conceal it at all then you do not need to RCD it (unless for another reason i.e. in a bathroom, TT etc)

Fine on new work, well, "workable" at least, but it's when renewing CU's that you get the conflict. Is it possible to buy a guaranteed long life no-leakage immersion? or a 3kW isolating transformer?

 

[hairyben]

Thanks for replies. I do have split load, and it's large enough for two rcds, isolator and all my mcbs (it must be 12 way after counting the isolator and rcd).

Will probably go for shower, boiler, immersion on one, rest on the other. The RCBO route would be too expensive in this case as I have all the MCBs and the box is big enough, so 1 RCD vs 3 RCBO.

Not fussed about the light on/off thing as the house is easy to navigate in the dark and have battery light that comes on when I open the cupboard door with the CU hidden inside.

So if the missus has a shock in the shower (ooh errr) then it won't knock the cooker off or spoil watching the telly. I'm sure that's what the 17th ed guy were thinking

The board layout with twin RCD is dictated by areas ie up lights & down sockets on one, up sockets and down lights on the other so there is still power of some sort to all areas when any one earth fault occurs.

RCBO's have plummeted in price recently BTW.

 

[Spark123]

You are not putting the circuit on an RCD because it in an immersion, you will need to protect it if an unprotected cable is installed concealled in a safe zone in a wall or partition less than 50mm from any surface.
If you can install your immersion by another allowable method for concealled cables or not conceal it at all then you do not need to RCD it (unless for another reason i.e. in a bathroom, TT etc)

Fine on new work, well, "workable" at least, but it's when renewing CU's that you get the conflict. Is it possible to buy a guaranteed long life no-leakage immersion? or a 3kW isolating transformer?

Not heard of a guarenteed life one. You are going to have a similar situation with most heating elements i.e. electric cookers. It is up to the installer to assess what he is putting on each RCD, maybe use RCBO for those loads? Still won't stop it tripping but will limit the inconvenience caused.
You can buy any size isolation transformer you like - might be a bit cost prohibitive tho.

 

[hairyben]

Not heard of a guarenteed life one. You are going to have a similar situation with most heating elements i.e. electric cookers. It is up to the installer to assess what he is putting on each RCD, maybe use RCBO for those loads? Still won't stop it tripping but will limit the inconvenience caused.
You can buy any size isolation transformer you like - might be a bit cost prohibitive tho.

which is why I say RCBO's are the way forward, but while the immersion will be the only circuit to trip it won't make the customer more understanding when they've got no hot water. What annoys me is I can't offer an effective, sensible solution. Earth leakage is inherent to many items... just the other day a customer of mine had to replace an 18 month old brand name dishwasher. I really think there should have been exceptions in the case of retro-fitting consumer units. I have a customer right now I'm considering trying to talk out of having CU's in flats he lets changed, which flies in the face of everything I've done up to now.

 

[trey]

As the rules require greater and greater protection or complexity so the cost increases. I worry about the people who can't afford to pay several hundred for a CU upgrade when an old fuse or circuit fails, and instead resort to something "inventive" like doubling up on fuses.

When we first bought our house a few years ago, it had a piece of 10mm2 copper across the main (MEB) fuse. It had a mis-wired RCD (and broken) and 4 mains circuits (kitchen radial, ring, loft radial, immersion) wired in to two 32A breakers, the ring had one wire in each! The worst thing is, you don't find this out from a basic survey of the wiring (ie check cable type, estimate cable age, check consumer box style and number of breakers). How many of you would hit the test button on the RCD when you're looking round a house - I know I would next time.

 

[hairyben]

As the rules require greater and greater protection or complexity so the cost increases. I worry about the people who can't afford to pay several hundred for a CU upgrade when an old fuse or circuit fails, and instead resort to something "inventive" like doubling up on fuses.

This is a common problem for people like me, especially with the increased liabilities of part P. A simple, easily fixed problem becomes majar work to bring the system up to standard, and that obviously plays into the hands of those who'll bodge it instead. Sometimes you have to leave people knowing they'll have bodged what you could have addressed but not fully to the latest regs, and within their budget.

All I can offer is work done by a proper spark will go the distance, and quite often will be cheaper in the long term.