CU for HMO

[ARW]

I think I have a solution that would please you. Give a bit of time I have just got in from work. It does away with the 100mA RCD main switch and only used SP RCDs.

[tabbypurr]

How would I feed the secondary(ies) from the first CU? Ie what electrical protection would the interconnecting cable need?

This 21 way might do the job, though it comes with the wrong RCDs:

NT

[tabbypurr]

I await with interest. There's one way I can think of to do that, TN-C, but I don't think that's allowed now. Not on T&E anyway, which everything is wired with.

NT

[John Rumm]

They would be fed from the return sub main from the meters. If they are also surfaces wired then they don't require 30mA trip RCD protection. So a type S 100mA trip device as main switch in primary CU. MCBs on feeds to meters. Returns from meters feeding whatever you decide to go with (RCBOs etc)

Might be better buying an empty CU and reusing what you can and buying what extra devices you need.

[tabbypurr]

I'm not sure that would work. It needs 4x RCDs total, doing the above would not farm any of the RCDs out to the 2nd box, so wouldn't enable use of 2x split CUs. I'd still need a box with 4x RCDs, and haven't seen any.

On second look I realised I could work it with the RCDs supplied, and 21 ways is just enough. So maybe I've found the right CU at last.

Didn't like the prices of industrial units.

NT

[John Rumm]

You can make your own sub divisions - you don't have to go with what is provided. If you start with a "high integrity" CU - that will normally have provision for at least three main switching elements (typically a main switch, some space for MCBs/RCBOs, then a pair of RCDs with their own bus bars in typical split load configuration).

I think you need to decide what topology of devices you are going to go with, then it will be easier to work out how to "package" what you need.

To an extent you get what you pay for. The industrial stuff tends to be more configurable, and flexible, better made, and more wiring space (not to mention capabilities for things like 3 phase, and high breaking capacity control gear).

[ARW]

I do apologise for the delay. The World Cup and having to travel to Durham everyday has left me little time. I also forgot that I had a suggestion to make because of the number of World Cup matches I have to watch in the pub.

Get rid of the 100mA main switch and just use a normal main switch with SP switching RCBOs for all circuits.

Feed the rooms meters off say a 32A or 20A RCBO (depending on what the socket circuit is powered at) you then either use the fused connection unit in the room for the lights that I suggested or back feed from the meter to a 6A MCB in the CU for the lights.

The rooms will share a RCD but who cares?

If you want DP isolation for the rooms then fit a cooker switch before the meter.

[tabbypurr]

we know how that works

I didn't realise one could omit RCD action for N-E faults. It would simplify things.

I'll go with fuses otherwise the required CU becomes 25 ways.

hmm, I'd not thought of commoning the meter supplies.

no need for that. N-E faults are almost always going to be in appliances.

If only SP RCD action is permitted this simplifies things. Cheers.

NT

[ARW]

You still have NE protection

I was still thinking of one RCD for each meter and each room, I meant that the lights and sockets in that room would share a RCD

It is allowed as long as every circuit is RCD protected and there is no

100mA mainswitch.

[tabbypurr]

yes, the penny has dropped. The relevant SP RCBO would disconnect the load, thought not disconnect the N-E fault.

yes will do that, using your FCU strategy

That would make life a lot easier. John reckoned DP RCD cover was required for TT - I may have to go reading.

Cheers

NT

[ARW]

The main switch is your DP cover. It need not be RCD DP protection.