CU for HMO

[tabbypurr]

Current fusebox has several fuseways each feeding a meter then a 2 way fuse box which powers each rented room. Plan is a new CU & new secondary 2 way C Us, the meters will be retained. The question is how may RCD protection be implemented. If done in the main CU with an RCBO, a trip results in a room losing all power including lighting. If done in the secondary small CU, the cable from main CU to meter to secondary would not be RCD protected. This wiring is surface clipped direct. Neither of those options strike me as ful ly satisfactory. How's good to do this?

NT

[Fredxx]

I thought it was normal to use a 100mA time delay RCD at the 'main' CU?

It does mean the wiring to each rented room would need to have mechanical protection or be surface wired, etc.

[John Rumm]

So not an acceptable solution...

If its clipped to the surface, then it does not require RCD protection. (assuming the earthing system is not TT)

If you need RCD protection for the submain, then use a type S device at the main CU.

[Tim Watts]

The link circuit is classed as a distribution circuit. It need not be RCD protected if it meets various criteria. Being visible is one. Another is to mechanically protect it, eg SWA, heavy steel conduit.

However, I have this setup for a sub-CU that deals with "outside things" (3 circuits). I opted for a single RCBO up the head end.

But I think in your case, as renters are more likely to cause trips and you don't want other renters being affected, a non RCD distribution circuit to the room and 2 RCBOs would give the best setup (lights don't go out of they trip the socket circuit).

[Tim Watts]

That's for TT earthing. 100mA Type S RCD does *not* provide "personal protection" which is the factor here.

[Fredxx]

I thought the 100mA was primarily for fire protection rather than just a requirement for TT earthing?

In this instance I had also assumed that each room would have its own CU with two RCDs or perhaps one with some form of emergency lighting.

Its also not unknown for lighting to be on a permanent, separate circuit, such it is not powered through a coin slot meter.

[John Rumm]

Not if there are downstream 30mA trip devices...

Both - either for installation protection, or for circumstances where the earth loop impedance is too high to reliably trip a MCB under fault conditions.

If they are independent, (e.g. RCBOs) then you can skip the lighting unless you are doing something particularly risky in there...

[ARW]

Just to add to that last bit. It depends on why you need the RCD protection. If you need RCD protection other than for a TT supply (ignoring stables etc) because the regs demand it due eg buried T&E then it must be 30mA non time delayed.

[ARW]

Surface clipped T&E does not need RCD protection. You can always trunk it if you prefer.

Ideally the rooms should have 2 RCBOs in their CU.

[ARW]

That would be 300mA RCD eg stables

That would be nice, but of no use if there is a 30mA RCD in series with it.

Yes, if done properly. Certainly for the communal areas. I am not sure what the regs are for em lighting inside a HMO room. It certainly had non on the last one I worked in.

[tabbypurr]

fusebox which powers each rented room. Plan is a new CU & new secondary 2 w ay CUs, the meters will be retained. The question is how may RCD protection be implemented. If done in the main CU with an RCBO, a trip results in a r oom losing all power including lighting.

econdary would not be RCD protected. This wiring is surface clipped direct.

I didn't think to mention the earthing: it's TT. So everything must be RCDe d.

Leaving the lights out of the metering would make life a lot easier, presum ably the secondary CUs could be dispensed with entirely. But it does create some problems down the line, so I'm not sure that'll be chosen.

NT

[tabbypurr]

The communal areas are unmetered.

Is it acceptable to use just one CU, like so: Shared 300mA RCD feeds an MCB for each room. Each MCB output goes to a meter and back into the main CU, where it feeds 2x RCBOs for lighting & sockets. There are then no secondary CUs.

NT

[John Rumm]

How many rooms?

You would need a separate set of busbars for each room then, in addition to a set for the main feeds to the meters. That may prove tricky depending on how many rooms there are. So you would need a main switch position with your type S RCD, plus a MCB for each room's meter feed. You then need a main switch for each room plus 2 x RCBOs.

So if it were say three rooms, that would be 5 ways on the supply side, plus 3 x 4 ways on the secondary side. 17 ways in total and 4 discrete sets of neutral bars. You may find some CUs designed for a 4 way split, Adam will probably be better able to advise.

Since its TT and you need all circuits RCD protected, you will have to ensure any submains are routed in such a way that you don't trigger the requirement that they be protected by a 30mA RCD, else you lose discrimination.

[Tim Watts]

100mA Type S at the head end 30mA RCBOs in the room.

But you will still need to treat the cable as non-RCD protected for the purposes of how it is routed as buried non mechanically protected cables need to have RCD protection at 30mA/40mS.

So to avoid confusion:

100mA Type S: Provides fault protection on the cable as L-E fault currents with a TT rod will generally not trip an MCB or fuse in the required time. 30mA/40mS RCD for personal (shock) protection.

If you are worried about the cable route, use SWA - it's going to be small for these circuits, it's cheap enough and you can run it how you like.

[Tim Watts]

Why 300mA RCD? 100mA is more usual for TT

I think you'd be wise to make the socket RCBO available in the room if these are rented in a small enclosure. This is what Premier Inn do for each room (in most of the ones I've been in) - then the guest can at least reset it if they trip it.

[ARW]

The 30mA RCBOs need to be DP switched.

[tabbypurr]

4

why a main switch for each room?

RCBOs means each circuit neutral fed from its own RCBO

they're surface wired, clipped direct

NT

[John Rumm]

You need DP isolation for TT, but the switch at the head of each sub section would achieve that would it not? Or am I missing something?

[John Rumm]

That's going to be pushing it a bit for one CU...

Its one way of getting round the limitations on the number of available neutral bus bars - you could return the circuit neutrals to the neutral pole of the switch (since there will only be two circuit). It also gives you a convenient method of isolation for the room.

and in turn those need to be fed from somewhere...

[1] Adam makes a good point that I did overlook though. Using single pole RCBOs like this potentially causes a loss of discrimination if you were to ever get a neutral to earth short. It would trip the local RCBO, but that would not clear the actual fault which would still be visible to the head end RCD. Hence it would trip the whole installation.

The only way round that would be using double pole RCBOs, or using another type S RCD in the place of each of the main switches.

So at the moment, no need for a 30mA RCD on the submains.

[Tim Watts]

And as a side note, Hager sell DP RCBOs (industrial range) but they are compatible with their consumer boards.