CU for HMO

[tabbypurr]

RCDed on this TT system, either at 30mA or 100mA time delayed.

yes, I misread that

equired, meaning both can run off the same RCD. That certainly simplifies t hings, as you say a garage CU would do the job. I was looking at a single l arger secondary CU with SP RCBOs because that would be cheaper & neater, bu t if SP RCBOs are no good then so be it.

yes, all power to a room being cut off in the event of credit running out i s standard practice.

CBOs sold? There's something I'm not clear on yet here.

gotcha Presumably for the same reason it wouldn't be well suited to TN-S either, o nly for PME supplies.

xing of cables in escape routes has come up. I presume the usual plastic ca ble clips will no longer be permissible, are we going to go back to metal b uckle clips? Is their lack of insulation ok today?

In this case the submain wiring is unsupported & there's no possible way to clip the cables using any known cable clip product. Insulated copper cable & wafer head screws should meet the requirements.

NT

[John Rumm]

I don't recall DP being a requirement for TN-S. While you are likely to see a bigger voltage between N & E at the premises, they are still tied together typically within a few hundred yards. The supply cable earth is also properly bonded to a substatial earth at the sub station, so there are not the same risks as there might be with a puny local earth rod.

[tabbypurr]

ok cheers. So the TN-S place can use the SP RCBOs for the final circuits. Do they still need to be fed from 2x DP RCDs to provide N-E fault protection?

NT

[ARW]

Well it's only a problem as you have a TT supply with a RCD main switch.

There use is fine. If using trunking there are special clips that fit inside the trunking.

[ARW]

As it's a TT supply then a 100mA RCD and surface mounted cable is fine.

There is no requirement for the lights and sockets to be on seperate RCDs apart from inconvenience in the case of a trip.

The communal lights should be on a separate RCD to the rooms.

[ARW]

It would have to be 100mA S type for the meters

I am trying to think of a way to simplify it.

[ARW]

Ways to simplify and keep down costs bearing in mind that it's an HMO and no one of any importance will live there.

Lets assume that we are now going to put the lights and sockets in each room on a shared 30mA supply.

1.Use a 5A unswitched fused spur in the room to power the lights from the sockets. This saves space in your CU and will not be a problem. 2.This now means you just need 1 DP switched RCD in the CU for each room, these can be DP or SP width depending on what you want to pay (the Wylex SP width DP switching ones I linked to were about £26) 3.Use a hi integrity split load CU with a 100mA main switch.

4.Put the RCBOs on the 100mA side of the board (this will take 4 to 8 ways up of the CU).

5.Feed the room meters from the RCBO and then from the meter to each room.

6.Use the 30mA RCD protected spaces to power the communal areas

So if using Wylex stuff about £250.

[tabbypurr]

CBOs sold? There's something I'm not clear on yet here.

xing of cables in escape routes has come up. I presume the usual plastic ca ble clips will no longer be permissible, are we going to go back to metal b uckle clips? Is their lack of insulation ok today?

Yup. It turns out there's no space for trunking or clips, the bundle tied w ith electrical singles screwed to the wall looks the only viable fire resis tant option. It will be out of view, but not to the extent that it becomes a drilling hazard.

NT

[ARW]

electrical singles?

explain please.

[tabbypurr]

what I meant was an insulated neutral conductor from a bit of cable. There's no room for other fixings & plastic cable ties won't hold it up in a fire.

NT

[John Rumm]

On TN-S NE fault currents are likely to be small in comparison to LE (from an infrastructure point of view they are typically a RCD nuisance trip hazard rather than a fire risk) so don't require explicit protection.

(obviously NE shorts can pose a shock risk to users and maintainers of the system - and may lay undetected on non RCD systems).

[dennis]

You can buy metal cable ties.

[tabbypurr]

yes, though they're less useful rather than more.

NT

[tabbypurr]

sebox which powers each rented room. Plan is a new CU & new secondary 2 way CUs, the meters will be retained. The question is how may RCD protection b e implemented. If done in the main CU with an RCBO, a trip results in a roo m losing all power including lighting. If done in the secondary small CU, t he cable from main CU to meter to secondary would not be RCD protected. Thi s wiring is surface clipped direct. Neither of those options strike me as f ully satisfactory. How's good to do this?

OK, here's where things are so far...

TT & metal CU means a 100mA S RCD - can that go in the CU or must it be ext ernal to protect against a tail to case short? If external can it be in pla stic or must be metal? And if metal does it need another before it to... :)

Rented rooms can all go on one DP S RCD, with one SP RCBO then feeding each meter, which feeds each socket circuit which feeds each lighting circuit v ia a 5A fused FCU. The purpose of the SP RCBOs is to give discrimination ag ainst L-E leakage trips.

Communal areas I take it must be fed from 2 RCDs not 1 because losing light ing could be an issue due to cooking being potentially hazardous.

Fire alarm circuit requires its own MCB or SP RCBO: does it need its own se parate DP RCD?

thanks.

NT

[John Rumm]

You may be able to get an extra insulation kit for TT installs to allow them to be done in a metal enclosure.

A SP RCBO will discriminate on L E faults, but not N & E faults as previously discussed. Since you are feeding both lights and sockets from the same circuit, then a normal DP RCD and MCBs would be adequate and also protect against tripping the head end RCD on a NE fault.

They also want to be independent of the rooms, so that you can "escape" to a light communal area, if a trip takes out all power to a room.

You don't want any other fault being able to disable it.

[tabbypurr]

Yes, but that isn't wanted. The expense of multiple DP MCBs is unwelcome, and the lack of discrimination of a single DP RCD is also not wanted. Hence the plan to split the protection: L-E discriminating, N-E not.

That takes the count up to a minimum of 4 main RCDs, effectively a 4 way split CU. Not going to be cheap.

NT

[John Rumm]

You might find that 1 x RCD + 2 x MCB is cheaper than 2 x RCBO.

Nope... then again if paying for the labour, then that would probably dwarf the material cost.

[tabbypurr]

I found that reusing the SP RCBOs is far cheaper

When I've got a bit of time I'll have to look again at what's available. It's going to be a monster of a CU at this rate.

NT

[tabbypurr]

was going to go for a Wylex NM CU, but with all the required RCDs there won't be enough ways left. If a different CU is used it will add hundreds to the cost due to need for new RCDs/RCBOs. So... could this lot be split into 2 CUs? How?

NT

[John Rumm]

Multiple CUs is probably the way I would go - the number does not really change the principle of what you are trying to do (much - although note that previous comment about having a terminal to use as a neutral).

You could probbaly do a large CU for the head end with sumbmain feeds, and say a couple of rooms. Then a second smaller one for the next two rooms.