So if the main switch for each room is done away with because we are using DP 30mA RCBOs then that is 2 ways for the main 100mA switch and assuming that the DP RCBOs are double width then a 21 way module CU will work.
Schneider do single module DP RCBOs if you want a smaller CU but only on a C curve.
There is now need for extra bus bars as the RCBOs neutrals would be directly fed from the neutral bus bar on the outgoing side of the main switch.
Sorry - NO need for extra bus bars
List price is north of £135+VAT though ...
Cheap then?
Just spotted these.
Not sure if the second link will work.
So Wylex are finally back in the game.
each room would need 4 ways for itself, so the rooms alone would take up 16 ways, then there's the rest of the circuits. It would take more than a 21 way.
A bunch of Wylex RCBOs are available to use, the cost of all new DP RCBOs i s not going down well.
There are some things I'm not seeing here. Why would socket & lighting need separate RCD action when the norm is to fit a split CU where lots of circu its all share each RCD?
NT
I have one and they're nothing like that in reality...
That would mean that you are taking your live feed to the RCBO from the return submain from the external meter, and the neutral feed from the origin of the supply submain to the meter.
Probably not a problem electrically (the neutral connections on the meter are likely just looped through anyway), but not quite as elegant as using the neutral return from the meter.
The neutral is only there to allow the meter to measure the live current.
A meter will work without a neutral return. Safest way to bypass a meter? - remove the neutral.
Indeed, I realise it will work ok, it was more a case of my being wary of doing something unexpected for a future maintainer.
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, some questions. Does there need to be separate rcd protection for lights & sockets so light ing isn't lost when sockets trip? Since SP RCBOs don't disconnect N-E faults, I presume the whole install sti ll requires 2x DP RCDs. Can these be 300mA time delayed or must they be 30m A? I take it there's no regulatory problem with using one secondary CU for 4 c ircuits rather than 4 separate CUs. Thanks everyone, getting close to plan 2 I think.
NT
They must be 30mA RCD.
No problem other than inconvenience with a fault.
You did make it sound like each meter was just supplying one room with just a light and socket circuit. So what are the 4 circuits?
Ideally, yes
If there are additional downstream 30mA trip devices, then they can in theory be 100mA type S devices. However you won't then be able to achieve discrimination with the RCD protecting the submain feeds to the meters. So you are no nearer a workable solution.
So that leaves you needing either 2 DP RCBOs per room, or two conventional 30mA RCDs and two MCBs (likely 6 ways in total per room).
The cheapest option may be a small CU for the head end - Type S RCD on its incomer, then 4 x regular MCBs for your submain feeds.
The submain feeds then return to a *pair* of basic 17th edition bundle deal style CUs (i.e. split load type with two normal RCDs and a selection of MCBs). Feed two rooms from each of the secondary CUs. You will probably get a good enough selection of MCBs with the CUs to meet your needs.
That bit is ok.
fusebox 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 protectio n 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 a s fully satisfactory. How's good to do this?
ighting isn't lost when sockets trip?
still requires 2x DP RCDs. Can these be 300mA time delayed or must they be 30mA?
If the cables feeding the meters require 30mA RCDs I'm seeing a problem wit h that. Either a) there is no RCD further downline, and any current imabalnce causes both sockets & lights to go out in a room, or b) there is another RCD, but no discrimination, so luck of the draw which t rips & still often both circuits go off on a leakage fault.
4 circuits rather than 4 separate CUs.The 4 feeds would be electrically unconnected to each other, wouldn't make any difference to fault behaviour. So the secondary CU would have 4 inputs, 1 from each landlord's meter, not the usual 1.
that's right, one return from the meter for each room. Each of which then f eeds socket & light circuits.
NT
fusebox 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 protectio n 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 a s fully satisfactory. How's good to do this?
ighting isn't lost when sockets trip?
the question is really whether it's required or not.
still requires 2x DP RCDs. Can these be 300mA time delayed or must they be 30mA?
I don't think I follow. The whole point of time delayed RCDs is to achieve discrimination. But if the 30mA RCDs are SP, of course there would be no di scrimination on a N-E fault. I gather that's not permissible.
those options look excessively expensive
we can put those in the main CU
IIUC you mean separate the 2 halves of the split CU so each RCD supplies on e room. That brings me back to the question of whether lighting/socket disc rimination within each room is a requirement. Life is easier if it isn't.
4 circuits rather than 4 separate CUs.thanks everyone, busy making progress on other work atm.
NT
It would probably be a a lot cheaper and easier to use a hi integrity dual split load CU with a 100mA main switch, 4 MCBs for the rooms fed from the mains switch and to use the 30mA RCD supplies for the communal areas and just fit a 2 way garage CU in each room.
Unless I'm mistaken the surface wiring to & from the meters must be DP RCDe d on this TT system, either at 30mA or 100mA time delayed. I think it would be ok to feed all 4 meters from the same RCD.
I take it discrimination between lights & sockets in each room is not requi red, meaning both can run off the same RCD. That certainly simplifies thing s, as you say a garage CU would do the job. I was looking at a single large r secondary CU with SP RCBOs because that would be cheaper & neater, but if SP RCBOs are no good then so be it.
If discrimination against N-E faults is required, why are single pole RCBOs sold? There's something I'm not clear on yet here.
I've also been reading about the 18th edition, such as it is so far. Fixing of cables in escape routes has come up. I presume the usual plastic cable clips will no longer be permissible, are we going to go back to metal buckl e clips? Is their lack of insulation ok today?
NT
Won't that mean the feeds to each room will not be 30mA RCD protected and will need to be surfaced wired, or in SWA.
Do you need the lighting in a HMO room to be on a separate RCD to mains outlets, or is the light in the landing on a different RCD circuit sufficient?
A 30mA trip RCD (i.e. shock protection) would only be required for the feeds to and from the meters if the cables are both hidden, and do not had sufficient protection (either mechanically or by virtue of burial depth). If they are surface wired, then you can do without 30mA protection.
They will need infrastructure / fire protection since its a TT install - but that can be via a type S RCD
Yup, if you look carefully at what Adam wrote "100mA" main switch rather than the more common "100A" main switch, I think he was implying that the main switch would be a 100mA trip type S device.
I would be happy with that since the chances of a fault on the submains is likely to be rare enough for it to be an acceptable risk / inconvenience.
This is one of the cases where the designer will need to make a judgement call based on the circumstances.
No discrimination between circuits in the room is less than the perfect ideal, but may be acceptable since the risks are low - i.e. no stairs, no rotating machinery etc, and its only one room etc.
A similar situation will exist with a 17th edition style split load unit
- one RCD trip will affect several circuits (although you normally arrange them such that the power circuit would not cause a trip on the same lighting circuit that serves the power circuit)
You could also mitigate with a £15 non maintained emergency light in the room if you wanted.
For TN-C-S installs there is unlikely to be any significant voltage difference between N & E at the origin of the installation (since they are joined there). So once the loads are disconnected there is unlikely to be any significant current flow in the CPC. What there is, will be conducted by the suppliers PEN conductor which is designed to be a current carrying conductor in normal operation (i.e. it is a "live" wire)
With TT, the neutral at the point of supply could be at a different potential to that of the local earth spike. So a persistent N E fault could drive significant current through the local earth spike (which in turn could cause heating, loss of moisture and rising external earth impedance).
So on balance a SP RCBO will still result in a significant reduction in serious injury risk for most installations - its just not as suitable for TT installs
(although I would highlight that I have a 16th edition style TT system here, with 100mA trip type S incomer, and then a DP 30mA normal RCD for power circuits. I also have a SP RCBO on my kitchen circuit (since that was nuisance tripping the 30mA RCD). I decided it was preferable to accept that a N&E fault on that one circuit could result in tripping the incomer RCD as well, and be rid of the nuisance trip issue that was there before). (I do have several non maintained emergency lights as well though)
Yup this is a significant (and long overdue) change especially for commercial / office installs.
(The main issue being that in many office / commercial fires things like suspended ceilings would be covered is a mass of wiring. The moment the flimsy ali frames fail in the heat, this is dropped onto the people below - typically fire fighters - where it will snag on their breathing apparatus and risks trapping them).
For the commercial installs, better use of cable trays, metal trunking etc will solve most of the problems.
For smaller installs and domestic situations, I would expect there will emerge a new range of non combustible cable restraint systems.
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