No space for a separate mainswitch. Mainswitch is CU incomer. Sub CU needed. Can the one mainswitch incomer on the existing CU also switch the sub CU? That is, one switch does the two CUs. If so, what is the usual way of doing it?
Sub Consumer Unit
- Thread starter hard-work
- Start date
Sponsored Links
- Joined
- 27 Jan 2008
- Messages
- 23,665
- Reaction score
- 2,666
- Location
- Llanfair Caereinion, Nr Welshpool
- Country
In the main a large MCB or RCBO is used, which depends on cable route and type, mother house had SWA around outside of house for kitchen, so MCB and RCBO's in kitchen consumer unit.
The problem with external main isolator and running the supply before going through any MCB/RCBO is cable is treated as main tails, so 16 mm² or 25 mm² and often needs protection so SWA cable which is not easy to route. I have not done it since 17th edition, I have used three phase switched fuse units with phases linked in the past to feed three local consumer units, but since that the rules on RCD protection have come in, and SWA cable is not easy to route.
Remember a CU is a type tested unit, so unless the manufacturer gives a provision to connect direct to bus bar without a MCB your not allowed to do it, keeping the type tested status. So if you want more than the maximum size MCB/RCBO you need an external isolator, which can then feed a henley block and the two consumer units, but if distance means SWA needs to be used, then it needs terminating, which is not easy with a henley block, which is why I used a fused isolator 160 amp as the metal casing allowed SWA cables to be terminated. However since that job I have been told any premises under the control of an ordinary person should use all type tested equipment, think HSE rule rather than BS7671 but not sure, so technically that 160A fused isolator did not comply as not type tested.
Personally I would ask the scheme provider, as clearly Part P notifiable, so it is up to scheme provider what they will permit, or of course the LABC if using that route. i.e. shift the blame if anyone in future does not like what you have done. After all what do you spend all that money for, if not to ask advice on things like this.
The problem with external main isolator and running the supply before going through any MCB/RCBO is cable is treated as main tails, so 16 mm² or 25 mm² and often needs protection so SWA cable which is not easy to route. I have not done it since 17th edition, I have used three phase switched fuse units with phases linked in the past to feed three local consumer units, but since that the rules on RCD protection have come in, and SWA cable is not easy to route.
Remember a CU is a type tested unit, so unless the manufacturer gives a provision to connect direct to bus bar without a MCB your not allowed to do it, keeping the type tested status. So if you want more than the maximum size MCB/RCBO you need an external isolator, which can then feed a henley block and the two consumer units, but if distance means SWA needs to be used, then it needs terminating, which is not easy with a henley block, which is why I used a fused isolator 160 amp as the metal casing allowed SWA cables to be terminated. However since that job I have been told any premises under the control of an ordinary person should use all type tested equipment, think HSE rule rather than BS7671 but not sure, so technically that 160A fused isolator did not comply as not type tested.
Personally I would ask the scheme provider, as clearly Part P notifiable, so it is up to scheme provider what they will permit, or of course the LABC if using that route. i.e. shift the blame if anyone in future does not like what you have done. After all what do you spend all that money for, if not to ask advice on things like this.
Strictly speaking, as I read them the regs actually require that there be a single switch/isolator controlling any/all CUs in an installation ("single point of isolation for whole installation").
However, I imagine that would most commonly be achieved by an external isolator feeding the two (or more) CUs, rather than by using one of the CU's main switches to feed the other - but, even if the latter were done, I would personally still want a main switch in the 'sub-CU', so that it (just it) could be 'locally isolated'.
Eh? No "type tested CU" is fed with a supply which has upstream protection from anything other than the DNO's service fuse. How does feeding it "from a bus bar" differ from feeding it directly from meter tails?
I've personally never heard of that one. As far as I'm aware, the main difference between a 'type-tested CU' and a 'non-type-tested' DB seems to be in relation to the required current ratings (e.g. 6, 10 or 16 kA) of protective devices - although I've never understood why that requirement should differ according to whether the person 'in control of it' is, or is not, an "ordinary person"!!
Kind Regards, John
Sponsored Links
- Joined
- 27 Jan 2008
- Messages
- 23,665
- Reaction score
- 2,666
- Location
- Llanfair Caereinion, Nr Welshpool
- Country
Bus bar I was talking about was the one inside the consumer unit which connects the RCD or Isolator to the MCB or RCBO not external, and I also find this in charge of ordinary person some what strange, but BS7671 rules are easy we have the book, but HSE rules are not so easy, the GS 38 is a good example once your given the number you can find it, but without the number one wonders where is came from.
Yes, I realise that, but I still don't really understand your point.
If Hard-work did as he suggested/asked, the second CU would presumably be fed from the load terminals of the first CU's main switch, not 'from a bus bar', wouldn't it? ... and I can't see that such a connection would invalidate the 'type-tested' status of either CU, would it?
Kind Regards, John
Can you show us some photos of your layout, please.
I find it rather difficult to believe that something (electrical) which was fully BS7671-compliant could be rendered 'unlawful' by whatever any 'HSE rules' might say.
Kind Regards, John
Found out, that there are stackable CUs, using only one mainswitch incomer - Proetus is one maker. This is a Wylex CU. I am not sure if Wylex do stackable CUs.
http://proteusswitchgear.com/media/wysiwyg/Proteus-Consumer-Catalogue.pdf
Last edited:
No need. Just one CU with no space either side and only space below that can just accommodate one CU. I have found it is acceptable, but how will it be done? I know the bus on the existing (master CU) can be drilled with the Line tail to the sub CU having a crimped lug with it bolted to the bus bar. But is there a sort of kit for for it?
Last edited:
Who knows, maybe it will spark off an idea.
Indeed. I think most of the mainstream manufacturers do stackable ones (although, since they are uncommon, one doesn't often see them advertised by retailers/wholesalers) - and, as I said, there is then a BS7671 requirement that there is only one main switch.
Not specific to my question in the 1st post. If a separate mainswitch isolator us fitted with two CUs, The CU incomer mainswitches are not needed and overkill. OK a henley block after the Mainswitch, then if the slots that occupy the CU mainswitch incomers are needed for RCBOs or mcbs then how do you connect the tails into the CUs? The N is easy right to the N bar. The L?
The 18th is a part of the Building Regs which are law (criminal law). BS are recommendations. BS are only law when a part of the 18th points to a BS number as being a part of the regs.
The stackables only use one Mainswitch in the master CU.
The only way I can see a non-stackable CU having a sub CU is drilling the bus bar on the master CU after the mainswitch. Other views welcome.
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links
