Feeding domestic db from 3phase db -advice please

[Rose Ironworks]

Hi All

So i have just got a job of doing a rewire on a small flat, which includes change of DB etc... Currently this is fed from a 3 phase DB in a commercial premises below. The feed cable is over 25m long (16mm T&E). the client has asked for a sub main tennant meter to be installed. At the moment the only downstream protection for the domestic DB is a 50amp single pole type b breaker..
I could do with clarification when i come to install the new tennant meter, if i should be fitting a double pole isolation switch after the meter. (thinking along the same lines as required protection for meter tails over 3 metres.. Ive had a rummage through the blue book of knowledge but can't seem to find the required answer..

thanks Chris

 

[pc1966]

If the T&E is not penetration-safe (i.e. is probably buried less than 50mm below the surface so could be hit by a nail/screw during building work or DIY) then the supply should be a 30mA RCD and that is a whole lot of pain for the client. In that case you should be looking at putting in SWA so it can come off a switched-fuse or (less desirable) a MCB with high instant trip point for selectivity.

What I would be concerned about is selectivity, and if the flat's occupant has access to the commercial DB in order to reset after a fault has tripped the supply.

Depending on the PFC & PSSC at the flat, it is possible that an upstream MCB is OK, but I doubt a B-curve will be. Generally you would want to look at C or D curve so the max PFC/PSSC on the final circuits wont hit the instant trip point., and that generally means the sub-main is working to 5s disconnection and is beyond the < 0.4s that usually you see for magnetic trip.

That is where an up-stream fuse is better, but if going that way you might need to see if you can fit links on the commercial DB so it feeds a fused-switch for the sub-main instead of relying on the MCB.

 

[pc1966]

To answer your origin question (I think) the sub-main already has protection and a means of isolation using the MCB in the commercial DB.

I am not saying it is currently a good means of protection, but it is there, and so it deals with the whole 3m thing (which is really only an issue from the DNO cut-out where the fuse is not under the client's control). Fitting a billing meter after the commercial DB's MCB (or change to switched-fuse) does not really change that.

 

[Rose Ironworks]

If the T&E is not penetration-safe (i.e. is probably buried less than 50mm below the surface so could be hit by a nail/screw during building work or DIY) then the supply should be a 30mA RCD and that is a whole lot of pain for the client. In that case you should be looking at putting in SWA so it can come off a switched-fuse or (less desirable) a MCB with high instant trip point for selectivity.

What I would be concerned about is selectivity, and if the flat's occupant has access to the commercial DB in order to reset after a fault has tripped the supply.

Depending on the PFC & PSSC at the flat, it is possible that an upstream MCB is OK, but I doubt a B-curve will be. Generally you would want to look at C or D curve so the max PFC/PSSC on the final circuits wont hit the instant trip point., and that generally means the sub-main is working to 5s disconnection and is beyond the < 0.4s that usually you see for magnetic trip.

That is where an up-stream fuse is better, but if going that way you might need to see if you can fit links on the commercial DB so it feeds a fused-switch for the sub-main instead of relying on the MCB.

Hello thanks for that. i doubt if the client would be wanting to pay for a complete new feed cable as the current one is well an truly hidden in the ceiling of the restuarant !. and the work involved would be huge ! the tenant is currently the chef, but i doubt if he knows where the supply comes from.
I had considered changing the MCB in the commercial board for a D curve and fitting a Wylex type switch fuse with a bs88 fuse...

 

[pc1966]

If the sub-main is not realistically at risk of being nailed during DIY then it is probably fine to leave it. In a restaurant roof is kind of that...

See what the max MCB is that you can get away with for the end of sub-main Zs to meet 5s. If you can change to a BS88 fuse you generally get a better combination of higher Zs for 5s and still enough 'delay' so a down-stream MCB clearing won't take it out.

 

[pc1966]

I had considered changing the MCB in the commercial board for a D curve and fitting a Wylex type switch fuse with a bs88 fuse...

Fitting a fuse after as high a trip MCB as possible might be easier than finding 'links' for the DB, but see what the figures for the sub-main Zs allow first.

 

[Rose Ironworks]

If the sub-main is not realistically at risk of being nailed during DIY then it is probably fine to leave it. In a restaurant roof is kind of that...

See what the max MCB is that you can get away with for the end of sub-main Zs to meet 5s. If you can change to a BS88m duse you generally get a better combination of higher Zs for 5s and still enough 'delay' so a down-stream MCB clearing won't take it out.

that sounds like the right solution to me. The 3p DB is a proteus and looking at there product range they do a only seem to do a C curve breaker unto 63amp... but come to think back to looking at the job today im sure the 50amp breaker is only a 6ka capacity... im also considering fitting the new tenants DB as a RCB board to minimise any chance of loosing multiple circuits, although there seem to be only 3 lighting and two radials

 

[pc1966]

that sounds like the right solution to me. The 3p DB is a proteus

You have my condolences...

and looking at there product range they do a only seem to do a C curve breaker unto 63amp... but come to think back

If it is a 3-phase board it should be the "commercial" range and they are 10kA and offer 63A D curve:

63A Single Pole 10kA D Curve MCB

Suitable for AX, ABX, BX, XL, CBX, MCX and PDU boards

www.cef.co.uk

But I have no idea of the board you are dealing with, that was just a quick search on CEF.

(To be fair to CEF/Proteus, they actually do decent data sheets for their products)

to looking at the job today im sure the 50amp breaker is only a 6ka capacity... im also considering fitting the new tenants DB as a RCB board to minimise any chance of loosing multiple circuits, although there seem to be only 3 lighting and two radials

Going RCBO is best if you can, and for a handful of circuits the cost is not that much compared to the time you have to charge for in order to fit and test it in any case.

 

[Rose Ironworks]

You have my condolences...


If it is a 3-phase board it should be the "commercial" range and they are 10kA and offer 63A D curve:

63A Single Pole 10kA D Curve MCB

Suitable for AX, ABX, BX, XL, CBX, MCX and PDU boards

www.cef.co.uk

But I have no idea of the board you are dealing with, that was just a quick search on CEF.

Going RCBO is best if you can, and for a handful of circuits the cost is not that much compared to the time you have to charge for in order to fit and test it in any case.

Yes its a commercial BX type so a 10KA will be spot on... (ps i think they are the ALDI of the db world !)