Three phase opinion please

[kilmo]

I'm after your opinions, I posted the other day and have since spoken to my electrician and it appears I got a few bits mixed up, basically I'm converting a pub into a single domestic dwelling.

The building has 3 phase and earth is supplied by the armour from the main suply head.

We have now came up with the following plan
A three phase distribution board in the cellar next to the meter.
Inside this On one phase
- sockets and lights in the cellar (all surface mounted)
- outside power
-CCTV
-smoke alarms
On another phase
- A sub main to ground floor consumer unit
On the 3rd phase
- A sub main to first floor consumer unit

Would give me the future option to connect 3phase equipment

Ground floor and first floor will be standard consumer units with a isolator and RCBOS on all ways

The distribution board will provide whole system isolation and RCD to the sub main tails as these will be chased into the wall at one point but will still be less then 50mm from the surface.

It was also suggested 3way swg could be used as the sub main tails and metal clad consumer units could be used

I'm after your thoughts our opinions

 

[malcolmsanford]

The RCD for the main distribution board is a bit of a stumbling block. Is the RCCB/RCD going to be a 3 phase one? if it is then any fault anywhere on the installation will take your whole house out.

Even if the DB is going to have a main 3 phase switch and each local sub main on each phase is supplied by a single phase RCD/RCCB then that will take out the whole board on that phase if you have a final circuit fault.

Also the RCD on the sub mains is not going to provide that sub main with overload protection.

By all means have that 3 phase DB in and sub main from it. IF you going to bury the cable then use SWA or MICC. Another way is to run conduit in the wall for singles or an earthed metalic covering another type of cable.

This way you could protect the sub main with either a fuse or an MCB giving you both overload and fault protection, and your sub board could contaion the RCBo's

 

[Deleted member 26818]

Not much point using RCBOs if there is front end RCD protection.
Could you not use another means of additional protection for the distribution circuits?

 

[Guest123]

As the chaps have said the drawback with having to provide addiotional protection to the distribution circuits via front end 30mA RCD means any fault anywhere in the building will cause ALL the power to be lost in the WHOLE building.

Not good if you're upstairs on the first floor, having to make your way down to the cellar to reset.

 

[kilmo]

Thanks, this is exactly the sort of opinions i was after. i have a basic understanding and needed a expert second opinion, even tho im not doing the work myself 99% of the conversion has been done by me but always leave lecky and gas to the pro but i like to understand whats been fitted fully.

i can see what your saying, its the protection on the distribution circuits causing the issues

do you think this idea could work with some corrections is is it better scrapping this and stating from scratch?

 

[malcolmsanford]

No the design is good.

Another way to do it is do away with sub mains to local boards and have say a 24way TPN+E board at origin. This would aliviate your protection of any submain around the property. It gives you the added bonus of just one board in one location, the only downside to it is longer cable runs for your final circuits, and the sparks will have to take a little more thought over design.

There are many ways to skin a cat, if you have a competant sparks then he will do it the safest and most economical way.

 

[johnboy6083]

i would go with SWA as your submain, and the you have no problems with what we call discrimination (ensuring only the fuse/mcb closest to the fault operates). as for the rest, it sounds good to me.

 

[RogertheBodger]

Sounds good to me too!
I would go with a TP&N board at source and I wouldn't balk at a main RCD.
It may be controversial but I think this is the way it's going to go in the future.
The reasons against are not convincing, spend all the money, worry about the Old Lady (God bless her) falling down the stairs in the dark, ...................... and then you get a power cut

 

[somersetsparks]

Well the regs. specifically state that you should have discrimination, so therefore a 30ma RCD Incomer is definately not recommended. Especially not with 20 odd circuits over 3 storeys, fed from the cellar.

 

[Deleted member 26818]

Sorry Roger, but the requirement divide an installation so as to minimize inconvenience in the event of a fault has been with us for some time.
Installing a front end RCD does not to my mind minimize inconvenience, in this case it would increase it.

 

[Guest123]

Sounds good to me too!
I would go with a TP&N board at source and I wouldn't balk at a main RCD.
It may be controversial but I think this is the way it's going to go in the future.
The reasons against are not convincing, spend all the money, worry about the Old Lady (God bless her) falling down the stairs in the dark, ...................... and then you get a power cut

So you would happily carry out works in direct contravention of BS 7671???