Repurpose submain

[grumpyjohn01]

Hi All, Have been asked to provide some outside socket outlets in a retail setting for market traders / catering vans. The premises is an old industrial site comprising various historic buildings etc. There is an existing 3 phase supply to the location that used to feed a building that has been demolished, it is a 4 core 10mm swa supplied from the origin (in another building) via an up front s-type rcd and then a small board with 100mA rcd and 3 pole 50A breaker, earthing is TT. Where the cable emerges from the ground (run of approx 30m) there is to be a wooden structure constructed into which I will install a cabinet containing all the equipment including sockets etc. I intend to terminate cable into a 4 pole rotary isolator and then take a single phase supply to a small DB with 30mA mainswitch, feeding the sockets from individual mcbs, (Initially the customer requests 1 x 32A c-form socket and 2 x 16A c-forms.) Also will provide an earthing rod at this point. My calculations indicate this will be fine, however couple of questions to the wise...

1. Is there any reason not to keep the 3 pole breaker supplying the circuit even though only one phase is being used for final circuits?

2. If the customer requests further sockets to be added (likely) could an additional entirely separate single phase circuit be combined into the 4 core? ie using 2 lines and 2 neutrals and combining the cpcs using the sheath? I would imagine at this point the 3 pole breaker supplying to be replaced by 2 x single pole. Whilst I realise this may not be 'good design' is it nominally compliant? sure I've seen it done before...

3. What about keeping the 3 phase circuit and fitting a 3 phase board to supply all the single phase sockets? I would worry about the neutral being overloaded and this is a little beyond my 3 phase design capabilities !

Any other suggestions? As always cost is a factor and the client is keen to reuse old kit wherever possible just now as the site is due to undergo some major refurbs and at that point there will be a new installation as the building housing the present supply will be demolished.

Am going down there today to test all the equipment I intend to reuse.
Thanks for your attention guys, looking forward to some input.. John

 

[westward10]

I would put a triple pole board in as it allows more flexibility and fit rcbos. Have you tested the existing cable and is it okay for connection to a 50A device.

 

[Richard Burns]

If you have a three phase supply coming into the proposed building then I would stay with a three phase supply, it gives the most flexibility.
You could have each socket circuit on a different phase, if this would not cause problems.
As your neutral is the same csa as the line it is very unlikely you can overload the neutral without serious harmonics.
The RCBOs idea is a sensible one.

1. Keep the 3P breaker, lose the 100mA RCD and have 30mA for each socket circuit.
2. I suppose you could do this, but why? when you have the three phase already.
3. Sounding good, get some advice on three phase installation and go for it, but do get it right.

 

[grumpyjohn01]

Ok Thanks for the replies. Did post yesterday but I guess it got lost in the ether..(server change?)
I went to the site yesterday and tested the cable, which is fine and according to my calcs will be ok on the 50A device.
As far as using a 3 phase board and rcbos, that was my first thought but my nagging doubt is regarding load balancing if the number of sockets rises due to client request. I would obviously spread the circuits across the phases but would have no control over which ones were used or for what. I have designed and installed 3 phase kit before but only ever when 3 phase loads made up the bulk of the circuits and single phase only used for a relatively insignificant part of the total. In this case we could end up with 8 or 9 single phase sockets all on there own circuit. My knowledge of 3 phase theory is rusty but can't help thinking that the neutral current could exceed the ccc of the cable...root 3 something..mumble..mumble..
Anyhow if I did go down that route, if there is a 4 pole isolator on the board will single pole rcbos be ok? TT and all that..cheers J

 

[westward10]

Lose the 100ma rcd?
Maybe I am misreading but I thought it was TT at the origin.

 

[Richard Burns]

Lose the 100ma rcd?
Maybe I am misreading but I thought it was TT at the origin.

As I understand it there is a 100mA type S at the origin and then a 100mA at the remote DB. Since all the circuits from that DB are planned to have 30mA protection the intermediate 100mA is unnecessary.

 

[westward10]

Maybe I am misreading, OP where are the existing 100ma rcd/s.

 

[grumpyjohn01]

Agree that the 100mA intermediate RCD is superfluous as there is indeed an 's' type up front however this rcd is the mainswitch in the board feeding the sub and is of an 'historic' type, so replacing it with a mainswitch of the correct type might be difficult and add to the cost of the job if the whole board had to be swapped out. It will probably stay in.
I have decided to install a small 3 phase board with 4p mainswitch and 6 single phase ways (2 x 3phase) therefore limiting the max number of sockets to 6 and at 2 per phase I reckon any imbalance would still be within the capacity of the cable, even if fully loaded on 2 phases and no load on the 3rd. As I received no answers to my questions on the stuff that really concerned me, I reckon this is a good compromise....final circuits will all be on SP rcbos. Have just been and installed earth rod(s) and have got a meaningful Ra so all good...Thanks to Westward & Richard for input. J

 

[davesparks]

With so little basic knowledge of three phase why are you working on it?

The neutral current from normal loads is effectively cancelled out in a balanced load. At a basic level a totally balanced load will have 0 neutral current, a totally unbalanced load (load on only one phase) will have a neutral current equal to the line current.

It's different if you've got harmonics in the neutral current, but that's unlikely to be an issue with market traders.

Have you considered whether this will fall under the scope of BS7909?

 

[grumpyjohn01]

My basic knowledge is fine, if I got my college books out and spent an evening working it all out, I am sure I could nail this scenario down, however its a little outside my recent experience (the specific scenario of exclusively multiple single phase socket outlets on a 3 phase supply) & if I had time for a hobby that wouldn't be it. So a quick consult on a forum is more expedient, I also had a chat with an electrical design engineer of my acquaintance today at Neweys, he didn't think I shouldn't be doing this job......
It doesn't fall under BS7909. Thanks for your concerns. J