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Topic Title: 3 phase in domestic consumer unit set-up
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Created On: 04 December 2009 01:32 PM
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 04 December 2009 01:32 PM
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kennedy123

Posts: 36
Joined: 02 December 2007

Hi,

After my recent post we've decided to build a wall to allow for the three phase consumer units.

My plan is to have the three phase meter installed, from that take 3 x 25mm tails to individual consumer units and a 25mm neutral to a henley block and take 3 x 25mm neutrals to each consumer unit ( 3 in total)

The in the c/u's I'l have lighting on one phase (db1), sockets on other phase(db2) and electric boiler (60A) on last phase(db3).

The system is TT so I am goign to install 100A 60947-3 in each c/u with indivdual rcbos for each circuit.

Can anyone tell me if I am planning this correctly or if there is an more cost effective way of doing this!?

Thanks in advance
 04 December 2009 01:41 PM
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Testit

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Doesnt sound that balanced to me, but only you are aware of the likely actual loadings.......

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 04 December 2009 02:25 PM
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OMS

Posts: 19835
Joined: 23 March 2004

Can anyone tell me if I am planning this correctly or if there is an more cost effective way of doing this!?


Yes, install a 3 phase DB - a 6 way TP&N would give you 18 single phase ways. Much more effective than 3 seperate con units - and more flexible in terms of a sensible load balance

Regards

OMS

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Failure is always an option
 04 December 2009 03:00 PM
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spinlondon

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Wouldn't bother with the 100mA RCD if all the circuits are to be protected with 30mA RCBOs.
 04 December 2009 03:48 PM
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kennedy123

Posts: 36
Joined: 02 December 2007

thanks all so far,

Do I need an RCD to protect the tails as this tpn d/b is a metal enclosure on a TT system?

And is my main switch to be a 125A 3 pole or 4 pole?

My max demand of whole install is 160A.

Cheers
 04 December 2009 09:05 PM
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tillie

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Hi, I was hoping some of you better informed than me were going to answer this final question.

I have only came across one TT installataion in my working life.

I would have thought that the tails going into the metal Distribution board would need protection by a 100ma time delayed rcd in an insulated enclosure in case of a fault on the tails going into the metal Distribution board.

If so then a neutral to earth fault would bypass the single pole 30ma rcbos and take out the whole installation.

What would be the best way to deal with this scenario.

Regards
 04 December 2009 09:24 PM
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slittle

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The best way to deal with it would be to ensure no N-E faults.... or find someone who makes DP RCBO's that fit a TPN board.

To have a metalclad board as an incoming board would be grounds for refusal of connection in my part of the world. As you've rightly said the RCD must be in a plastic enclosure to protect the tails into the metal board.

Also OMS is always right, I think I'd consider the three boards route unless of course someone does a TPN board which is plastic ??. or can you ask the dno nicely and get PME/TNCS as this solves the incoming rcd problem.

I'd also be careful about the "lighting on one phase / power on one phase" situation as it would be easy to end up with two phases in close proximity which although not a problem in skilled hands could be in domestic.
How big is the dwelling ??, can you not do a phase per floor or similar and then the one that doesn't have the kitchen / utility etc can have the heating on it.


Stu
 04 December 2009 09:42 PM
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tillie

Posts: 795
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Hi , thank you for the reply Stu.

It was actually OMS who suggested installing the three phase board the op was originally going to install three single phase boards.

That was what got me thinking about the need to install the rcd before the Distribution board.

Am I right in thinking that it would be a four pole 100ma time delayed rcd.

Also I wish we could guarantee no neutral to earth faults but who can tell what will happen in the future to the installation.

I think the DP rcbos are the only way to go in this situation.

Regards
 04 December 2009 10:14 PM
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slittle

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Sorry, it's been a long week. Been off sick and I'm getting a bit bored of being stuck at home..

Yep, if it's a metal board you are spot on, it would be 4 pole, time delayed. I might be tempted to push it up to 300ma though as you're only protecting the tails really and of course limiting the touch voltages around the installation, so if Ra is low enough, then 300ma it would be.

It's a tough one when you're TT, but I've got several farms which sit happily on a 100mA TD RCD even when it's raining !,

Stu
 05 December 2009 06:57 AM
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normcall

Posts: 8162
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The practical choice is either a single RCD to protect whole installation or separate DBs
3 single phase supplies would allow insulated fuse boxes either in a stacked board or 3 phase board.
Once that decision has been made, the rest follows naturally.

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Norman
 07 December 2009 04:07 PM
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AJJewsbury

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And is my main switch to be a 125A 3 pole or 4 pole?

Can't omit N isolation on TT, so 4-pole.
- Andy.
 08 December 2009 01:19 PM
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OMS

Posts: 19835
Joined: 23 March 2004

Try Hagar - I believe thier Invicta range comes with a GRP option to IP 65 in thier Type B boards.

Essentially a plastic box. Coupled with double insulated tails and suitable cable entry that should solve any concerns over faults to the metalclad DB.

Regards

OMS

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 08 December 2009 07:30 PM
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sparkingchip

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If there will not be any three phase equipment, why not ask for three single phase supplies individually metered, I can think of a number of installations like that such as a local home for more mature people.

Andy
 08 December 2009 07:32 PM
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OMS

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Wouldn't the standing charges alone make that uneconomic, Andy

Personally, I would have seriously reviewed the design load and probably concluded that a single phase 100A was just fine

Regards

OMS

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 08 December 2009 08:37 PM
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sparkingchip

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I would have to have a chat to a supplier regards costs.

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