Two supplies to a building

[Phoenix]

Quick one to see if anyone has any further thoughts about it...

Firm I work for is doing the electrics for a new all electric kitchen in a commerial building, existing supply is maxed out, so the plan from the main contractor along with the DNO (the DNO themselves seem happy enough about this, which I did wonder when it was looking like it would get suggested), is to bring another feed from the local 11kv tx on the site boundry into the outside switchroom, where it will feed a separte head and CT chamer, this feed is to go to a switchfuse which will then feed the submain into the kitchen area

There was some debate about the suitability of this, my opinion is that its not that uncommon in larger buildings, but some issues to be addressed; both supplies will have to be tied into the same earth bar... DNO needs to be happy with this (I assume its something they have already considered, but ought to check), good practice to ensure that the supplies done 'cross over' and feed the same rooms in the building, and to ensure that cables do not share the same cable trays, etc. Suitable notcies to be affixed to new distboards, and those forming part of the existing installation located in adjacent rooms warning as well as within the inside switchroom* of this fact. Careful labeling in the outside switchroom to make it clear that there are two feeds from the public supply and therefore two main switches.

*Existing setup is the supply comes into the outside switchroom, feeds a BS88 panelboard with two circuits, each of the circuits feeds 1busbar chamber in the inside switchroom

Anyone think I've missed anything? would you be happy with this setup?

 

[Andy™]

probably the best you can do in this situation

 

[steptoe]

are both supplies 3phase.? (I would assume so)

I dont really see any issue with this apart from the fact that you may need to investigate on whether you are required to put the worst case readings on your certs or not,

ie: where 2 supplies are present record the highest(or lowest) value measured,

more intended for back up supplies, but may well need to be considered in this case as well.

 

[Phoenix]

Not sure the worst case value thing is relevant here... its more to do with if you have a backup generator feeding the installation (i always limitation it on a PIR, but draw attention to where the zs values are 'tight enough that there 'could' be an issue)

But this is like two installations, so would be two EICs (would be if they were both new anyway), so no issue there, only one figure for each box, but two separate certs as it were

Both supplies are three phase, yes, existing is 400A, proprosed additional supply is 200A (its a big kitchen!)

 

[steptoe]

Im still not convinced you have 2 separate supplies, not entirely anyway,

an earth fault on one is going to effect the other,

you have 2 separate supplies and one equipotential zone, I dont see how you can get around just having 2 entirely different certs,

an earth fault on either system is always going to take the path of least resistance,

what supply have you got,? TNCS? herein lies a problem of a shared neutral/earth.

 

[Phoenix]

I understand what you are saying, theres going to be parallel paths in the earthing... (that would be the same though if it were two separate sites but shared a metallic gas main!), of course ze tested with supply earth in isolation from earth bar, bonding, other elec service etc

Earths are going to have to be bonded together to form eqipotenial zone, as as mentioned going to have to check this does not present issues on DNO side with circulating currents etc

I'm picturing it as, it would be the same if they built another site next to it and put a supply into there, but just put into the existing building alongside existing supply

I think the exisitng is tncs, and nre is very likely to be too

 

[steptoe]

I understand your next door unit scenario,

but you are not next door, you are in the same EZ with two supplies, two neutrals and two earths, and this needs to be considered,

DNO wont care what you do once they present it to you, thats your responsibility.

 

[Phoenix]

DNO wont care what you do once they present it to you, thats your responsibility.

I don't know about that, seem pretty well on the ball with reagrds to parralele earth and neutral paths in this area, had then refuse to give TNCS to a row of industrial units before now, on account of shared steel work (I wasn't resposnible for designing that as though it would be available tho >.&gt

 

[kme]

So you`ve effectively got a TPN "summation" supply.

You cannot, IMO, issue seperate certs, unless you are prepared to class the kitchen as its own installation, i.e. a seperate entity from the rest of the building. If that is the case, then I don`t foresee a serious issue, bearing in mind the comments you`ve made above.

However, if you want to share the whole building between the two supplies (you did say the 400A supply was on limits..?), then the issue gets a little more confusing. However, the prime issue for you is that you cound NOT issue seperate EICs for each supply, when they serve a common premises. You`d have to fill the cert., as Albert said, with the "worst case" figures, and explanation of the supply system.

The best solution, IMO, to the earthing problem, is if DNO will tie the PEN conductors "at the HEDs". That would give you one earthing conductor. The other option is to request a non-PME supply, explaining your concerns. Most DNOs will be open to your comments, some will even provide advice.

HTH

KME