2 electricity supplies in one building, 1 water main

[Jim2287]

What should be done in the following scenario.. ?

two shops divided by a wall..

"Shop A" 3 phase 100 amp TNCS
"Shop B" single phase 100 amp TNCS

The wall was knocked down making just one Shop with 2 TNCS supplies.

Both Shops had a seperate water main, but now, "Shop B" water main supplies "Shop A" also.

Treating the shops as seperate buildings with seperate supplies ...
If the water pipe is bonded at the stop cock for "Shop B" and where the water enters "Shop A" .... would this cause PME neutral current to flow through the water pipe ?

If yes, would this be dangerous and how would you resolve the problem ?

Bond at the same point for both electricity supplies ?
Link the two bonds using 10mm2 earth ?

any help or feed back would be great

 

[Steve]

I once worked at a shop with two supplies. Both were in the shop's cellar. One was 3 phase PME for the shop, the other was single phase TNS, for the flat above the shop.

Both supplies were earthed together, and to the water main, which also supplied both properties.

 

[JohnD]

with a certain amount of luck, the two neutrals would be coming of the same main in the street... but you never know. I don't see any question that they should be bonded together using 16mm. I presume the 3ph DB comes off 25mm tails?

It's surely just a matter of main equipotential bonding of all the metallic services that enter the building and could introduce a potential from outside.

BTW, don't think of it as two buildings. It's one building now they've been knocked together. You'd have to main-bond to the obsolete water main as well, as you never know when it might come back into use, or be touched by someone.

p.s. It would be quite hard to provide a main switch that can be depended on to isolate the entire installation. I bet circuits will creep between the floor areas of the two previously independent shops.

Metering costs would also be lower if the 3PH supply was used for both, and the SP was declared redundant (until next time the shops are re-divided).

 

[Jim2287]

Yes its all 25mm tails, 16mm main earth, 10 mm main equipotential bonds

Where do i bond on the old water main if there is nothing coming off the consumer side of the stop cock ? we arent supposed to bond on supply side of services right ?

 

[JohnD]

isn't there a stub with a stop-end on it? If not, won't the shop flood if some dummy turns it on?

Get a plumber or someone to cap it off, and, I suppose, bond to the brass cap.

 

[Jim2287]

the areas supplied by the seperate supplies are well divided

changing to just the 3phase supply is a good idea and much safer thing to do as isolation by one isolator ... but wouldnt the water still need bonding by the both supplies still whether its in use or not ?

 

[JohnD]

yes, you should bond all the incoming services.

 

[bernardgreen]

Seek the advice of the DNO

Specifically ask about the two neutrals into the building. If they are from the same neutral running along the street then there should not be a problem linking them via the bonding.

But if they are from different neutrals along the road then the link via the bonding will have problems. If one of the neutrals floats or is carying heavy currrent due to out of balance on the phases then the bonding will carry some of thet current.