If there is equipment from two supplies that is simultaneously accessible - i.e. a person could be between the two CPCs via the metal casings of bits of kit, then they need to be solidly bonded.

From you description this will almost certainly be the case, if it is not already - if both supplies are already attached to the same water mains and heating systems, structural steels etc.they are probably already informally linked.

 If both supplies are TNC-s you will need to think a bit about the potential for diverted neutral currents that may be quite large. Can you tell if both supplies come from the same DNO transformer originally ?
Mike.

Hi Mike 

Thanks for the response, I can confirm that one is a TNS and the other is TNC-S. 

I have a note for this same scenario in my regs book for "buildings with multiple supplies" - 542.1.3.3 Earthing and 544.1.1 for Bonding.

Thanks, So it says ".....any protective conductors common to any of these installations shall either be capable of carrying the the maximum fault current likely to flow through them....."

So does that mean as long as you have done the correct cable calculations you can have two separate supplies within the same room?

Thanks Dan

I wouldn't be so sure on that. It is safest to assume that any public TN-S supply may have been converted to TN-C-S.

As Mike says, presumably the two supplies come from the same transformer, but you can never be sure without asking the DNO.

Apart from the earthing considerations there is also the question of the lack of a single means of isolation.  Whilst this isnt a specific 7671 requirement it could be an issue in this sort of environment.  Labelling and notices will be OK for a situation where people have the time and inclination to read and understand them but in an emergency such as fire or electric shock it needs to be instantly clear how to turn off the power in my view.  Tin hat on....

This may be a situation where the 'my oven  is on fire' sort of isolation needs to operate appliances on both supplies , or be to isolators side by side so it is obvious on inspection how to turn off which item- the argument  'no local isolation needed it can be switched at the MCB' looks weak if it is in another building or unclear which board.

Mike

As has been pointed out, Regulation 411.3.1.1 required simultaneously accessible exposed-conductive-parts to be connected to the same earthing arrangement (i.e. earthing system).

But also, as has been pointed out, Regulation 542.1.3.3 means you simply can't bond two installations together without considering ANY currents that might flow between the two earthing systems.

mapj1 said:

 If both supplies are TNC-s you will need to think a bit about the potential for diverted neutral currents that may be quite large. Can you tell if both supplies come from the same DNO transformer originally ?

Not quite:

• if ANY (one or more) of the installations is TN-C-S you need to think about diverted neutral currents
• as some repairs have been carried out to TN-S supplies using CNE cables, if ANY (one or more) of the installations is TN-S, you might also experience diverted neutral currents.

tattyinengland said:

I have a note for this same scenario in my regs book for "buildings with multiple supplies" - 542.1.3.3 Earthing and 544.1.1 for Bonding.

Regarding 544.1.1, as an absolute minimum, the CSA of ALL main bonding conductors for BOTH installations would be dictated by supply requiring the largest CSA (whether or not PME). For example, if a large TN-S installation requiring say 16 sq mm main protective bonding conductors were connected to a TN-C-S installation requiring 10 sq mm, at least 16 sq mm would perhaps be required for both installations ... and the bonding between them.

BUT

This is where things start to get tricky. The smaller installation's DNO supply cable may not be suitable for the bonding size (among other considerations) ... all meaning the DNO may not permit you to simply bond the two installations together (and it may not be safe to do so).

Chris Pearson said:

presumably the two supplies come from the same transformer, but you can never be sure without asking the DNO.

And yes, added to the above, if the supplies are from separate HV transformers, there may be other considerations due to HV networks in the area.

In general, you can't always take it upon yourself to make the decision, and the DNO may not like the two installations bonding together (it may be against their conditions of supply to do so).

Thanks for replying there will be an isolator, correct cable used, MCBs etc  

I’ve spoken to the DNO and have a site visit soon to discuss.

After speaking to the DNO they confirmed that both supplies go back to the same transformer