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).

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).

the DNO may not like the two installations bonding together

Agreed, but keep in mind that if the two supplies feed the same building (or a pair of closely associated buildings) - the existing supplies are likely to be already solidly bonded together via bonding to common metallic pipes etc. Provided there's no evidence of overheating, that might give you an arguing point that things would be no worse than they are already, regardless of the DNO's latest rules. Of course the DNO might want to take the opportunity to improve things - but if that's their choice, there's an argument that it should be at their cost.

   - Andy.

AJJewsbury said:

Agreed, but keep in mind that if the two supplies feed the same building (or a pair of closely associated buildings) - the existing supplies are likely to be already solidly bonded together via bonding to common metallic pipes etc.

I don't think that connecting the two installation by metal pipes or steel-framed building is directly analogous, because unlike insulated wire, current may also flow through the ground from them. It all depends on local soil resistivity and how damp the conditions are around the pipe etc, as to how alike the two situations are.

So, yes if there are shared extraneous-conductive-parts, this might help the argument.

BUT - there's something else for Sparky Dan  to consider

In a lot of instances if the DNO actually know (or get to know) that there are shared extraneous-conductive-parts between two supplies in the same building, especially if there are large contact areas (e.g. steel-framed buildings) one of the two installations will not be offered a DNO earthing terminal. It may be against their conditions, so I'm not 100% clear what the situation would be here?

one of the two installations will not be offered a DNO earthing terminal.

Alternatively the DNO may make the N/PE split at a common point outside the building and run the two supplies with separate N and PE conductors from there to the two cut-outs. Certainly that seems to be the standard approach for some DNOs for new supplies into multi occupied steel frame buildings these days.

   - Andy.

AJJewsbury said:

Certainly that seems to be the standard approach for some DNOs for new supplies into multi occupied steel frame buildings these days.

Agreed ... some DNOs, but not all, and may depend on the configuration of the local network.