So, as they are private transformers, are the secondaries wired as TN-S ? Or if they are not could at least one of them be (or PNB, which achieves the same end)?

The earths must be interlinked, and may well be already - it is important that there is no chance of anyone straddling two earth zones at different potentials, and in this case there is no way to keep them far enough apart not to be simultaneously within reach.

If you are not paralleling the secondary windings, then apart from making it really clear where various things are isolated, then  there is no sensible current path that will give a diverted neutral problem, unless you had that already when they were separated.

It will be necessary to keep the circuits from the two transformers apart, and a very good idea to avoid things fed from the two ends from sharing panels or switchrooms or very very clearly marked if they must.

I'm sure others will chip in with further observations.

Mike.

Hi Mike, thanks some good points. 

The secondary from each transformer is TN-S. The earth mat at each transformer are linked, therefore my thinking is that they share the same potential. We will verify this on site via testing between. So effectively earth on the MV side and by default on the LV side are one and the same. 

Secondaries are not paralleled, each transformer feeds a separate switchboard at opposite ends of each building so there is physical separation and circuits from each switch board are suitably protected back to their source. 

Signage, warnings and labeling is covered and will be detailed in the O&M's. 

Each Separate Sw Bd should have the same earth potential, hence outgoing circuits common in that respect and protected individually as you'd expect for any normal installation. I am currently of the opinion to not link the earths on the LV side as that could lead to issues with earth leakage, and possible neutral currents from one to the other, appreciate your thoughts on this? 

Hi Mike, thanks some good points. 

The secondary from each transformer is TN-S. The earth mat at each transformer are linked, therefore my thinking is that they share the same potential. We will verify this on site via testing between. So effectively earth on the MV side and by default on the LV side are one and the same. 

Secondaries are not paralleled, each transformer feeds a separate switchboard at opposite ends of each building so there is physical separation and circuits from each switch board are suitably protected back to their source. 

Signage, warnings and labeling is covered and will be detailed in the O&M's. 

Each Separate Sw Bd should have the same earth potential, hence outgoing circuits common in that respect and protected individually as you'd expect for any normal installation. I am currently of the opinion to not link the earths on the LV side as that could lead to issues with earth leakage, and possible neutral currents from one to the other, appreciate your thoughts on this? 

Two TNS transformers is very good news, as the diverted neutral current /TNC/ PME  questions now vanish.

If you like it or not the two LV earths will interconnect in a single building, if only via plumbing and building steels, so there will be cases where if a fault occurs in a fitting in the middle of the building, when current flows from the live of TX1 and into the earthing of both TX2 and TX1 - this is to be expected. It is therefore essential to make sure that it can get back to TX1 from the TX2 earthing, and that the largest credible fault current does not cause a problem if/when this happens.

I know youd do not really want it to take the long path, but some fraction of the current will do so - it will split in the ratio of the resistance, well impedance really,  of the two routes. Final circuit CPCs you can separate, but the incidental paths of things screwed to the wall or plumbing cannot be avoided, and so the METs are much better solidly linked than half-heartedly so via something that might not handle hundreds of  amps quite so gracefully. (what is the LV PSCC actually ?)

This may mean adding additional deliberate bonding between the two 'MET's. or you may have a large enough cross-section of steel/copper already.

Mike