If the building(s) are steel framed and all interconnected, then it sounds like you have a single earthing system whether you like it or not, so probably makes sense to design it that way from the outset. As Mike says, even if the earthing system is common, the neutrals should be separate (unless paralleled) as long as you adopt a TN-S rather than TN-C distribution arrangements - which you're probably obliged to do anyway under the ESQCR if the whole LV side is owned by the "consumer".

I think 542.1.3.3 is aimed more at the situation where small c.p.c. circuits might create a bridge between separate buildings that could be on different earthing arrangements  - think for example an outside light between two buildings with a requirement for 2-way switching with one switch in each building. Normal rules may well require the c.p.c. to be interconnected to both buildings earthing systems - and so the c.p.c. might then be obliged to carry very substantial currents between the two earthing systems (under fault conditions, but also diverted N currents under normal conditions if PME is involved). Earthing conductors associated with a supply are likely to be easily big enough to cope with any fault currents (or if the two supplies are very different sizes, the smaller might possibly need a small upgrade in size if it may carry a large proportion of fault currents from the larger installation).

100m does feel to be a long length of conductor to interconnect things - others may have a better idea of what's normal for this kind of thing. Maybe look to having a single "MET" at the building end (even if that's two terminals and a shorter interconnecting conductor).

   - Andy.

If the building(s) are steel framed and all interconnected, then it sounds like you have a single earthing system whether you like it or not, so probably makes sense to design it that way from the outset. As Mike says, even if the earthing system is common, the neutrals should be separate (unless paralleled) as long as you adopt a TN-S rather than TN-C distribution arrangements - which you're probably obliged to do anyway under the ESQCR if the whole LV side is owned by the "consumer".

I think 542.1.3.3 is aimed more at the situation where small c.p.c. circuits might create a bridge between separate buildings that could be on different earthing arrangements  - think for example an outside light between two buildings with a requirement for 2-way switching with one switch in each building. Normal rules may well require the c.p.c. to be interconnected to both buildings earthing systems - and so the c.p.c. might then be obliged to carry very substantial currents between the two earthing systems (under fault conditions, but also diverted N currents under normal conditions if PME is involved). Earthing conductors associated with a supply are likely to be easily big enough to cope with any fault currents (or if the two supplies are very different sizes, the smaller might possibly need a small upgrade in size if it may carry a large proportion of fault currents from the larger installation).

100m does feel to be a long length of conductor to interconnect things - others may have a better idea of what's normal for this kind of thing. Maybe look to having a single "MET" at the building end (even if that's two terminals and a shorter interconnecting conductor).

   - Andy.

Hi Andy, thank you for commenting. Similar to other responses the system is TN-S. Understand the reasoning in 542.1.3.3 for final circuits, I am comfortable in this case that the two buildings will share earthing characteristics and are suitably protected. However, just didn't fully understand the rationale to earth within one installation only. It would be almost impossible to insulate between the two earthing systems in this instance. 

Would you still suggest MET's were interconnected (LV side) given the transformer earthing mats are interlinked with bare buried conductor?