I think that the CPC must accompany the live conductors.

You must not have two simultaneously accessible different earthing systems. If one is TN-C-S and the other is genuinely TN-S, then I would say that they are different. However, if they are connected to the same transformer, and one is TN-C-S, the apparently TN-S one is also effectively TN-C-S.

100 m apart is quite a long way. Are you certain that the two service cables are branches of the same street mains?

How far apart are the buildings? If the supplies are both perpendicular to the road, that suggests 100 m, in which case voltage drop and EFLI might be considerations.

I think that the CPC must accompany the live conductors.

You must not have two simultaneously accessible different earthing systems. If one is TN-C-S and the other is genuinely TN-S, then I would say that they are different. However, if they are connected to the same transformer, and one is TN-C-S, the apparently TN-S one is also effectively TN-C-S.

100 m apart is quite a long way. Are you certain that the two service cables are branches of the same street mains?

How far apart are the buildings? If the supplies are both perpendicular to the road, that suggests 100 m, in which case voltage drop and EFLI might be considerations.

I think it's highly likely the supplies come from the same transformer but will check with the DNO. I agree with the points about voltage drop and EFLI.

However, if they are connected to the same transformer, and one is TN-C-S, the apparently TN-S one is also effectively TN-C-S.

In theory, you could have a genuine TN-S and TN-C-S from the same transformer. Simple enough if the transformer had multiple secondary windings, but even if it had a single secondary, as long as the TN-S earth was kept apart from the PEN conductor all the way to the star point/substation electrode, I think it would still qualify as TN-S. In practice these days though, the DNOs will almost certainly treat any shared main as PME and feel free to interconnect N and PE during any alterations and repairs, so while any older cut-outs may look like TN-S, the normal position is to "treat as PME".

You must not have two simultaneously accessible different earthing systems. If one is TN-C-S and the other is genuinely TN-S, then I would say that they are different.

And conversely, even if you have two obviously TN-C-S supplies, that only means you have two systems of a common type, not one common system. Even if they happen to be connected together in the street at the moment, there's no guaranteed it'll stay that way - it's far from unknown for the DNOs to chop a few customers off the end of one main and extend the cable to be supplied from the next one. If you want to make them common, you need to connect them together yourself.

   - Andy.