It rather depends what the final circuit CPC is connected to at the load end. A self contained class I item like a car on rubber tyres, I agree with you, under non-fault conditions, there is no path for current along the CPC to get back, and only current flowing originates  from the live supply to that load.

But things with plumbing, or other metalwork stuck in the ground - including cameras mounted on some designs of metal fence post, antenna masts and lamp-posts and so on, in effect have another electrode stuck in the ground at the load point.

Now the regs do not make a clear distinction, but there is a huge difference in the potential fault  current that can flow in the CPC of something that metallically bridges to another earthing system - like a water main shared with the house over the road, and one that only connects through several ohms of terra-firma - such as a pipe to a private tank in the yard that goes no further.

But in any case once you have two electrodes, or connection to another earthing system,  however that occurs,  there is nothing to prevent currents that are nothing to do with your installation, popping up one electrode, through your CPC and off down another, using your  CPC and wiring as a low impedance short cut to parallel up with an otherwise quite resistive chunk of mud.

The problem is also that these currents remain even when the intended load is switched off.

So the safest thing is to avoid there  being such voltage slopes across the ground in the first place, and to do that one has to be out of the near field of any PME earthed hardware.

In the case of the OP with PME earthed street lights etc nearby , that is likely to be impossible to avoid totally, and the question is how good is good enough

ATEX is a significant complication..

Mike.

Chris Pearson said:

not in final circuits

Even in TN-C-S, final circuit load current still has to flow in the TN-C portion of the system.

The issue in PFS, is that they don't want any current flowing through the ground at all ... because if it flows through the bonding grid that can be dangerous.

It does depend geographically how the TN-C bits are arranged, so if it's "linear down a path" the current flowing through the ground can be managed more easily ... but in reality, in a lot of installations, bits of TN-C go all over a site, so ground currents can still occur by shortest path through Earth.

gkenyon said:

Even in TN-C-S, final circuit load current still has to flow in the TN-C portion of the system.

Yes, I get that.

As you say, it probably is all in the geography. There must be plenty of PFSs that have adjacent street lamps and dwellings with metallic services on a PME supply

Chris Pearson said:

As you say, it probably is all in the geography. There must be plenty of PFSs that have adjacent street lamps and dwellings with metallic services on a PME supply

Well, I've seen this in ["private"] installations and had to take a view ... and make recommendations to remedy the issues that can result - and nothing to do with explosive atmospheres.