Classification of an earthing arrangement where an interconnected earth electrode system provides the earth fault return path — TN-S or TT?

One point that may help the discussion is the concept of electrical independence.

Although the fault current leaves the installation through an earth electrode, it does not return through the general mass of earth alone. The installation earth electrode is intentionally bonded to the transformer earth electrode by a buried bare copper ring, creating a permanent low-impedance metallic return path.

From that perspective, the buried copper ring effectively forms part of the protective conductor network. The earth electrodes mainly provide the connection to the soil, but the designed fault return path is via the interconnected earthing system rather than through independent electrodes.

This seems to differ from a conventional TT system, where the installation earth electrode is electrically independent of the source earth electrode and the fault current returns predominantly through the earth.

Would it therefore be more accurate to classify this arrangement as a TN-S system with an interconnected earth electrode network, assuming the metallic interconnection is permanent, adequately sized, and intended to carry earth fault current? Or is there a specific requirement in BS 7671 or IEC 60364 that would still classify it as TT despite the intentional metallic interconnection?

I’d be interested to hear how others interpret this, particularly with reference to BS 7671 definitions and IEC guidance.

Terry osifo said:

the concept of electrical independence.

Then there is the practical and legal aspects... which don't care about the basic science. 

Legal frameworks can be 'dumb' (say nothing) about the whys & wherefores of their scientifically 'stupid' (apparently irrelevant) requirements. Often the reasons are buried in historic failings and blinkered legal responses [in the horse racing sense] (*)

More earthing is usually better, not enough is almost always worse. They trade off one set of 'abbreviations' for another, TN; TT; ADS; etc. The 'fault current' safety conductor ends up needing to be maintainable/inspect-able/close-by so that it's maintainable/inspect-able/close-by.. 

(*) reminds me of a legal tale from the US where (in Chicago) headlights were required when it was raining. This became a legal decision that operating windscreen wipers meant the driver knew it was raining, and ultimately that automatic windscreen wipers needed to automatically turn on the headlights... or some variant of a similar tale.

Terry osifo said:

the concept of electrical independence.

Then there is the practical and legal aspects... which don't care about the basic science. 

Legal frameworks can be 'dumb' (say nothing) about the whys & wherefores of their scientifically 'stupid' (apparently irrelevant) requirements. Often the reasons are buried in historic failings and blinkered legal responses [in the horse racing sense] (*)

More earthing is usually better, not enough is almost always worse. They trade off one set of 'abbreviations' for another, TN; TT; ADS; etc. The 'fault current' safety conductor ends up needing to be maintainable/inspect-able/close-by so that it's maintainable/inspect-able/close-by.. 

(*) reminds me of a legal tale from the US where (in Chicago) headlights were required when it was raining. This became a legal decision that operating windscreen wipers meant the driver knew it was raining, and ultimately that automatic windscreen wipers needed to automatically turn on the headlights... or some variant of a similar tale.