If both are TNC-s, then as the CPCs of the 2 supplies link to building steels etc, you can get supply neutral currents flowing in the structure of the building between the to intake positions.
https://www.tangle-tamers.com/tncs/TNCS-DN-1-RD-V1.6.pdf

has a nice illustration that may help.

Mike

mapj1 said:

If both are TNC-s, then as the CPCs of the 2 supplies link to building steels etc, you can get supply neutral currents flowing in the structure of the building between the to intake positions.

Yes, this is the case.

HOWEVER, if subsequent supplies to such a building are TT, they are an extreme case of high prospective earth fault currents in TT systems, as illustrated in IET Guidance Note 6 Protection against overcurrent where the prospective earth fault current (after connection of extraneous-conductive-parts) is similar to the L-N prospective fault current:

In fact, prospective earth fault currents in TT systems are rarely as low as that predicted for the 'worst case' used for ADS, and therefore:

• RCDs should never be relied upon for protection against earth fault current ... it is the 'overcurrent protective circuit-breaker' element of an RCBO that should be considered. (NOTE: Section 1.5 of GN6 discusses devices that are suitable for protection against overcurrent) ; and
• Care must be exercised when trying to determine maximum prospective fault currents in TT systems ... even if it's not an extreme case as Figure 6.3 of GN6 (above), earth electrode resistances of the earth electrode itself, and the equivalent for extraneous-conductive-parts, will vary at different times of the year, so may well be much less on some days, than any values you measure on the day of testing.

Couldn't you get that sort of situation between neighbours, especially with metal water and gas supplies to both premises?

What about a pair of semi-detached houses, or a terrace?

I have two supplies. I am 99% sure that they have a common earth, but they are not within touching distance of each other and never will be.

Chris Pearson said:

Couldn't you get that sort of situation between neighbours, especially with metal water and gas supplies to both premises?

Absolutely, that's why a water pipe-looking object is used as an example in GN6 ... but the steel-framed building is perhaps the most extreme example of this type of case in TT systems.

Cheers Mike.  Thanks for the response.  Good report, very helpful.

Thanks Chris.  Helpful addition to the report as provided by Mike.

Thanks Chris.  Helpful addition to the report as provided by Mike.