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.

Apart from the earthing considerations there were also the old Electrciity Supply Regulations.  One of these e.g. 25.1 of the 1988 regs could be read mand was sometimes quoted   as requiring only a single point of supply.  It was standard practice in some DNOs (probably as a result) at least to insist on supplies having a single point of isolation for safety purposes so that there was no doubt about how to turn off the supply in an emergency.  This lead to significant costs for customers (either expensive switchgear or the need to go to an HV supply if more than 400A at LV was required in some cases.  Other DNOs would cheerfully provide multiple supplies so there was little consistency.   

you can get supply neutral currents flowing in the structure of the building between the to intake positions.

Although some DNOs now have a policy of putting the N-PE spit outside multi-unit buildings and then running separate earth wire cabling from there to each intake position.

   - Andy.

There are other more cynical reasons to prefer or require only a single supply to a building.

The electricity supply industry is under pressure to improve reliability of electricity supply. The usual metric is "lost customer minutes" If the supply fails for an hour to six shops, that is 360 lost customer minutes. If instead only a single DNO supply was provided, with sub-metring by the managing agents, then only 60 customer minutes are lost for a similar failure.

There is also pressure to reduce losses in distribution. If a customer can be required to take a supply at high voltage, then that eliminates the transformer losses, at least from the DNO point of view. The losses still occur but are now part of the customers load, and no longer a DNO loss.

If customers with physically expensive premises, such as a large hospital or university can be given one central point of supply, then that removes losses in the perhaps lengthy cable from the DNO account.

Look how well we are doing ! getting greener and more reliable, year on year.

haha - the problem of separating what really should be a joined up view of the losses problem.

As ever thoughtless changes to the rules encourage silly behaviour.

Although  personally I would support the careful use of higher voltage within customer side of large installations (hundreds of meters linear dimensions)  until much nearer the load, as being a very real 'green' solution - perhaps not always at 11 or 33 kV, at least in anything less than the tallest skyscrapers, but certainly encourage more use of 690 and maybe consider permitting 1k2 as 'normal' voltages for large indoor spaces needing a lot of power.  Modern insulation means that while 250V to earth was considered the upper bound in the era of cotton covered rubber and so on, kit for higher voltages is now really not so difficult to manufacture . (and that would mean more use of the U0>400V  column in the table of maximum ADS breaking times for the regs ) electronic sensing of earth leakage and even electronically assisted breakers can make doing  things safely quite practical - the hurdles are more a mixture of designer inertia and regulation.

Mike