Peter Hartley said:

Why is this not defined in BS7671?

Basically, public LV supply is ESQCR ... and BS 7671 is not permitted to repeat legislation in its requirements or definitions - although there are notes etc. along the lines "for public supplies in accordance with ESQCR ..."

Other supplies are "not public LV supplies" by pure logic,simply that, so why does BS 7671 need to define that?

I think just a minor mis-understanding of "standards speak" and what standards can and cannot do.

Thank you. I think BS 7671 needs to be clear. I understand the response from Chris Pearson above and I understand your explanation regarding a standard cannot be seen to contradict legislation. 

My interpretation has always been that a private supply is "privately generated" as there is no distinction made between HV grid point of connection and LV grid point of connection. Thus one can be excused for thinking that the private transformer is just part of the installation, such as cables, switchgear etc. 

I incorrectly assumed that it is due to the difficulty of keeping the privatly generated supply voltage constant. Thus some tweaking of the words would be helpful or include the explanation in the text.

Peter Hartley said:

My interpretation has always been that a private supply is "privately generated" as there is no distinction made between HV grid point of connection and LV grid point of connection.

Well, there's a huge difference (legally) between a public LV supply, and one derived from a private transformer, where the public supply is provided at HV.

In general, supplies received at HV are outside the scope of BS 7671 (by definition).

"grid-independent" is another issue again.

Peter Hartley said:

My interpretation has always been that a private supply is "privately generated" as there is no distinction made between HV grid point of connection and LV grid point of connection.

Well, there's a huge difference (legally) between a public LV supply, and one derived from a private transformer, where the public supply is provided at HV.

In general, supplies received at HV are outside the scope of BS 7671 (by definition).

"grid-independent" is another issue again.