Depends on the situation, but for a typical domestic install, I'd suggest that discrimination isn't necessary - even if the DNO fuse blows, you'd be in the same situation as if you'd had short tails in the first place (or the DNO had accepted responsibility for their fuse providing protection for longer tails).

If carefully routed, the chances of a fault on the extended tails/submain should be pretty small.

   - Andy.

Sure, here's a polished version of your reply:

Hi Andy,

Thank you for your message.

The installation pertains to a multi-apartment residential block where the Distribution Network Operator (DNO) equipment is situated in a separate room. The estimated length of the tails is 15 meters, terminating into the BNO equipment, which includes a multi-service distribution board or rising bus bars via an unmetered supply.

Given the limited space around the DNO equipment, it's challenging to accommodate additional protective equipment. Hence, I'm considering the possibility of locating the devices field end of the cable and potentially encapsulating the tails for added mechanical protection along the route to minimize impact damage.

However, since this approach would deviate from BS 7671 standards, I'm seeking clarification on whether obtaining consent from the DNO to use their devices for a certain distance would suffice. Additionally, I'd appreciate any insights you might have regarding other factors I should consider in this scenario.

Looking forward to your guidance.

In fact it does not really deviate from BS7671 - the company fuse can and will protect suitably sized  tails just fine. It may well deviate from your DNOs protection advice, where they do not want to see long tails relying on their fuse - even if in a race between fuses, theirs may well finish first anyway especially when yours and theirs are the same rating.

The '3m' figure from '7671 is intended to relate to situations when there is no upstream protection to speak of, and might be for example when there is a tap line onto a 1000A bus for a 4mm2 wire supplying  one light and a socket that is fused down to 32 A or whatever only once it gets outside the location of the bus chamber. The wiring for the 'light switch in the substation' situation,,,, 

Some DNOs invoke this clause, treating the situation as if their fuse may at any time be replaced by a 4" nail - when it would indeed be a problem and against B7671, but at that point  you lose also your 16ka PSSC rating for your box of  6kA breakers as well.

Mike.

In fact it does not really deviate from BS7671 - the company fuse can and will protect suitably sized  tails just fine. It may well deviate from your DNOs protection advice, where they do not want to see long tails relying on their fuse - even if in a race between fuses, theirs may well finish first anyway especially when yours and theirs are the same rating.

The '3m' figure from '7671 is intended to relate to situations when there is no upstream protection to speak of, and might be for example when there is a tap line onto a 1000A bus for a 4mm2 wire supplying  one light and a socket that is fused down to 32 A or whatever only once it gets outside the location of the bus chamber. The wiring for the 'light switch in the substation' situation,,,, 

Some DNOs invoke this clause, treating the situation as if their fuse may at any time be replaced by a 4" nail - when it would indeed be a problem and against B7671, but at that point  you lose also your 16ka PSSC rating for your box of  6kA breakers as well.

Mike.