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.

You'd have to check with your local DNO, but the usual solution for flats etc is for the DNO just to provide the intake fuses (1/phase) and then the BNO owns he Ryefield unit with one fuse/flat - as that's the BNO's rather than the DNOs, it's not subject to the DNO's 3m llimit and so no further protection is needed (just a 'red head' cutout before the individual flat's meter, if it's far downstream).

   - Andy.

Some DNOs invoke this clause, treating the situation as if their fuse may at any time be replaced by a 4" nail

Or just a higher rated fuse. Say a customer has a 60A fuse and asks for an upgrade to 100A - how can the DNO satisfy their duty of care that they're not creating additional risks by their actions if they can't see what it'll be protecting (16mm2 tails? 25mm2 tails in thermal insulation, lord knows what as it's buried in the structure of the building..) Hence most DNOs not only apply a 3m limit but ask the cables are visible too.

  - Andy.

true, but the choice of 3m is an arbitrary coincidence with the second part of the following, which covers the 'no fault protection case'

I have no problem with a DNO having sensible house rules that require 3m,or indeed visibility, or any other such requirement but the situation described is not the same as the one covered by this particular reg in BS7671 and to assume it against the regs as such  would be incorrect.

433.2.2
The device protecting a conductor against overload may be installed along the run of that conductor
if the part of the run between the point where a change occurs (in cross-sectional area, method of installation, type
of cable or conductor, or in environmental conditions) and the position of the protective device has neither branch
circuits nor outlets for connection of current-using equipment and fulfils at least one of the following conditions:

(i) It is protected against fault current in accordance with the requirements stated in Section 434

(ii) Its length does not exceed 3 m, it is installed in such a manner as to reduce the risk of fault to a minimum,
and it is installed in such a manner as to reduce to a minimum the risk of fire or danger to persons (see also
Regulation 434.2.1).

In terms of short circuit faults the company fuse will operate so we are in a the longer length clause (i) condition, not the length limited clause (ii) case where we tolerate a small risk of a cable fire - the 'self clearing fault' where the wire burns back.

Mike.

Thank you both for providing such insightful information and for clarifying that our proposed approach does not deviate from the guidelines outlined in BS 7671. Your expertise and clarity on this matter are greatly appreciated.

Moreover, it's reassuring to learn that our plan to comply with regulation 434.3 (iv) aligns with industry standards. Understanding that obtaining approval from the Distribution Network Operator (DNO) is sufficient in this regard provides us with confidence moving forward.

433.2.2 (ii) seems a bit superfluous, as it's just a re-wording of 434.2.1 - so already an option under (i) (unless I've missed something - always possible).

It's 433.3.1 (iii) and 434.3 (iv) that make it more of a challenge if you're outside the DNO's rules.

  - Andy.

yes - you do need the nod from the DNO that it meet their requirements,- and  maybe more usefully to know for sure what their fuse rating is and the largest it could ever be.

If the tails are sooo loooong that a dead short at the end won't blow the company fuse in a sensible time then there are other more serious issues to consider.

the  meter tails should not be so thin as not to be afforded both overload and fault cover by the largest company fuse likely to be fitted - probably 100A

Mike.

probably 100A

OP said it's a "multi-apartment residential block" so possibly larger than that - for comparison the DNO version of that Lucy insulated MSDB came with 315A incomers.

   - Andy.