Can a DNO Service Cut-out be used as the primary source of fault current protection, when the DB it supplies is more than 3m from the intake?

Physically, it's probably OK (provided all loop impedances are reasonable, TN system etc.)

Compliance wise, it's more difficult - BS 7671 only allows omission of fault protection for up to 3m (with other conditions), so you're reliant on the the DNO agreeiing that their fuses provide protection ... so you'll have to ask them. Usually they either refuse alltogether and advise relying on BS 7671's 3m rule, or set their own limist (which is frequently 3m or thereabouts), but your milage may vary.

    - Andy.

Thanks for your reply Andy, very helpful. 

If the DNO does not agree to its fuse providing fault protection, how do they prevent it from doing so?

And how does that sit with any additional fuse fitted likely being of the same rating?

From a practical point of view it should be fine, provided that the cables are correctly sized in relation to the fuse.

The DNO may however say otherwise, which is their prerogative. It COULD also be argued that the installation is not compliant with BS 7671, due to the omission of fault protection to conductors longer than 3 m

This in MY VIEW is incorrect, I would argue that the cut out fuse does provide fault protection to a correctly sized downstream cable. This may be confirmed by reference to fuse data and cable ratings. The fact that that the DNO will not confirm this is not in my view relevant, provided that the installer is satisfied. In what way does the ownership of the fuse affect the fault protection offered ?

Does anyone really believe that the cut out fuse [owned by the DNO] wont protect a suitably selected cable, but that a second identical fuse owned by the consumer will protect the cable ?

That said, if I was performing the installation, I would add the second fuse, to "tick a box" not because it does any good.

If an existing installation lacked the second fuse, then this would not worry me, provided that published data showed that the installed cable was properly protected by the fuse used, without concern as to whom owned the fuse. If the fuse carrier was sealed and the fuse size not reliably known, then I would presume the worst case.

In what way does the ownership of the fuse affect the fault protection offered ?

I guess the DNO's argument is that they may wish to change the fuse characteristics at some point in the future and don't want to be responsible for re-evaluating the design of the customer's installation if they do (at least not anything beyond looking at what's obviously next to the cut-out and compare with some very simple rules). Many homes have increased from 60A to 80A or 100A over recent decades and as there is a tendency to 'electrify' even more (space heating an EV) it's reasonable to assume that the trend will continue.

  - Andy.