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Thoughts on 3M max tails protection with SPD & isolator in tails?

justinneedham
DNO allowance for tails protection from their fuse is commonly 3M. - 3M is deemed AFAICT to extent through any in-line isolator, whether or not that isolator is property of DNO or is consumer fitted part. (The latter is not spelled out by DNO SSEPD who  just state the 3M tails figure without further qualification).
If said isolator begins to sprout additional complexity such as MCB and SPD as a shunt addition, my interpretation (Definitions, part 2) is that it still doesn’t yet qualify as a consumer unit, hence the 3M rule could be said to still run straight through it as far as the farthest CU.

- Any thoughts?

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    A length limit of 3m after a 100A fuse  is not a BS7671 requirement, it is a DNO house rule that sets the length of tails . Given that you can then have another fuse of the same rating, and then as much cable as you like, it does not stand scientific scrutiny So long as any damage to the tails would blow the company fuse promptly, as far as BS7671 goes it is satisfactory. Subject to the usual things like rating, suitable routes etc.

    It may be worth a note that it is 'unusual', and the DNO may not like to connect to it if they did it now, but fails no BS7671 requirement.
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    A length limit of 3m after a 100A fuse  is not a BS7671 requirement, it is a DNO house rule that sets the length of tails . Given that you can then have another fuse of the same rating, and then as much cable as you like, it does not stand scientific scrutiny So long as any damage to the tails would blow the company fuse promptly, as far as BS7671 goes it is satisfactory. Subject to the usual things like rating, suitable routes etc.

    It may be worth a note that it is 'unusual', and the DNO may not like to connect to it if they did it now, but fails no BS7671 requirement.
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