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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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    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.



    The rule will run as far as the conductors that are only protected from overcurrent by the DNO's fuse - so the furthest switchfuse or CU that's directly connected to the supply. Remote CUs fed via a switch-fuse or a way in a CU next to the intake wouldn't be a concern.


      - Andy.
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    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.



    The rule will run as far as the conductors that are only protected from overcurrent by the DNO's fuse - so the furthest switchfuse or CU that's directly connected to the supply. Remote CUs fed via a switch-fuse or a way in a CU next to the intake wouldn't be a concern.


      - Andy.
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