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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 does sort of 'align' with a BS 7671 regulation though - i.e. the 3m requirement of 434.2.1. I'm sure at least one DNO (I can't remember which off the top if my head) has it written in their technical documentation that they offer no guarantees at all about what their fuse will provide for the consumer's installation, and so suggest using the 434.2.4 approach which allows a short length of BS 7671 circuit not to have overcurrent protection against faults. What that implies fo ADS with metal CUs isn't entirely clear.

       - Andy.
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  • 0

    A length limit of 3m after a 100A fuse  is not a BS7671 requirement,



    It does sort of 'align' with a BS 7671 regulation though - i.e. the 3m requirement of 434.2.1. I'm sure at least one DNO (I can't remember which off the top if my head) has it written in their technical documentation that they offer no guarantees at all about what their fuse will provide for the consumer's installation, and so suggest using the 434.2.4 approach which allows a short length of BS 7671 circuit not to have overcurrent protection against faults. What that implies fo ADS with metal CUs isn't entirely clear.

       - Andy.
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