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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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    The 3m distance (or in my employer’s case 2.5m) takes you to the customer’s first over current device, whether this is a fuse switch or the consumer unit. Note the word “tails”. This is important, as the entire distance before the over current device must be in single meter tails. If you need to use SWA or some other multi core cable, then you will need overcurrent protection immediately before it. The reasoning is, the distance is used in the DNOs design calculations for both the normal arrangement and back feed options. The DNO cannot calculate each sub-main in each property, so it is easier to set a standard, and say that is what it must be. 


    Regards,


    Alan.
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  • 0
    The 3m distance (or in my employer’s case 2.5m) takes you to the customer’s first over current device, whether this is a fuse switch or the consumer unit. Note the word “tails”. This is important, as the entire distance before the over current device must be in single meter tails. If you need to use SWA or some other multi core cable, then you will need overcurrent protection immediately before it. The reasoning is, the distance is used in the DNOs design calculations for both the normal arrangement and back feed options. The DNO cannot calculate each sub-main in each property, so it is easier to set a standard, and say that is what it must be. 


    Regards,


    Alan.
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