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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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    So we can use the DNO main fuse to protect up to three metres of meter tails.


    But presumably that is from the actual fuse, so includes the tails into the suppliers meter as well as the tails out to the consumer unit.


    We can also use it to protect the main switch along with the internal tails and busbars in the the consumer unit, in addition we can use the suppliers fuse as backup if the prospective fault current is too high for the MCBs with the PFC being up to 16.5 kA with a device that is rated at 6 kA.


    Now in some installations the DNO fuse is being used as the protective device for surge protection devices and their connections, with the SPDs mounted in the consumer unit, a separate enclosure or within a main switch enclosure.


    It doesn’t actually matter if the meter tails are three metres long or less than a metre, the implications and risks are the same.


    Andy Betteridge
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  • 0
    So we can use the DNO main fuse to protect up to three metres of meter tails.


    But presumably that is from the actual fuse, so includes the tails into the suppliers meter as well as the tails out to the consumer unit.


    We can also use it to protect the main switch along with the internal tails and busbars in the the consumer unit, in addition we can use the suppliers fuse as backup if the prospective fault current is too high for the MCBs with the PFC being up to 16.5 kA with a device that is rated at 6 kA.


    Now in some installations the DNO fuse is being used as the protective device for surge protection devices and their connections, with the SPDs mounted in the consumer unit, a separate enclosure or within a main switch enclosure.


    It doesn’t actually matter if the meter tails are three metres long or less than a metre, the implications and risks are the same.


    Andy Betteridge
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