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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 internal 6mm wiring to the shunt device is technically a "circuit" protected by the main fuse. Not sure how this fits with the DNO view.



    I might be tempted to consider any factory fitted internals as being covered by the product standard rather than BS 7671 (or DNO rules) - as long as we use (say) 25mm² to the device then it's not our concern what's inside. Similar to the situation with bus-bars on many CUs that are often somewhat less than 25mm² c.s.a. (especially where they connect to the incoming device).


       - Andy.
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    the internal 6mm wiring to the shunt device is technically a "circuit" protected by the main fuse. Not sure how this fits with the DNO view.



    I might be tempted to consider any factory fitted internals as being covered by the product standard rather than BS 7671 (or DNO rules) - as long as we use (say) 25mm² to the device then it's not our concern what's inside. Similar to the situation with bus-bars on many CUs that are often somewhat less than 25mm² c.s.a. (especially where they connect to the incoming device).


       - Andy.
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