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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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    That is as I'd always thought it.

    I picked out the "Fuse Box" brand F1M2SPD as potentially a very good  value way to provide up-front SPD for a garage installation which now has 4 CU's (EV, PV, heat pump, main house). Henley Block, no isolator. My part is technically only replacing the main CU. Fitting one of these isolator/SPD in series in the tails (ie not shunted off the Henley) before the Henley block appears to cover all bases with the bonus the SPD operates on the whole installation. 

    However, it also struck me that though the SPD device is rated by the manufacturers as not needing MCB, (ie OK right on suppliers fuse), 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. (TNCS install BTW).

     


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  • 0
    That is as I'd always thought it.

    I picked out the "Fuse Box" brand F1M2SPD as potentially a very good  value way to provide up-front SPD for a garage installation which now has 4 CU's (EV, PV, heat pump, main house). Henley Block, no isolator. My part is technically only replacing the main CU. Fitting one of these isolator/SPD in series in the tails (ie not shunted off the Henley) before the Henley block appears to cover all bases with the bonus the SPD operates on the whole installation. 

    However, it also struck me that though the SPD device is rated by the manufacturers as not needing MCB, (ie OK right on suppliers fuse), 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. (TNCS install BTW).

     


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