Lights Supplied by 30 Amp Ring Final. B.S. 3036 Fuse.

I think most are of the opinion that having two fuses creates two further circuits - so with a common N that's technically a contravention of BS 7671. I agree that in this particular circumstance the actual danger is minimal - but as usual (as with the tails though thin steel plates) BS 7671 tends to have a blanket requirement based on a simple principle and doesn't bother distinguishing between different practical situations. If it was two different circuits from a CU or even two different FCUs with a common N downstream, it would maybe be a more obvious contravention but I think, given BS 7671's wording, the same rules apply here. Like I said, not an immediate calamity, but perhaps something to keep in mind if you're putting your name on a piece of paper claiming it did conform to BS 7671.


From the point of view of a 3A fuse being there to limit the amount of current availble to start a fire (presuming we beleive the manufacturers) then maybe having two 3A supplies to the same smouldering box rather defeats the intention (depening on how the perhaps charred innards allow current to flow from both L and SL under the wrong conditions) - as a worst case it might be thought that two 3A supplies might be no better than a single 6A supply...


As others have said, the usual solution is a single 3A fuse upstream of both the light, switch and fan - although granted that's usually far less convenient to retrofit.


   - Andy.

I think most are of the opinion that having two fuses creates two further circuits - so with a common N that's technically a contravention of BS 7671. I agree that in this particular circumstance the actual danger is minimal - but as usual (as with the tails though thin steel plates) BS 7671 tends to have a blanket requirement based on a simple principle and doesn't bother distinguishing between different practical situations. If it was two different circuits from a CU or even two different FCUs with a common N downstream, it would maybe be a more obvious contravention but I think, given BS 7671's wording, the same rules apply here. Like I said, not an immediate calamity, but perhaps something to keep in mind if you're putting your name on a piece of paper claiming it did conform to BS 7671.


From the point of view of a 3A fuse being there to limit the amount of current availble to start a fire (presuming we beleive the manufacturers) then maybe having two 3A supplies to the same smouldering box rather defeats the intention (depening on how the perhaps charred innards allow current to flow from both L and SL under the wrong conditions) - as a worst case it might be thought that two 3A supplies might be no better than a single 6A supply...


As others have said, the usual solution is a single 3A fuse upstream of both the light, switch and fan - although granted that's usually far less convenient to retrofit.


   - Andy.