Does anyone on here know why the time/current characteristics published for BS1361 "plug top" fuses have two curves illustrated both for the 3 amp and the 13 amp sizes? It gives an "operating zone". To the best of my knowledge, no other overcurrent protection devices have two time/current curves. The BS1362 time/current chart isn't in BS7671 because (I think) it's not part of the fixed installation so the chart I have came off the internet. Not a major issue in itself - just a bit of a puzzle to me. The reason for my question is that I "considered the manufacturer's instructions" when connecting up a recent new boiler, and a 3 amp supply fuse was recommended, which I duly installed in an FCU. However, the radial circuit is on a 6amp Type B RCBO, so I was subsequently checking for selectivity ( I should have done that first - there isn't any!)
The two curves represent the 'all fire' and the 'never fire' conditions - real fuses have a considerable spread due to wire tolerances and variation in alloy mixture and so forth, and then on top of that the environment, i.e. how well the fuse caps are cooled, has an effect at on the exact boundary shape - if the fuse is running hot, then it takes less of an overload (time or duration) to melt it.
Actually all fuses and breakers are like this really, just for BS 7671 we are more interested in the 'all fire' condition, and concentrate on that curve, and often assume that the other limit might as well be a single value, the rating on the fuse body.
However the let through energy you need to get good discrimination between final circuits and distribution sort of relies on the same idea - just expressed differently.
Oh, and at 7 A there will be selectivity - the 6A breaker wont, the 3A Fuse will, after any duration that could be up to about 20 mins... I agree for a (near) short circuit the breaker will win up to about 500A, the the fuse will take over again as the breaker mechanics cannot go supersonic... . But do we really need to care that much? because regardless of which operates, the cable being protected will be fine.
Edit.
Once upon a time we just assume double or treble - so the 'never fire' curve for a 30A BS3036 hot wire fuse, may be assumed to be somewhere between 30A and 60A forever, and 60-120A for some minutes. etc.
And before anyone winces - 13A fuses discriminate really well with 20 and 30A fuse wire, and that in turn discriminates really well against a 60A or higher company fuse - and for the best part of half a century that was the preferred line up, and in many buildings in daily use, it still is. There is a reason there is so much slack in the cable ratings, and kettle leads can be 0.75mm without over heating, when according the annex they are way undersize.
The two curves represent the 'all fire' and the 'never fire' conditions - real fuses have a considerable spread due to wire tolerances and variation in alloy mixture and so forth, and then on top of that the environment, i.e. how well the fuse caps are cooled, has an effect at on the exact boundary shape - if the fuse is running hot, then it takes less of an overload (time or duration) to melt it.
Actually all fuses and breakers are like this really, just for BS 7671 we are more interested in the 'all fire' condition, and concentrate on that curve, and often assume that the other limit might as well be a single value, the rating on the fuse body.
However the let through energy you need to get good discrimination between final circuits and distribution sort of relies on the same idea - just expressed differently.
Oh, and at 7 A there will be selectivity - the 6A breaker wont, the 3A Fuse will, after any duration that could be up to about 20 mins... I agree for a (near) short circuit the breaker will win up to about 500A, the the fuse will take over again as the breaker mechanics cannot go supersonic... . But do we really need to care that much? because regardless of which operates, the cable being protected will be fine.
Edit.
Once upon a time we just assume double or treble - so the 'never fire' curve for a 30A BS3036 hot wire fuse, may be assumed to be somewhere between 30A and 60A forever, and 60-120A for some minutes. etc.
And before anyone winces - 13A fuses discriminate really well with 20 and 30A fuse wire, and that in turn discriminates really well against a 60A or higher company fuse - and for the best part of half a century that was the preferred line up, and in many buildings in daily use, it still is. There is a reason there is so much slack in the cable ratings, and kettle leads can be 0.75mm without over heating, when according the annex they are way undersize.
