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!)
