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!)
Actually it not just an accident, it is essential for sanity that MCBs and fuses are not too trippy and have a well defined load dependant time delay - many domestic devices have significant inrush currents that persist while motors are spinning up (Fridges may take 50-100A until the rotor has got moving) or while hot wires are getting hot.The old filament lamp is a classic for that temperature dependant resistance - a 60W lamp takes about 1/4 of an amp when hot, but has about 10% of the resistance when cold. so takes more like 2 and a bit amps when cold - so dimmers on the really dim setting are always dealing with thin slices of many times the current you might expect, so you find you need a 10A triac in a 200W dimmer...
Modern LED lights with switch mode supplies introduce a new issue, of a bridge rectifier feeding a capacitor, where on switch on (mains - 2 lots of 0.7V ) finds itself charging a flat capacitor of many microfarads, so you may get 100A plus inrush for the first part cycle.
The breaker curves are deliberately designed to approximate a fuse-like time current integral up to the point the magnetic near instant part takes over to allow things like motors, lamps etc to be used without too much hassle.
Actually it not just an accident, it is essential for sanity that MCBs and fuses are not too trippy and have a well defined load dependant time delay - many domestic devices have significant inrush currents that persist while motors are spinning up (Fridges may take 50-100A until the rotor has got moving) or while hot wires are getting hot.The old filament lamp is a classic for that temperature dependant resistance - a 60W lamp takes about 1/4 of an amp when hot, but has about 10% of the resistance when cold. so takes more like 2 and a bit amps when cold - so dimmers on the really dim setting are always dealing with thin slices of many times the current you might expect, so you find you need a 10A triac in a 200W dimmer...
Modern LED lights with switch mode supplies introduce a new issue, of a bridge rectifier feeding a capacitor, where on switch on (mains - 2 lots of 0.7V ) finds itself charging a flat capacitor of many microfarads, so you may get 100A plus inrush for the first part cycle.
The breaker curves are deliberately designed to approximate a fuse-like time current integral up to the point the magnetic near instant part takes over to allow things like motors, lamps etc to be used without too much hassle.
