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!)
Don't manufacturers' data for MCBs have similar zones? I must admit that I find them confusing.
Putting two and two together, an MCB will not trip in under an hour if I < 1.15 x In but we should not take advantage of this (433.1). The situation might be unmanageable if an MCB tripped the moment that it's rated current was exceeded.
Equally, an MCB must trip within an hour when I > 1.45 x In assuming that the lowest CCC of any conductor equals the rating of the MCB (433.1.1(iii) ).
In reality, any particular MCB will trip somewhere within the zone. Precisely where cannot be said because of manufacturing tolerances.
Don't manufacturers' data for MCBs have similar zones? I must admit that I find them confusing.
Putting two and two together, an MCB will not trip in under an hour if I < 1.15 x In but we should not take advantage of this (433.1). The situation might be unmanageable if an MCB tripped the moment that it's rated current was exceeded.
Equally, an MCB must trip within an hour when I > 1.45 x In assuming that the lowest CCC of any conductor equals the rating of the MCB (433.1.1(iii) ).
In reality, any particular MCB will trip somewhere within the zone. Precisely where cannot be said because of manufacturing tolerances.
