So just now I was adding an extra light over my day bed it's a vintage 60 watt bulb looks like an old radio valve it even has the little glass pip on top. Anyhow as I was wiring it up two whiskas of wire touched there was barely any sound but it popped the 5 amp fuse in the plug adaptor and tripped 16 amp MCB how is this even possible even under partial short circuit conditions surely a 5 amp fuse should clear before a 16 amp MCB?
is this even possible even under partial short circuit conditions surely a 5 amp fuse should clear before a 16 amp MCB?
Almost all over-current devices have(at least) two stages to their operation - firstly the "commit" to opening but the actual stopping of the current happens some time later. Fuses first melt a small gap in the fusewire but the current arcs over that gap and so still flows in the circuit - only when the gaps gets large enough (or we get to the zero point in the a.c. cycle) does the arc fail and the current really get interrupted. MCBs have an even longer sequence - first 'de-latching' then physically moving the contacts apart before starting on the arcing phase before finally properly breaking the circuit. In each case once the first stage is reached, the device will eventually be left in an open state, even if some other device in the circuit gets to the final stage first.
MCBs are notoriously bad at discriminating - they de-latch relatively quickly (usually faster than a fuse will completely open) but take much longer to interrput the current - so allowing other devices plenty of time to see and start responding to the same fault current. MCBs pretty much don't discriminate at all with other MCBs (excepting where the fault current is too low to trip the larger rated MCB at all), and are pretty poor with downstream fuses. Fuses upstream of MCBs have a much better chance - but usually have to be significantly larger rated (say 60A+) even for small MCB ratings.
is this even possible even under partial short circuit conditions surely a 5 amp fuse should clear before a 16 amp MCB?
Almost all over-current devices have(at least) two stages to their operation - firstly the "commit" to opening but the actual stopping of the current happens some time later. Fuses first melt a small gap in the fusewire but the current arcs over that gap and so still flows in the circuit - only when the gaps gets large enough (or we get to the zero point in the a.c. cycle) does the arc fail and the current really get interrupted. MCBs have an even longer sequence - first 'de-latching' then physically moving the contacts apart before starting on the arcing phase before finally properly breaking the circuit. In each case once the first stage is reached, the device will eventually be left in an open state, even if some other device in the circuit gets to the final stage first.
MCBs are notoriously bad at discriminating - they de-latch relatively quickly (usually faster than a fuse will completely open) but take much longer to interrput the current - so allowing other devices plenty of time to see and start responding to the same fault current. MCBs pretty much don't discriminate at all with other MCBs (excepting where the fault current is too low to trip the larger rated MCB at all), and are pretty poor with downstream fuses. Fuses upstream of MCBs have a much better chance - but usually have to be significantly larger rated (say 60A+) even for small MCB ratings.
