Hi! Can anyone clear this question up for me.
Based on Design current (Ib) < Protective device rating (In) / rating factors < Tabulated CCC (It), to protect the circuit cable from reaching it's limiting / hazardous temperature:
Why and when is it OK to use a cable that doesn't comply with Ib<In<It? You see it with a 2.5mm cable for an oven on a 32A cooker circuit (maybe on a cooker plate with a hob for example). You see it with lights where the cable goes from say 1.5mm to a 0.75mm flex. Socket spurs can be wired in 1.5mm if I remember right.
On plugged in appliance it's different as there's the fuse to create a new Ib<In<It (or max load<fuse<It).
I see how the main circuit cable needs to withstand the entire circuit design current whereas a flex to a light just has the current load of the light, for example. But if that meant it was safe, why do plug in appliances have fuses and not the other examples I mentioned?
Also, yes a low impedance short circuit or earth fault will trip the MCB/RCBO/RCD quickly if the Zs is low enough, but what if there was a fault letting through enough current to melt the small cable but not to trip the MCB/RCBO/RCD?
Thanks!
