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When cables are downsized without a fuse, question

Jimi

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!

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  • 0

    Accessory terminals can be something else - quite often the current that we can think of as coming down the wire as  a more or less full cylinder of current, i.e using the full area of conductor, is asked to bunch up to one side or the other to get on or off via the small area at the point of a screw and the rather odd shaped splodge just opposite it where two curved surfaces meet, somteimes  with very different radii.
    There is a reason that very high current terminals use flat palm braids at high frequencies or an all-round hex crimp and a flat bolt area for DC/50Hz stoff , to be sure that the contact area is several times the cross-section of the cable coming in, so you see no pinch points if you  cut across the current anywhere along its journey.

    Mike

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  • 0

    Accessory terminals can be something else - quite often the current that we can think of as coming down the wire as  a more or less full cylinder of current, i.e using the full area of conductor, is asked to bunch up to one side or the other to get on or off via the small area at the point of a screw and the rather odd shaped splodge just opposite it where two curved surfaces meet, somteimes  with very different radii.
    There is a reason that very high current terminals use flat palm braids at high frequencies or an all-round hex crimp and a flat bolt area for DC/50Hz stoff , to be sure that the contact area is several times the cross-section of the cable coming in, so you see no pinch points if you  cut across the current anywhere along its journey.

    Mike

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