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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
    Jimi said:
    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?

    So, BS 7671 has two requirements for overcurrent protection:

    The first is protection against fault current. In this situation, a fault of negligible impedance only is considered. The cross-sectional area (csa) of the cable must meet the adiabatic equation, although a more complex alternative calculation is also permitted.

    The second is protection against overload current. Overload current may be omitted under certain circumstances, usually for final circuits where accidental overload is not considered possible (usually fixed resistive loads).

    BS 7671 does not consider resistive faults in the way you describe - however, some equipment, such as those with certain heating elements, may occasionally develop faults which might lead to overload rather than full fault current. In some cases, the manufacturer of an appliance may specify the maximum rating of the overcurrent protective device for the appliance, and if that is the case, it might be appropriate to rate the final circuit cable according to the overcurrent protective specified by the manufacturer, if you are in any doubt.

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  • 0
    Jimi said:
    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?

    So, BS 7671 has two requirements for overcurrent protection:

    The first is protection against fault current. In this situation, a fault of negligible impedance only is considered. The cross-sectional area (csa) of the cable must meet the adiabatic equation, although a more complex alternative calculation is also permitted.

    The second is protection against overload current. Overload current may be omitted under certain circumstances, usually for final circuits where accidental overload is not considered possible (usually fixed resistive loads).

    BS 7671 does not consider resistive faults in the way you describe - however, some equipment, such as those with certain heating elements, may occasionally develop faults which might lead to overload rather than full fault current. In some cases, the manufacturer of an appliance may specify the maximum rating of the overcurrent protective device for the appliance, and if that is the case, it might be appropriate to rate the final circuit cable according to the overcurrent protective specified by the manufacturer, if you are in any doubt.

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