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Armour bonding: Effect on cable rating

Jam
Hi all,

Inspired by this recent post (but not wanting to digress too far), the advantage of single-bonding cable is ostensibly the reduced circulating current in the armour and hence increased capacity in the line conductors... But can anyone tell me if there’s a way to quantify this benefit, without recalculating on first principles per IEC60287? Is it as “simple” as calculating the induced voltage using the method in the IET calculations book and then “adding back” the avoided current that would have otherwise ensued to the tabulating rating?

Also I most commonly see this applied at private transformer tails... but surely there they’re normally so short it’s of little benefit unless you’re mainly concerned with fault current rating?

Thanks as always.
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    Morning all.

    To clarify I'm thinking of induced current in the armour of single core AWA cables (and similar). Think typical transformer LV tails (though in that case the runs might be too short to induce much current). Most cable datasheets for general purpose applications specify current carrying capacity for two-point bonding - though not all. We currently just use that rating and then any gain from reduced eddy currents are a bonus.


    Calcs for induced voltages in single point bonded cables are in the IET "Calculations for Electricans and Designers"'. You could use V=IR with the resistance of the armour and return path(s) to estimate the resulting current... and then subtract that from the current carrying capacity or is there a more considered formula available*? Indeed is that even the conservative case, given that the heat is now in a different place from the core?
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  • 0
    Morning all.

    To clarify I'm thinking of induced current in the armour of single core AWA cables (and similar). Think typical transformer LV tails (though in that case the runs might be too short to induce much current). Most cable datasheets for general purpose applications specify current carrying capacity for two-point bonding - though not all. We currently just use that rating and then any gain from reduced eddy currents are a bonus.


    Calcs for induced voltages in single point bonded cables are in the IET "Calculations for Electricans and Designers"'. You could use V=IR with the resistance of the armour and return path(s) to estimate the resulting current... and then subtract that from the current carrying capacity or is there a more considered formula available*? Indeed is that even the conservative case, given that the heat is now in a different place from the core?
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