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Armour bonding: Effect on cable rating

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.
Parents
  • Hi Jam


    The point is that there is no circulating current on the armour (at least of any magnitude) because the magnetic field from the conductors currents sum to zero. This is the same as any conductors passing through a hole in a magnetic material, through the same hole the eddy current loss is small (and in the material thickness) whereas through one hole per conductor the loss is larger, depending on the material thickness. Thus the effect on the current rating is not significant. SWA cables do not get hot due to armour losses provided that the circuit is all contained in one cable. If one must use singles then aluminium or copper armoured cables are available, which minimise the losses.


    Regards

    David CEng etc.
Reply
  • Hi Jam


    The point is that there is no circulating current on the armour (at least of any magnitude) because the magnetic field from the conductors currents sum to zero. This is the same as any conductors passing through a hole in a magnetic material, through the same hole the eddy current loss is small (and in the material thickness) whereas through one hole per conductor the loss is larger, depending on the material thickness. Thus the effect on the current rating is not significant. SWA cables do not get hot due to armour losses provided that the circuit is all contained in one cable. If one must use singles then aluminium or copper armoured cables are available, which minimise the losses.


    Regards

    David CEng etc.
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