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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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    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?

    I can't help with a known formula I'm afraid, but I would have thought it was more a case of subtracting (or adding) the heat produced, rather than the current. The two will of course be related, but a the armour and core are likely to be of different materials as well as possibly different c.s.a.s I might suspect that the same current wouldn't produce the same heat in both.


       - Andy.
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  • 0
    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?

    I can't help with a known formula I'm afraid, but I would have thought it was more a case of subtracting (or adding) the heat produced, rather than the current. The two will of course be related, but a the armour and core are likely to be of different materials as well as possibly different c.s.a.s I might suspect that the same current wouldn't produce the same heat in both.


       - Andy.
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