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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.
  • 0
    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.
  • 0
    I suspect Jam's is thinking of the case of single core armoured cables - where even non-magnetic armours can end up with some circulating currents if they form part of a loop. (I suppose it's a bit like a coreless 1:1 transformer with two copper windings).

       - Andy.
  • 0
    I have a feeling Jam is thinking of EMI in VFD cables. When you put a link to a post, the forum just links to the thread, and EMC was mentioned in that thread.
  • 0
    Try and keep magnetic an electrical effects mentally distinct.

    'Eddy currents' are electric currents that always want to flow in a direction parallel  to the current that creates them, they create heating in the normal electrical resistance (I2 R) sort of way


    . Magnetic field lines want to form rings around the currents that create them and are the direction that the little atomic scale compasses in the chunk of iron or steel keep trying to swing round to  align with, and generally at 50Hz they more or less keep up, but friction (magnetic hysteresis) means they create some heat doing so.


    When you know which mechanism you are dealing with you know which way things need to be cut or stranded or grouped.

  • 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?
  • 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.
  • 0
    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.


    Ach! Yes of course that's obvious when you put it that way, yet hadn't even crossed my mind, thank you.?

    Hmm...


  • 0
    My recollection is that for single point bonded armoured cables you can use the ratings for the unarmoured version of the same cable.

    The extra layer of plastic adds a bit to the thermal resistance and the greater diameter adds more surface area so the two effects nearly cancel out.

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