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S.W.A. Puzzler.

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You are to use two three core S.W.A. cables connected in parallel. Same size conductors and same length. The supply from source to load is single phase. If just using the steel wire armour as the C.P.C.s how would you arrange the six live cores to use them all for the single phase supply avoiding problems?


Z.
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    For AC supplies the rules require that you need the net current inside the steel armour to add to zero. Now if you don't then you do at least  need to consider two effects that come into play.

    The first is heating of the steel due to the magnetic hysteresis of the armour, though unlike a steel conduit the magnetic path around the cores is not complete, as the steel wire to wire contact is imperfect so this effect will not be as serious as it is for conduits and trunking.

    The second is the same problem you see with AWA cables, which is heating due to circulating currents in the two armours if they are glanded at both ends, so that when all is done up tight they form an circular loop.

    If the current inside that loop of conducting tube is not net-zero then you have created a shorted one turn transformer of sorts.

    To to be pure and meet the regs, you use L +N in each, and nothing else.

    In practice LNN and LLN would probably be just fine with a bit of thought about glanding.

    Mike.
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  • 0
    For AC supplies the rules require that you need the net current inside the steel armour to add to zero. Now if you don't then you do at least  need to consider two effects that come into play.

    The first is heating of the steel due to the magnetic hysteresis of the armour, though unlike a steel conduit the magnetic path around the cores is not complete, as the steel wire to wire contact is imperfect so this effect will not be as serious as it is for conduits and trunking.

    The second is the same problem you see with AWA cables, which is heating due to circulating currents in the two armours if they are glanded at both ends, so that when all is done up tight they form an circular loop.

    If the current inside that loop of conducting tube is not net-zero then you have created a shorted one turn transformer of sorts.

    To to be pure and meet the regs, you use L +N in each, and nothing else.

    In practice LNN and LLN would probably be just fine with a bit of thought about glanding.

    Mike.
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