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Wonder if there's any paper justification regarding HV/LV armoured cable earthing?

Marcus Ting

In short, armoured cable earthing is to mitigate stray current induced within the cable, but we were told that rule of thumb practice (compliant to IEC) is that 3C TCWB cables shall be earthed at both ends, while 1C TCWB cables shall be earthed at 1 end. Why is 1C TCWB cables earthed only at 1 end? Because if the 1C cables are laid in trefoil arrangement, isn't it resemble 3C arrangement too? My guess is that it is a minimum requirement to earth at 1 end for 1C TCWB cables, where earthed at both ends are totally fine, Eg. cables for instruments are earthed at both ends due to highly sensitive equipment they have. 

I'm still looking around for written justification, any input or discussion will help significantly

P.S Edited from SWA to TCWB to clear the clouds.

I have found the source in ABS 4-8-4/21.7.2 "Single Conductor Cables" (iii) The cable armoring (to be non-magnetic) or any metallic protection (non-magnetic) is to be earth at mid span or supply end only;

Is there any clarification for earthing at supply end only?

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    Because if the 1C cables are laid in trefoil arrangement, isn't it resemble 3C arrangement too?

    Not entirely. With 3x 1-core cables each individual conductor is surrounded by the armour, whereas with a 3 core cable, the three conductors have a single armour around all of them - so in the 3C cable instance the magnetic fields from each core get a chance to cancel each other out before inducing any current in the armour, whereas with 3x 1C cables the current is induced in each of the three armours - and then we have the harder problem of having to cancel out the currents (by interconnecting the ends and having currents flowing around everywhere, generating heat).

    Generally the problem is reduced by using non-ferrous armouring (not steel) on single core cables, but not entirely eliminated, as even with copper "armour" you still have what's in effect a rather inefficient 1:1 single-turn transformer.

       - Andy.

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  • 0

    Because if the 1C cables are laid in trefoil arrangement, isn't it resemble 3C arrangement too?

    Not entirely. With 3x 1-core cables each individual conductor is surrounded by the armour, whereas with a 3 core cable, the three conductors have a single armour around all of them - so in the 3C cable instance the magnetic fields from each core get a chance to cancel each other out before inducing any current in the armour, whereas with 3x 1C cables the current is induced in each of the three armours - and then we have the harder problem of having to cancel out the currents (by interconnecting the ends and having currents flowing around everywhere, generating heat).

    Generally the problem is reduced by using non-ferrous armouring (not steel) on single core cables, but not entirely eliminated, as even with copper "armour" you still have what's in effect a rather inefficient 1:1 single-turn transformer.

       - Andy.

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