Single Core Cables

A1: heat dissipation is greater in flat formation, therefore current carrying capacity is greater.

refer to the following thread

http://cr4.globalspec.com/thread/76571/SS-Glands-for-Single-Core-Cables

Tri-foil limits magnetic interference but has the disadvantage of lower current.

Aluminium armouring is bonded at one end only to provide earthing but at the same time prevent circulating currents which would occur if both ends were bonded.

Hi

The reason that single core cables are laid in trefoil formation is to reduce the effects of harmonics that happen in this modern age, due to switch mode power supplies and VSD's.

If supplies with harmonic issues are not laid in trefoil formation the voltage drop across the cable and also has effects on the neutral current, check out http://en.wikipedia.org/wiki/Harmonics_(electrical_power)

Cheers

Joe

1. In multicore cables, the resultant magnetic field around the cable is nullified as it will be the vector sum of the phase(s) current and the neutral current. As such there would not be any induced emf on to the cable armour. Whereas in single-core cables there is a possibility of such induced emf on to the cable armour which could circulate currents thro the armour and thus cause additional heating in the armour & hence premature failure of the cable. One of the ways to avoid the emf induction, is to lay the single core cables in trefoil formation.

2. In single core cable, there is sheath current as explained above.

3. Bonding the armour/sheath at both ends would cause circulating currents (if the single core cables are laid in a non-trefoil formation)

Bonding the armour at one end, would definitely avoid circulating current, bute there could be dangerous touch voltages developed in the shetah, which could harm anybody coming in contact with the same.

please elaborate to me the difference between multicore cable and 3-single core cables (forming 3 phase) in the resultant field ?

why do you say that multicore cable resultant field will have zero resultant field,while 3-single core cable resultant field isn't zero,althought the sum the 3 phase current in both cases is zero so there should'nt be resultant field

Well! You need some basic training in flux linkages. In muli-core cables, the cores are laid in trefoil form. As such, the flux linkages between the circumferential flux generated due to the current flowing in each core with respect to the cable armour of the other cores would be radial (i.e.) at 90 degrees. Hence, no emf will be induced in the armour due to this circumferential flux. This is also true in three single core cables laid in trefoil formation externally. Whereas, in single core cables laid in falt formation, the flux linkages will be axial and hence there would be induced emf in the armour.

Hope this is clear now!