Grounding

To avoid circulating currents usually earthing at one is practiced.

If it is a single core cable laid flat, then earth the cable armour at one end only.

If it is a multicore cable or single core cable laid in trefoil formation then you may earth the cable armour at both ends.

dear sir,

Electrical Earthing at 2 point is always better. Ther would not be circulating current, but fault current shall be divided in 2 parts, reducing the loading on Armour. Wether it is single core / multi core / trfoil or flat laying, Electrical fault energy always divides in 2 paths with 2 earth points, reducing damages to the conducting portion of the armour.

What was said in earlier replies, may be applicable for Electronics earth, where noise interferences are to be elimintaed , say in signal cables for PLC , RS485 cables, communication cables etc... there, single earthing is used only to eliminate the noise levels ( noise - electronic electrmagnetic - RF interference / )

For a single core cable which carries AC current, the armor shouldn't be grounded at both ends. The alternating nature of the current flowing through the conductor induced voltage along the length of the armor and there exists a voltage difference between the both ends of the armor. If it is grounded at both ends, a circulating current always flows through the armor and keeps the cable additional heated. To prevent it, only one end (generally the source end) is grounded and the other end is insulated. The CEC (Canadian Electric Code) rules 4-008 and 10-304 applies restricting grounding both ends for this case.

For the multi-conductor cable, the ultimate effect of alternating current is zero (cancelling out due to phase neutral or 3 phase system) and there is no induced voltage along the length of armor.

See also http://cr4.globalspec.com/thread/40950/Armored-Cable-and-Earthing

- MS

You are terribly wrong, I am afraid! It is not the fault current that we are discussing. If you have a single core cable, when the cable conductor carries an alternating current, it would set up a flux around the conductor. The cable armour, if it is made up of a magnetic material, would cut this flux and an EMF would be induced in this armour. If the armour is earthed at both ends, it becomes a "closed loop" (like that of a short-circuited transformer secondary winding) and definitely a current would circulate in the armour thus heating up the armour and the insulation. If you earth at only one end, EMF will be there but there would not be any circulating current (as in a open circuited transformer secondary).

In muli-core cables or in trefoil-laid single-core cables, all the three-phase (and the neutral, in three and a half or in four core cables) fluxes surround the armour and the vector sum is zero and there would not be any induced EMF in the armour. Hence no circulating current in the armour and you can earth the same at both glanding ends.