Armored Cable and Earthing

To eliminate ground loops.

Single conductor armoured cable does not contain a ground wire. The sheath is for protection only. Grounding a both ends would make the sheath a grounding source. Interconnection to ground. The idea behind grounding only one end is to bleed any voltage that might be induced into the sheath without creating a circulating current or have the sheath as a ground.

Because...

In single core armoured cables, you do not have the magnetic fields generated by the other two phases. In a three phase cable, the sum value of the three magnetic fields is zero. In a single core cable, there is no "cancelling" effect, so... a voltage will be induced in the armour by the magnetic field surrounding the conductor.

The impressed voltage is directly tied to the size of the magnetic field generated by the current flowing in the cable. The impressed voltage can be quite high. When both ends are grounded, a "circulating sheath current" can flow between the armour, to ground then back to the armour. The magnitude of the circulating current is a product of the impressed voltage and the impedance of the sheath.

This circulating current can cause considerable heating in the armoured cable and will require a de-rating of the cable ampacity be applied.

To prevent this from being a problem. Single conductor metallic armoured cable (armoured with lead, aluminum, stainless steel, steel or copper) are bonded to ground, usually at the source end, then isolated from ground along its length.

If such isolation can not be done, under the Canadian code, a de-rating down to 70% of original ampacity is required.

For further reading... see CEC rules 4-008 and 10-304.

How about a 3C cable + 1grnd is it also required to ground at both ends the armoured wire?. Load is most likely unbalanced. The distance is ~ 400meters of cable armoured 3.5C (directly buried) between two substation (Main and distribution substation). It was a switchgear to switchgear connection with the same voltage level. We are suspecting heating of cable in the source (MV swithcgear 34.5KV). Can anyone give some ideas or experience on how they handle the problem.