What limitations are there on using the steel wire in armoured cable as the earth conductor?

Answer 1

This may depend on the regulations you are working to, but generally the earth resistance must be no more than 50% of the phase conductor resistance. since steel does not conduct as well as copper, the total csa needs to be somewhat higher.

Some cables have some of the steel wires replaced with copper wires to give the required path. These need to be specified.

Many installers don't take the risk and will use a dedicated conductor either included in the cable or ran separately.

Your cable supplier should be able to guide you suitably. If not consider buying someplace else.

Answer 2

The Steel wire must not be used as an earthing conductor. The reason being that the surface area of the steel wire may be insufficient for the amperage your running and may be venerable to skin effect.

Secondly you can never guarantee the condition of the gland steel wire interface.

A separate earth conductor should be run if no other conductors are available. If you have a spare way then buy some earth tape and mark it appropriately if not then you need run a separate earth cable tying to the main run, the earth should be the same size as the cross section of the armoured cable i.e. 16mm armoured then its a 16mm earth.

Good electrical practice dictates that you run the separate conductor anyway particularly if the Cable is under ground etc.