Alternating current flowing in a conductor sets up an alternating magnetic field which is much stronger if the conductor is surrounded by an iron-rich material, for example if it is steel wire armoured or if it is installed in a steel conduit. The currents in a twin cable, or in two single core cables feeding a single load, will be the same. They will exert opposite magnetic effects which will almost cancel, so that virtually no magnetic flux is produced if they are both enclosed in the same conduit or armouring. The same is true of three-phase balanced or unbalanced circuits provided that all three (or four, where there is a neutral) cores are within the same steel armouring or steel conduit.
Hence, it is important that all conductors of a circuit are contained within the same cable, or are in the same conduit if they are single-core types
For this reason, single-core armoured cables should not be used. If the single core cable has a metal sheath which is non-magnetic, less magnetic flux will be produced. However, there will still be induced EMF in the sheath, which can give rise to a circulating current and sheath heating.
The induced EMF causes circulating currents / eddy currents to flow in metallic enclosures, gland plates etc particularly if the conductor armourings are purposely commonly bonded / earthed at both ends (separate single bonding can eliminate this) , creating a potential equipment failure and fire / explosive atmosphere ignition hazard.
When I think of circulating currents associated with single core cables, I think of the induced voltages in the cable screens / armourings, caused by current flow in the conductor itself. That induced voltage can cause a circulating current to flow, if the cable is earthed at both ends. That circulating current can be quite high, causing localized heating at ferromagnetic gland plates, any associated tray work, metallic trunking, conduit etc.
If the cable is SWA, the problem is a whole lot worse, because SWA is magnetic. That is why single core cables, particularly when designed to carry above 400A, should not be SWA. Using AWA cable reduces this problem because aluminum is non-magnetic (as is CWA).
These circulating currents also generate eddy currents at the gland plates etc that create further heating effects. There are also hysteresis losses caused by this in the ferromagnetic material.
One way to mitigate this uncontrolled current is to earth only one end of the single core cable, but this then introduces a standing voltage at the unearthed end, which can be high enough to be hazardous. Using cross bonding with transposition helps reduce the circulating current / standing voltage.
Correct spacing and use of non-magnetic materials for armours, gland plates etc all contribute to decreasing the problems. Another way to reduce eddy current heating at gland plates is to put a saw cut between the single core cable entry holes/glands - that breaks the path and prevents eddy current circulation.