4.2.4  Cables carrying alternating currents

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.

An alternating flux in an iron core results in iron losses, which result in power loss appearing as heat in the metal enclosure. It should be remembered that not only will the heat produced by losses raise the temperature of the conductor, but that the energy involved will be paid for by the installation user through his electricity meter. Thus, 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 (see {Fig 4.4}).

Fig 4.4 Iron losses in the steel surrounding a cable when it carries alternating current

a) twin conductors of the same single-phase circuit - no losses
b) single cone conductor- high losses

A similar problem will occur when single-core conductors enter an enclosure through separate holes in a steel end plate {Fig 4.5}.

Fig 4.5 Iron losses when single-core cables enter a steel enclosure through separate holes

 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 e.m.f. in the sheath, which can give rise to a circulating current and sheath heating.

If mineral insulated cables are used, or if multi-core cables are used, with all conductors of a particular circuit being in the same cable, no problems will result. The copper sheath is non-magnetic, so the level of magnetic flux will be less than for a steel armoured cable; there will still be enough flux, particularly around a high current cable, to produce a significant induced e.m.f. However, multi-core mineral insulated cables are only made in sizes up to 25 mm² and if larger cables are needed they must be single core.

{Figure 4.6(a)} shows the path of circulating currents in the sheaths of such single core cables if both ends are bonded. {Figure 4.6(b)} shows a way of breaking the circuit for circulating currents.

Fig 4.6 Circulating currents in the metal sheaths of single core cables
(a) bonded at both ends (b) circulating currents prevented by single point bonding

[523-05-01] calls for all single core cable sheaths to be bonded at both ends unless they have conductors of 70 mm² or greater. In that case they can be single point bonded if they have an insulating outer sheath, provided that:

i)   e.m.f. values no greater than 25 V to earth are involved, and
ii)   the circulating current causes no corrosion, and
iii)  there is no danger under fault conditions.

The last requirement is necessary because fault currents will be many times greater than normal load currents. This will result in correspondingly larger values of alternating magnetic flux and of induced e.m.f.