Single-core cables through a steel gland plate — why all conductors of one circuit must share the same opening

The guide on cable screen earthing covers how a cable's screen is earthed. This article covers a different, less well-known physical effect that specifically occurs with single-core cables (cables with one conductor per sheath) in an AC circuit: eddy-current heating in ferromagnetic material surrounding the conductors, and the installation requirement meant to prevent it.

The underlying effect

An AC current in a conductor generates an alternating magnetic field. In a three- or multi-phase circuit with balanced loading, the magnetic fields of the individual phase conductors largely cancel each other out, provided all conductors of that circuit run together through the same ferromagnetic material (steel). If the conductors of one circuit instead pass individually through separately drilled holes in a steel gland plate, trunking, or duct, a net alternating magnetic field remains per conductor, which induces eddy currents in the surrounding steel — resulting in local heating of that steel, independent of the cable's own loading.

The installation requirement: sharing the same opening

§521.5 (IEC 60364-5-52) therefore requires that where conductors of an AC circuit pass through a ferromagnetic wall or plate, all conductors of that circuit — including the protective conductor — are passed together through the same opening, so that they are collectively surrounded by the same piece of steel instead of individually. This applies to:

• a steel gland plate on a switchboard through which several single-core cables are routed;
• a steel duct or armour through which the outgoing and return conductor of the same circuit run;
• any other ferromagnetic enclosure that surrounds part of a circuit without the rest of that same circuit.

A non-magnetic gland plate (aluminium, plastic, brass) does not have this problem and is for that reason often the simpler solution in a new installation with multiple single-core cables.

Why single-core cables with steel armour are not used for AC

The same principle explains why a single-core cable with steel wire or tape armour is not used in an AC circuit: the armour of that one cable surrounds only a single conductor and therefore itself forms the ferromagnetic enclosure that causes eddy-current heating — independent of any gland plate further along the route. For a single-core application where mechanical armouring is desired, aluminium armour is therefore recommended, which is non-ferromagnetic and does not exhibit this effect. This is not an issue with a multicore cable (all phases in one sheath), because the armour then surrounds the whole circuit together — comparable to the shared-opening requirement above.

Practical relevance

When designing or assessing a cable entry into a steel switchboard, distribution board, or building wall with multiple single-core cables, it should be checked that all conductors of the same circuit pass through one shared opening — not each through its own separately drilled hole — and that any armour applied to a single-core cable is of a non-ferromagnetic material. A switchboard that feels unusually warm at a steel gland plate, without the cables themselves being overloaded, is a practical sign of this effect.

Common mistakes

1. Routing each single-core cable of a circuit through its own, separately drilled hole in a steel gland plate instead of together through one opening.
2. Using a single-core cable with steel wire or tape armour in an AC circuit, resulting in eddy-current heating of the armour itself.
3. Attributing unexplained local heating of a steel gland plate or duct to cable overload, when the actual cause is eddy currents from separated routing of conductors of the same circuit.
4. Replacing a non-magnetic gland plate with a steel variant (for example during a repair) without re-establishing the shared-opening requirement.

Related

• Cable screen earthing — single-point or both ends
• Conductor classes — IEC 60228 Class 1-6
• Armoured cable — armour as protective conductor
• Busbar trunking systems