Concentric (PEN) cables — the wave-formed concentric conductor as combined neutral-and-earth, and why it is not a shield

The armoured-cable-as-protective-conductor guide covers using steel wire armour as a PE conductor, and the cable-screen-earthing guide covers earthing a metallic screen at one or both ends. This article covers a construction that looks superficially similar but serves a functionally different purpose: the concentric conductor found on Dutch low-voltage underground service cables, typically designated NAYCWY or NYCWY, where the "C" indicates the concentric conductor and the "W" indicates its characteristic wave (golf) formation.

What the concentric conductor actually is

Around the insulated phase core(s) of a concentric cable, a layer of copper wires is applied not straight along the cable axis and not as a simple helical wrap, but pressed into a wave pattern: each wire follows a sinusoidal path around the circumference as it runs along the cable length. This wave formation is deliberate — it gives the concentric conductor layer significant longitudinal elasticity, so that when the cable is bent, pulled, or subjected to thermal expansion and contraction over its service life, the concentric wires can flex without work-hardening and fatigue-cracking the way a straight or tightly helical layer would. The wave-formed concentric conductor sits underneath the outer PVC or PE sheath and is designed to carry full combined neutral-and-earth (PEN) working current continuously, not just transient fault current.

Why it functions as a PEN conductor, not a screen

This is the point most easily confused with a cable screen or an armour layer used as PE: a screen (as in the cable-screen-earthing guide) is primarily an electromagnetic/electrostatic control layer, normally carrying only capacitive charging current and fault current, and earthed at one or both ends depending on the screening strategy chosen. Steel wire armour (as in the armoured-cable-as-protective-conductor guide) can be used as a PE conductor but is not sized to also carry normal load neutral current. The concentric conductor in a PEN cable is different again: it is the combined neutral-and-protective (PEN) conductor for the circuit, meaning it carries the full unbalanced neutral current under normal load conditions in addition to acting as the earth-fault return path. Its cross-sectional area is therefore selected as part of the cable's electrical design to match the phase conductors' current-carrying requirements under IEC 60502-1, not merely sized as a fault-clearing path the way a bonding conductor might be.

Sizing and continuity implications

Because the concentric conductor is a genuine working PEN conductor, its cross-sectional area is specified on the cable's data sheet alongside the phase conductor cross-section, and reduced-cross-section concentric conductors (smaller than the phase conductors) are only acceptable where the design current and fault-current withstand have both been verified for the reduced size, consistent with the general PEN-conductor sizing rules in NEN 1010 §543.4. Continuity testing of the concentric conductor along the full cable run, and at every joint and termination, is not optional maintenance housekeeping — it is a direct safety requirement, because a break in the concentric conductor simultaneously interrupts the neutral return path and the earth-fault protective path for everything downstream of that point, unlike a break in a screen (which mainly affects EMC/fault-current behaviour) or in an armour PE conductor running in parallel with a separate neutral.

Practical relevance

When working with NAYCWY/NYCWY-type service cables, correctly identifying the concentric conductor as a load-bearing PEN conductor — rather than treating it like a screen that can be safely left unconnected at one end for EMC reasons, or like an armour PE layer that only matters during a fault — is essential during joint-kit selection, during earthing/bonding verification at the meter cabinet, and during periodic inspection where continuity and cross-sectional integrity of the concentric layer must be confirmed rather than assumed from the cable type designation alone.

Common mistakes

Treating the concentric conductor as an EMC screen and leaving it unconnected at one end "to avoid earth loops," which breaks the PEN function that the installation actually depends on for neutral current return and earth-fault protection.
Assuming the concentric conductor's cross-section always equals the phase conductor cross-section without checking the specific cable's data sheet, when reduced-cross-section variants exist and have different current-carrying and fault-withstand limits.
Skipping continuity verification of the concentric conductor at joints and terminations, on the assumption that "it's just like armour" and only matters during a fault, when in fact it also carries continuous neutral current in normal operation.
Confusing the wave (golf) formation with ordinary stranding, overlooking that the wave pattern exists specifically to provide mechanical flexibility for the concentric layer, which affects repair and jointing technique compared with a straight-laid concentric or braided screen.