Earthing and bonding of metal cable support systems — bonding versus use as a protective conductor

The guide on cable support systems covers the mechanical requirements of NEN-EN-IEC 61537 for cable trays and cable ladders: load-bearing capacity, deflection, and material classification. This article covers a different aspect of the same standard that is often underestimated in practice: the electrical continuity between the individual sections of a metal support system, and the distinction between two different, frequently confused applications of it.

Two separate questions

For a metal cable tray or ladder made up of multiple, coupled sections, two separate questions actually need to be answered:

Does the support system itself need to be equipotentially bonded? A metal support system that runs through a large part of a building is an extraneous-conductive-part that, if not bonded, could take on a different potential than its surroundings — for example through induction from the cables lying on it, through accidental contact with a live part, or through a static charge. Equipotential bonding prevents an accessible, conductive object from being able to build up a hazardous potential difference relative to its surroundings.
May the support system be used as the protective conductor (PE) for the circuits it carries? This is a fundamentally more far-reaching application: the support system then itself becomes part of the fault-current path that must ensure disconnection of the protective device in the event of an earth fault, and must therefore comply with the cross-section requirements for a protective conductor — not merely a continuity requirement for equipotential bonding.

These two applications are regularly confused in practice: a support system that is correctly bonded for the first application is not automatically suitable or approved as a protective conductor for the second application.

The continuity requirement from IEC 61537

IEC 61537 sets a concrete limit value for the electrical continuity of the support system itself: the contact resistance across a joint (the connection between two sections) may not exceed 50 mΩ, and the resistance of the material of the support system itself may not exceed 5 mΩ per metre, excluding the joints. These limit values are intended to demonstrate that the system is suitable as part of an equipotential bonding circuit — not that every construction automatically meets the considerably stricter requirements for a protective conductor under NEN 1010 §543.

Why a painted or powder-coated joint is a risk

Many cable trays and ladders are hot-dip galvanized or powder-coated for corrosion protection. A non-conductive powder coating on the joint face between two sections can introduce a significant contact resistance well above the 50 mΩ limit value, even though the mechanical connection (for example with bolts) looks solid. To prevent this, joints are typically fitted with a separate, explicit bonding connection (a bonding jumper or braided strap between the sections) rather than relying on the contact between the coated metal parts alone.

Practical relevance

A metal cable support system that runs through a large part of a building or growing operation must be connected to the main earth bar or a bonding bar at the start (and, on long runs, at regular intervals) — not solely via the incidental contact between the coupled sections.
When extending or modifying an existing support system (an extra section, a wall penetration), the continuity of the bonding must be re-checked — a new section that is only mechanically, but not electrically, well connected can introduce a break in the bonding circuit.
Deliberately using the support system itself as a protective conductor for the circuits it carries is a separate design decision that requires explicit verification against the cross-section requirements of NEN 1010 §543 — this must not simply be assumed based on the continuity requirement from IEC 61537 alone.

Common mistakes

Assuming that every installed cable support system automatically has sufficient electrical continuity — particularly with powder-coated or galvanized joints, the contact resistance can exceed the 50 mΩ limit of IEC 61537 by a wide margin without a separate bonding connection.
Treating the support system as a protective conductor (PE) without explicitly verifying this against the cross-section requirements of NEN 1010 §543 — a continuity connection that meets the IEC 61537 requirement for equipotential bonding is not automatically suitable as a protective conductor.
Not checking the continuity of a new section when extending an existing run — a mechanically solid but electrically poorly conducting joint can go unnoticed until a fault occurs.
Not periodically checking the contact resistance at joints on a support system in a corrosive or damp environment — corrosion on the joint face can gradually increase the contact resistance.