Main equipotential bonding & earthing of large steel greenhouse structures

A large greenhouse complex typically has one continuous, electrically connected steel frame that extends over hundreds of meters and interconnects multiple sub-distribution boards, screen-motor rails, and irrigation metalwork. This raises practical main-bonding questions that do not arise with a regular, compact installation.

The §544 cross-section rule

§544.1 of NEN 1010 sets the minimum cross-section of the main equipotential bonding conductor:

• Not smaller than half of the cross-section of the largest protective conductor (PE) in the installation.
• Never smaller than 6 mm² (copper).
• Not larger than needed: for copper, a cross-section above 25 mm² is not required from a conductivity standpoint (for a different metal the conductivity-equivalent cross-section applies).

What must be connected

All bonding, earthing, and PE conductors — and, where applicable, functional earth conductors — must come together at one main earthing busbar. This explicitly includes extraneous-conductive-parts that can introduce a potential from outside the installation, including metal structural parts and load-bearing steel structures.

Practical application at greenhouse scale

With a large, continuous steel greenhouse frame that spans multiple electrically separate sub-distribution boards, the structure effectively functions as one large conductive surface. The general §544 principles then translate into concrete design questions:

• At how many points must the frame be connected to the main earthing busbar(s) of the respective sub-distribution boards, so that no unintended parallel earth-fault path arises between two parts of the same installation?
• How is bonding secured during extensions of the greenhouse complex, where a new greenhouse section is electrically connected to the existing frame?

Note: there is no NEN-published, greenhouse-specific elaboration of a multi-point bonding strategy for one continuous steel frame that spans multiple sub-distribution boards. The above is an application of the general §544 principles at greenhouse scale, not a literally citable greenhouse-specific clause — have the concrete bonding strategy assessed per project by a recognized designer/installer.

Common mistakes

1. Assuming one fixed cross-section (for example always 6 mm²) without determining the actual largest PE cross-section in the installation — the §544 rule is relative (½ of the largest PE cross-section), not a fixed number.
2. Not explicitly treating the steel greenhouse frame as an "extraneous-conductive-part" — a load-bearing steel structure that can introduce a potential from outside the installation must be connected to the main earthing busbar, even if it is not perceived as "part of the installation."
3. Not explicitly assessing the bonding of the new section when extending the greenhouse complex — a new greenhouse section that is electrically connected to the existing frame can affect the existing bonding strategy.
4. Confusing equipotential bonding with functional (signal) earthing — §544 governs protective bonding; functional earthing (for example for sensitive climate-computer electronics) has its own additional requirements and must not replace protective bonding.

Related

• Earthing systems & PE — §411
• Greenhouses and Horticultural Enterprises
• Lightning protection for greenhouse complexes