Assimilation lighting — group division, RCD type and protection

Assimilation lighting (grow light) in greenhouse horticulture combines large numbers of luminaires, high power density and a humid, corrosive environment — a combination that requires specific attention to group division, residual current protection and cable sizing. This guide builds on the brief mention in the "Top 10 errors in greenhouse wiring" guide and works out the topic in more detail.

Leakage current per driver and group size

Every LED driver has an inherent leakage current to earth, typically in the order of 0.5–1.5 mA per driver. In a group with many luminaires, this leakage current adds up cumulatively within the group. A commonly used rule of thumb is to keep the group's combined background leakage current under roughly 30% of the RCD's rated tripping current (so around 9 mA for a 30 mA RCD), leaving enough margin for normal variation and ageing without nuisance tripping. Depending on the actual leakage current per driver, this works out in practice to roughly 6 to 18 luminaires per 30 mA group — considerably fewer with drivers at the high end of the range (1.5 mA) than with drivers at the low end (0.5 mA). Calculate this per project based on the actual driver documentation rather than relying on one fixed number.

RCD type: when type B is mandatory

Not every RCD type is suitable for modern lighting groups:

• Type A RCDs are insufficient when the driver or ballast does not have built-in detection of DC leakage currents — a situation that occurs with certain older HF control gear (SON-T) and with some LED driver designs.
• In those cases a type-B RCD is required, because only this type also reliably detects smooth DC fault currents.

If in doubt, check the technical documentation of the driver or ballast used for the required RCD type designation, rather than defaulting to type A.

Protection characteristic: B- versus C-curve

For regular lighting groups a B-curve circuit breaker is typically used. However, with assimilation lighting involving many LED drivers, the inrush current of the combined group can be considerably higher than with traditional lighting, which can cause nuisance tripping when the group is switched on. In that case a switch is often made to a C-curve breaker, which allows a higher momentary inrush current without losing the protective function in the event of an actual short circuit.

Sizing: Ib ≤ In ≤ Iz, with humidity correction

The regular NEN 1010 sizing rule remains applicable:

$$I_b \leq I_n \leq I_z$$

where Ib is the design current of the group, In the nominal current of the protective device, and Iz the permissible current-carrying capacity of the cable. For greenhouse cabling, the derating of Iz under high humidity and condensation is particularly relevant — see the existing guides on IP classification and cable cross-section calculation for the correction factors involved. The practical design current of a lighting group follows from the installed power per m² multiplied by the illuminated area, distributed across the available phases.

Note: there is no generally applicable, ready-made comparison table between SON-T and LED cable cross-sections — the actual design current must be calculated per project from the installed power, not read from a fixed table.

Common mistakes

1. Making a group as large as possible without accounting for the cumulative driver leakage current — with large numbers of luminaires, the combined leakage current of the drivers alone can already consume a considerable part of the 30 mA threshold.
2. Applying standard type-A RCDs without checking the driver documentation — for drivers without built-in DC detection, a type-B RCD is required.
3. Sticking with a B-curve breaker for a large LED group with a high combined inrush current — this can lead to repeated nuisance tripping; a C-curve is often more suitable in that case.
4. Forgetting the humidity correction on the cable cross-section — a greenhouse environment with high humidity and condensation requires a lower permissible Iz than the same cable in a dry environment.

Related

• Top 10 Mistakes in Greenhouse Wiring
• Lighting Installations — §559
• Cable cross-section & current-carrying capacity