The system runs for months without issue — then someone grabs the metal casing of a pump with a wet hand and gets a shock. "Everything was working" does not mean "it was safe." In a space where water, metal structures, and 230 V coexist, wiring errors produce no symptoms — until the moment they do.
Quick glossary: RCD (Residual Current Device) — an automatic switch that detects current leaking to earth and cuts power within 20–40 milliseconds; protects a person from electric shock when insulation is damaged or a live casing is touched. Earthing — connecting the metal casings of equipment to earth via a conductor; if insulation fails, current flows to earth rather than through a person, and the RCD or circuit breaker trips. PE conductor — the protective earth conductor in a cable (yellow-green), mandatory in TN-S and TN-C-S networks.
Why a Greenhouse and Grow Room Are High-Risk Environments
Water and electricity together multiply the risk of electric shock dramatically. Wet skin reduces body resistance from ~100 kΩ (dry) to 1 kΩ or less — at the same voltage, current through the body increases a hundredfold. Metal shelving, structural profiles, pump and chiller casings, pipes — all of these become potential conductors if insulation is damaged anywhere.
Water is constantly present in a grow room or greenhouse: condensation droplets, irrigation spray, drain water. Even equipment rated IP44 is not designed for continuous wetting and the mechanical wear that accumulates over time. A cable lying on a wet floor for a year without incident does not mean it isn't a hazard.
RCDs: What They Protect Against — and What They Don't
An RCD trips when there is a difference between the current that left through the live conductor and returned through the neutral — meaning some current has taken a different path, for example through a person into the earth. Within 20–40 milliseconds the circuit is cut — the safe threshold for contact with 30 mA before the heart fibrillates.
RCD sensitivity for wet locations: 10–30 mA. An RCD rated at 100–300 mA is fire protection, not personal protection — 100 mA through the heart is fatal within seconds.
What an RCD does not protect against: shock between live and neutral (when a person bridges two conductors — no earth leakage occurs and the RCD does not trip); overload and short circuit (that is the function of the circuit breaker). An RCD and a circuit breaker are not substitutes for each other — they are complementary.
A separate RCD for each consumer group is not over-engineering. If a single RCD on the whole panel trips, the entire greenhouse loses power — including ventilation and temperature control. With group circuits, a fault on one line does not shut down the rest.
Earthing: Why "It Works Fine" Is Not an Argument
Earthing serves two functions simultaneously: when insulation fails, it provides a safe path for current (through the PE conductor to earth, not through a person) and creates the conditions for the RCD or circuit breaker to trip.
Without earthing, a pump casing with damaged insulation is live but no current flows — the system "works," the RCD does not trip, until someone touches it. With earthing — the moment insulation fails, a leakage current appears through the PE conductor, the RCD trips and isolates the line before anyone makes contact.
Checking earthing is straightforward: a multimeter between live and the casing of earthed equipment reads ~230 V. Between neutral and casing — close to 0 V. If there is no voltage between live and the casing when earthing is disconnected — insulation is currently intact, but that is not a reason to omit earthing.
Common Wiring Mistakes in Wet Locations
Cables without mechanical protection on floors and in wet zones. Cable in conduit or a cable trunking — the minimum. A cable lying on the floor or hanging next to the irrigation system will have its insulation damaged — the question is when, not whether.
Twist connections in a wet environment. A bare twist without a sealed terminal block or adhesive-lined heat shrink oxidises, increases contact resistance, and generates heat. In a wet room this takes months, not years. Use only sealed connections or junction boxes rated IP65 or higher.
Extension leads and sockets without moisture protection. A domestic extension lead rated IP20 in a greenhouse means open contacts in condensation droplets. Minimum rating for wet locations — IP44; for zones with direct water contact — IP65.
No labelling or wiring diagram. During a fault or upgrade, it becomes impossible to quickly locate the right circuit. Every group should be labelled in the distribution board; the wiring layout should be saved in physical or digital form. This is part of the overall greenhouse documentation system.
Three Mistakes That Cost the Most
One RCD for the whole panel instead of group RCDs. A trip means the entire greenhouse loses power. Plants without ventilation and temperature control — stress or loss within hours in summer. Group RCDs solve both the safety question and the reliability question.
A "temporary" solution that becomes permanent. An extension lead on the floor "until I do it properly" lasts for years. Every temporary solution in a wet room is a risk that accumulates. Installing equipment in the greenhouse correctly from the start costs less than reworking it after an incident.
Not testing RCDs regularly. Every RCD has a "Test" button. Press it once a month and confirm the device trips. An RCD that does not trip on test is faulty and provides no protection — even if the lights are on.
How to Know the Electrical Installation Is Safe
RCDs rated 10–30 mA on every consumer group, tested monthly. Earthing confirmed with a multimeter on every metal casing. All connections in wet zones in sealed IP65 boxes. Cables in mechanical protection. A wiring diagram exists and is current.
For deeper understanding: The Greenhouse as a System: Equipment, Services, and Failure Points — explains how electrics fit into the overall infrastructure and where hidden risks most commonly appear when expanding or upgrading the system.