Surge protection for controllers is specified after the second failure on most farms, not the first. A storm passes, the climate computer returns on default setpoints, two EC probes read 0.4 mS/cm high for a week, and a dosing driver board dies three weeks later. The storm is blamed and the parts are replaced. What actually kills greenhouse electronics is rarely a direct strike: it is the switching surge from a compressor contactor, the spike induced on a 30 m sensor cable beside a pump feeder, and the earth potential difference between a wet manifold and a dry cabinet. All three cost less than the probes.
What Actually Fails After a Storm
Post-storm diagnostics find damage in a fixed order: power supplies, then communication ports, then sensor front ends. Mechanical parts rarely fail electrically: the pump motor survives while its 24 V driver does not.
- Switched-mode power supplies: the input rectifier and capacitor absorb the surge; failure is instant or shows up later as resets.
- RS-485 and Ethernet transceivers: long runs collect induced energy, and a dead port is usually blamed on software.
- EC and pH transmitters: the amplifier drifts, so readings stay plausible and wrong; see EC, pH and DO probe maintenance.
- Dosing driver boards: low-voltage boards on long runs, often the costliest item in the panel.
- Contactor coils: contacts weld or fail to drop out, which can run a pump dry.
Two other modes look like surge damage: brownout, when the utility sags for hours, and water ingress into junction boxes. Logging each event makes the pattern visible, and alarm thresholds and escalation covers what to record.
Surge Protection for Controllers: SPD Types 1, 2 and 3
Surge protective devices are classified by test waveform and by where they sit. A correct rating in the wrong place protects very little.
| Type | Waveform | Sits at | Rating | Job |
|---|---|---|---|---|
| Type 1 | 10/350 µs | Main distribution board | 12.5 to 25 kA | Partial lightning current |
| Type 2 | 8/20 µs | Greenhouse sub-boards | 20 kA nominal, 40 kA peak | Switching surges, residual energy |
| Type 3 | 8/20 µs combined | Within 10 m of the equipment | 3 to 10 kA | Fine protection for electronics |
| Data line | 8/20 µs, 10/700 µs | Both ends of data runs | 0.5 to 5 kA per pair | Stops energy on signal cable |
Voltage protection level, called Up, comes first: Category II equipment withstands about 2.5 kV, and controller boards are safer below 1.5 kV. Coordination comes second, needing roughly 10 m of cable between the Type 2 and Type 3 device or a designed decoupling inductance; without it the downstream device takes the whole event.
Every SPD needs a backup overcurrent device sized from the manufacturer table and a visible status flag, since a unit that has opened its thermal disconnect still looks like a unit.
UPS Sizing for Hydroponic Controls: Bridge Time
UPS for hydroponic controls is bought in minutes, not hours. The question is how long the controls ride through before the generator carries the board: transfer start delay of 5 to 15 seconds and crank of 5 to 10 seconds. That chain lands between 30 and 90 seconds, and longer when the first crank fails.
| Load | Draw | On UPS | Note |
|---|---|---|---|
| Climate computer | 50 to 150 VA | Yes | First on the list |
| Dosing controller | 100 to 400 VA | Yes | Check motor inrush |
| Network and gateway | 30 to 100 VA | Yes | Carries the alarm path |
| 24 V sensor supplies | 50 to 200 VA | Yes | Keeps records continuous |
| Solenoid valves | 20 to 60 VA holding | Yes | Inrush is real |
| Circulation pump | 1 to 3 kVA | Rarely | Generator duty, not UPS |
Add the column, then 25 to 30 percent headroom for inrush and aging. Internal batteries in a 1 to 2 kVA tower are typically 7 to 9 Ah and give roughly 8 to 12 minutes at 500 VA; external cabinets extend that to 30 to 90 minutes. Treat batteries as consumables: capacity drops about 20 percent by end of life, and every 10 °C above 25 °C halves life.
Topology matters with a generator. Line-interactive units transfer in 4 to 8 ms, which most switched-mode supplies ride through; double-conversion units cost more and run hotter but isolate the load from generator frequency drift and harmonics. Confirm the input frequency window, since some units drop to battery beyond 3 Hz of drift.
Which Loads Get UPS Power and Which Do Not
The rule: anything that must not lose its memory or setpoints goes on the UPS, and anything that only needs to restart goes on the generator.
- On UPS: climate computer, dosing controller, sensor supplies, network equipment, alarm path and solenoid valves.
- Generator only: circulation and transfer pumps, blowers, chillers, heat loop pumps, vent fans and lighting.
Keep motor loads under about 30 percent of UPS rating. A dosing pump at 4 to 6 times inrush trips the inverter the moment the grid fails. A second small UPS costs less than upsizing the first.
Earthing and Bonding Around Irrigation Water
Equipotential bonding is the cheapest surge measure on a wet site and the one most often skipped.
On farms with repeat sensor failures the electrode is usually fine and the bonding is not. A steel frame, a stainless manifold and a control cabinet at three different potentials push current through whatever connects them, and the cheapest connection is a sensor shield.
Metal pipework, the frame, pump bodies and cable trays all bond back to the main earth terminal with 6 to 16 mm² copper, sized to run length. Where the irrigation system is entirely plastic there is no bonding path, and induced voltage on long runs gets worse; those sites need a local electrode and an equipotential mesh in the floor screed.
Electrode resistance is commonly specified below 10 ohms, and anything above 20 ohms deserves investigation. Residual current devices sit at 30 mA on socket circuits and the controller supply, with type A or B selected to match the drives. Wet-area detail is covered in electrical safety in wet hydroponic areas.
Sensor Cable Practices That Reduce Induced Noise
Most induced noise is solved at installation, not at the panel:
- Separation: keep signal cable 300 mm from power cable on parallel runs, crossing at 90 degrees.
- Screen earthing: connect the drain wire at one end only, at the controller.
- Bus limits: keep RS-485 under 1,200 m, terminate 120 ohms at both ends, and avoid stubs.
- Signal type: prefer 4-20 mA over 0-10 V beyond about 20 m.
- Routing: never run signal cable along the steel frame, which is the down conductor in a strike.
- Ferrite: fit a common-mode choke on transmitters sharing a tray with a drive output.
- Outdoor runs: arresters at both ends, earthed to the local electrode.
The network deserves the same attention, since the gateway carries every alarm; see IoT and remote monitoring.
Testing and Replacement Intervals
- SPD flag: monthly and after every storm; replace when the window is not green.
- SPD units: every 5 to 10 years, and after any nearby strike.
- Electrode: measured yearly at the end of the dry season; investigate above 20 ohms.
- RCD trip: button test quarterly and instrument test yearly.
- Bonding: yearly, with anything above 0.5 ohms re-terminated.
- UPS battery: impedance check yearly, replaced every 2 to 3 years on hot sites.
- UPS runtime: twice a year on real load, logged in minutes.
After a nearby strike, walk the site with a checklist instead of waiting for failures: which boards lost power and how long the UPS held. Compare that against the panel and circuit layout. Where generator start-up sets the gap, backup generator sizing for hydroponic farms defines the bridge time that surge protection for controllers and the UPS have to cover.
Frequently Asked Questions
Does a Type 2 arrester at the main board protect controllers?
A: Not alone; Type 3 devices near the controller and arresters on data lines bring let-through down to what the electronics tolerate.
How much UPS runtime does a control panel need?
A: About 10 to 15 minutes, covering a normal generator start plus one failed crank.
Why do EC probes keep failing when the panel is protected?
A: The damage usually arrives on the signal cable, not the power cable, so separation and single-end screen earthing matter more than arrester rating.
Can the UPS run the dosing pumps too?
A: Only while motor inrush stays under about 30 percent of the rating; otherwise a second unit or generator duty is safer.
How often should surge arresters be replaced?
A: Every 5 to 10 years and immediately after a nearby strike, since varistors degrade with each event.
Request a Surge and UPS Layout
The decision is settled by four numbers: controller and sensor VA, measured earth resistance, the longest signal cable run, and generator bridge time. Panel space decides the rest, since a three-stage scheme needs room beside the main switch. To tell us which controllers you run and we will suggest a surge and UPS layout that fits your panel budget, include panel photos and cable routes.
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