BS EN 62305 — the UK lightning risk standard
BS EN 62305-2 is the UK standard for lightning risk assessment. It defines a probabilistic methodology calculating four risk components against tolerable thresholds:
Lightning and surge protection also decide where the inverter can sit and how it is earthed; the siting rules that insurers and RC62 apply are set out on solar inverters for farm buildings.
- R1 — risk of loss of human life
- R2 — risk of loss of service to the public
- R3 — risk of loss of cultural heritage
- R4 — risk of loss of economic value
For commercial UK solar farms, R4 typically dominates the calculation — the economic value of generation downtime, inverter replacement, and module damage from a direct or nearby strike. The assessment outputs a recommended Lightning Protection Level (LPL I, II, III, or IV) which determines the air-termination, down-conductor, earthing, and surge protection requirements for the site.
When LPS is required vs not
For most UK ground-mount solar farms above 250 kWp, the BS EN 62305 calculation typically does require an LPS. Smaller installs and rooftop installs on existing buildings have different considerations:
- Rooftop installs on existing buildings inherit the host building\'s LPS, which usually needs upgrade work — additional air-terminations to protect the PV array, bonding of the module frame to existing down-conductors, transient surge protection on DC and AC.
- Small ground-mount installs below 250 kWp on low-strike-density sites may not require an LPS, with SPDs alone providing sufficient protection.
- Sites above 5 MWp almost always require full LPS regardless of strike density, driven by the R4 economic calculation.
Surge Protection Devices (SPDs) — locations and types
SPDs limit transient over-voltages from direct or nearby lightning strikes. They\'re fitted at three locations on a solar farm:
- DC side of every string box and inverter input — protects the PV array from induced surges travelling along DC cabling.
- AC side of every inverter output — protects downstream equipment including transformers, switchgear, metering.
- Main LV switchgear and SCADA / monitoring equipment — protects control systems and the grid interface.
SPD types: Type 1+2 combined SPDs for direct-strike risk; Type 2 only for induced-surge risk. SPD lifecycle 5-15 years depending on local strike density — high-strike-density sites in Wales, the North-West, and Highland Scotland typically need SPD inspection every 2-3 years.
Earthing arrangement
Standard UK solar farm earthing uses TT or TN-S systems with site-wide bonding:
- Continuous earth strip around the perimeter at 600mm depth
- Bonded earth rods at corners and every 30m along the perimeter
- Module frame bonding via daisy-chained ring terminals (M6 or M8)
- Inverter chassis bonding to dedicated earth electrodes
- SCADA / monitoring earthed via separate electrode bonded to main earth at the substation
Earth resistance target typically below 5 ohms. For sites with high soil resistivity (rocky, dry, chalk-based ground common in parts of southern England), achieving sub-5-ohm resistance may require deep-driven rods or chemical earth electrodes.
What a feasibility study should cover for lightning risk assessment
A project normally starts with a desk-based feasibility study from half-hourly meter data and building dimensions, which should give an indicative system size, generation forecast and self-consumption ratio before anyone visits. Ask each installer what the study includes, what it costs, who carries it out and what the structural and electrical survey covers. This site is independent guidance, not an installer: send your details through the quote form and we reply by email.
Related pages
UK solar farm lightning risk assessment — common questions
What standard governs lightning risk assessment for UK solar farms?
BS EN 62305-2 is the UK standard for lightning risk assessment. It defines a probabilistic risk methodology calculating four risk components: R1 (loss of human life), R2 (loss of service to the public), R3 (loss of cultural heritage), R4 (loss of economic value). For commercial solar farms the R4 calculation typically dominates. The assessment outputs a tolerable risk threshold and a recommended Lightning Protection Level (LPL I, II, III, or IV) which determines air-termination, down-conductor, earthing, and surge protection requirements.
Is lightning protection required on a UK solar farm?
Lightning Protection is required by BS EN 62305 when the calculated risk exceeds the tolerable threshold. For most UK ground-mount solar farms above 250 kWp, this calculation typically does require an LPS. Rooftop installs on existing buildings inherit the host building's LPS, which usually needs upgrade work to accommodate the PV array (additional air-terminations, bonding of frame to existing down-conductors, transient surge protection on DC and AC sides). Without LPS the asset cannot be insured for lightning damage in most underwriting markets.
What does a lightning risk assessment cost?
There is no single price for an install of this kind. The official DESNZ series for 2025/26 gives a mean installed cost of £1,266.84 per kW for 10–50 kW systems (£1,167.29 for non-domestic installations in that band), but it covers MCS installations up to 50 kW across all building types, so larger farm systems are priced from written quotes. The farm solar cost guide sets out the series and what moves a farm quote.
What are SPDs and where do they go on a solar farm?
Surge Protection Devices (SPDs) limit transient over-voltages from lightning strikes (direct or nearby). They're fitted at three locations on a solar farm: (1) DC side of every string box and inverter input, protecting the PV array from induced surges; (2) AC side of every inverter output, protecting downstream equipment; (3) Main LV switchgear and any monitoring / SCADA equipment. Standard Type 1+2 SPDs for direct-strike risk; Type 2 SPDs for induced surge risk only. SPD lifecycle 5-15 years depending on local strike density.
What's a typical solar farm earthing arrangement?
UK solar farm earthing typically uses TT or TN-S earthing systems with site-wide bonding. Standard arrangement: continuous earth strip around the perimeter at 600mm depth; bonded earth rods at corners and every 30m along the perimeter; module frame bonding via daisy-chained M6/M8 ring terminals; inverter chassis bonding to dedicated earth electrodes; SCADA and monitoring earthed via separate earth electrode bonded to main earth at the substation. Earth resistance target typically below 5 ohms.
Do you provide lightning risk assessments yourselves?
Yes — we hold BS EN 62305 competency and deliver lightning risk assessments as part of our design service. For larger sites (above 5 MWp) or projects requiring independent assessment (e.g., for lender or insurer review), your installer works with specialist third-party assessors and project-manage the engagement. See our design service and UK farm solar lightning protection blog post.