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:
- 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.
How we deliver lightning risk assessment
For projects we design directly, lightning risk assessment is included in our design service at RIBA Stage 2-3. For projects requiring independent third-party assessment (typically for lender or insurer review on sites above 5 MWp), we work with specialist BS EN 62305-competent assessors and project-manage the engagement. Typical cost £1,200-£3,500 for standard sites; £3,500-£8,000 for larger sites.
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?
A BS EN 62305-2 risk assessment for a UK solar farm typically costs £1,200-£3,500 depending on site size and complexity. For sites above 5 MWp, expect £3,500-£8,000. The assessment is typically commissioned at design stage (RIBA Stage 2-3) so that any required LPS or SPD work can be integrated into the construction package rather than retrofitted. Retrofit LPS on a commissioned site typically costs 3-5x what design-stage integration would have cost.
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), we work with specialist third-party assessors and project-manage the engagement. See our design service and UK farm solar lightning protection blog post.