BS EN 62305 Lightning Protection for Solar Farms: A UK Farm Guide

A field-mounted solar array sitting on open farmland is one of the most exposed structures on a typical UK holding — large metallic surface area, long DC cable runs, and a direct connection back into your steading. BS EN 62305 is the British and European standard that tells you, in measured terms, how much that exposure actually matters and what (if anything) you need to do about it. This guide translates the standard into plain English for farmers and agricultural developers, covers when a full lightning protection system is genuinely required versus surge protection alone, and gives realistic 2026 UK costs.

What BS EN 62305 actually is

BS EN 62305 is the four-part UK standard for protection against lightning. Part 1 sets the general principles, Part 2 covers risk management, Part 3 deals with physical damage and the lightning protection system (LPS), and Part 4 covers electrical and electronic systems inside the structure (your inverters, monitoring, comms). For solar, the standard rarely demands a cathedral of air rods and down conductors — the modern emphasis is on a properly evidenced risk assessment under Part 2, followed by surge protection devices (SPDs) sized and placed under Part 4.

The standard is not building-regulations-mandatory for most rural ground-mount arrays, but it is the reference your DNO, your insurer, your installer’s MCS paperwork and any future buyer will expect to see. Skipping it doesn’t save money; it just moves the cost to your first storm-season claim.

The Part 2 risk assessment: R1 to R4

Everything starts with a lightning risk assessment for solar farms carried out to BS EN 62305-2. The assessment calculates four primary risk values and compares each against a tolerable threshold:

• R1 — risk of loss of human life (or permanent injury). The headline figure; the tolerable limit is 10⁻⁵ per year.
• R2 — risk of loss of service to the public (relevant where your export underpins a wider supply or a community scheme).
• R3 — risk of loss of cultural heritage (rarely material on a working farm, but listed barns can trigger it).
• R4 — risk of loss of economic value — the one most farms actually care about: damage to the array, inverters and the income they generate.

The assessor models your site’s collection area, the local ground flash density (Ng) for your postcode, the structure’s characteristics, existing protective measures and the consequential loss. Where a calculated risk exceeds its tolerable value, protection measures must be specified until it doesn’t. The output is a documented, defensible figure — not a guess — and it is what determines whether you need a full LPS or just SPDs.

When you need a full LPS versus SPDs only

This is the question that decides your budget. In practice:

• SPDs only is the common outcome for a free-standing ground-mount array in open farmland with no occupied structure within the strike zone. R1 (life safety) is usually already below threshold because nobody lives on the array; the residual risk is economic (R4), best handled by coordinated Type 1+2 surge protection and good earthing.
• A full external LPS (air termination, down conductors, earth termination) becomes necessary when the array shares or sits on an occupied or high-value structure — solar mounted on a dairy parlour roof, a manned grain store with combustible dust, or any building where R1 or R2 can’t be brought under threshold by surge protection alone.

The honest answer for most field arrays is that a competent Part 2 assessment shows SPDs are sufficient. The risk is paying for an unnecessary external LPS because nobody ran the numbers — or, worse, fitting nothing because someone assumed a field can’t be hit.

Type 1 and Type 2 SPD placement

SPDs are the workhorses of solar lightning protection, and BS EN 62305-4 (with BS 7671 / the IET Wiring Regulations) governs how they’re coordinated. A correctly protected array uses SPDs at three points:

1. DC string side — Type 1+2 (or Type 2) SPDs at the inverter DC input, and on long string runs, a second set out at the combiner or array end. Long DC cabling is the single biggest induced-surge risk on a field array; the further the run, the stronger the case for SPDs at both ends.
2. AC side — a Type 1+2 SPD at the inverter AC output and a Type 1 (or 1+2) device at the main LV distribution board / point of connection, protecting the grid interface and everything downstream in the steading.
3. Monitoring and comms — signal-line SPDs on data, Ethernet and metering cables. These are routinely forgotten and are the most common surge-claim failure point, because a strike that the power side survives still cooks the monitoring through an unprotected data line.

Where an external LPS is fitted, a Type 1 SPD capable of handling partial lightning current is mandatory at the service entrance — Type 2 alone is not sufficient on an LPS-protected structure.

Earthing: the foundation under 5 ohms

SPDs and any LPS are only as good as the earth they discharge into. UK solar installations are typically TT or TN-S earthed, and the working target for a solar array earth electrode is a measured resistance below 5 ohms — lower on sandy or rocky ground where rods alone struggle and a ring or radial earth conductor may be needed.

Critically, all metalwork — module frames, mounting rails, inverter enclosures, DC and AC trunking — must be bonded to a common earth so that a surge raises the whole structure’s potential together, rather than flashing across between a bonded and an unbonded part. Equipotential bonding is the detail that separates a system that survives a nearby strike from one that arcs internally and burns out an inverter. On the agricultural side, this dovetails with the wider electrical safety regime your agricultural solar panel installers are already certifying under BS 7671.

Typical UK costs in 2026

Lightning protection for farm solar spans a wide range, driven almost entirely by whether you need SPDs only or a full LPS:

• SPD-only package (coordinated DC + AC + comms SPDs, properly earthed) on a typical farm-scale array: roughly £1,200–£3,000 supplied and fitted.
• Larger or split-array sites with multiple inverters and long string runs needing SPDs at both ends: £3,000–£5,000.
• Full external LPS plus SPDs on an occupied or high-value structure: £5,000–£8,000, occasionally more on listed or large-footprint buildings.

A standalone Part 2 risk assessment typically runs a few hundred pounds and is the single most cost-effective spend you can make — it frequently shows you need less protection than a salesperson would quote. Either way, lightning protection qualifies as plant and machinery, so the spend can sit inside the 100% Annual Investment Allowance alongside the array itself (subject to the £1m AIA cap), and folds neatly into the overall project cost case.

Design-stage versus retrofit

The cheapest lightning protection is the kind you design in. At the planning stage, SPD locations, earthing runs and bonding are drawn into the single-line diagram, the assessment informs the DNO G99 connection application, and the whole package is commissioned and MCS-certified in one visit. Retrofitting protection to a live array means isolations, re-commissioning, sometimes lifting modules to add bonding, and a second round of certification — routinely 30–50% more expensive for the same protective outcome.

If you’re already planning a roof — for example a combined re-roof plus solar project, where CAR 2012 asbestos rules and structural work are in scope anyway — that is precisely the moment to specify lightning protection, because the access and electrical works are already paying for themselves. The same logic applies whether you own the system outright or take it via a PPA for farms: make sure the contract spells out who owns the lightning risk assessment and who pays for protection.

Getting it right for your holding

BS EN 62305 isn’t red tape — it’s the difference between an insurable, financeable asset and an uninsured liability sitting in your top field. Start with a documented lightning risk assessment for solar farms, let the R1–R4 numbers decide between SPDs and a full LPS, and build the protection into the design rather than bolting it on after the first near-miss. For sector-specific detail, our guides on solar for agricultural buildings cover how this fits the rest of your installation.

Ready to protect your array properly? Request a no-obligation quote and we’ll scope the right level of lightning protection for your farm.