Lightning Protection and Surge Arrester Specification for UK Solar PV Systems

Lightning Protection and Surge Arrester Specification for UK Solar PV Systems: What EPC Contractors Must Get Right

Lightning protection and surge arrester specification for UK solar PV systems is responsible for more project failures, warranty disputes, and grid connection rejections than almost any other single discipline on utility-scale sites. According to data from the UK Met Office, England and Wales experience an average of 300,000 lightning strikes per year, with ground flash density (Ng) values reaching 2.5 flashes per km² per year in parts of the East Midlands and East Anglia regions that have seen significant ground-mount solar development over the last decade. Yet a large proportion of EPC contractors still treat surge protection as a procurement afterthought, selecting devices from a datasheet rather than from a coordinated risk assessment. The consequences range from nuisance SPD failures and inverter damage to full grid connection rejection by the DNO at the G99 technical review stage.

This guide works through the full specification process: from IEC 62305 risk assessment through SPD type selection, DC-side sizing parameters, equipotential bonding requirements under BS EN 62305-3, and the most costly design errors seen on real UK utility-scale projects. It is written for electrical engineers, EPC contractors, and design consultants who need technically defensible protection systems not just a box checked on a commissioning form.

IEC 62305 Risk Assessment Applied to UK Solar PV Sites

Understanding Ground Flash Density and Its Role in the Decision Tree

The foundation of any technically sound lightning risk assessment solar project begins with IEC 62305-2, which defines the annual number of dangerous events affecting the structure. For a solar PV installation, the relevant parameter is Ng the ground flash density expressed in flashes per km² per year. In the UK, Ng values are available from the Met Office and range from approximately 0.1 in parts of northern Scotland to around 2.5 in the East Midlands. This number feeds directly into the collection area calculation, which for a ground-mount array is not simply the array footprint. It includes an equivalent border zone defined by the structure’s equivalent height typically taken as the tilt height of the rear row for ground-mount systems.

A worked example helps illustrate why geometry matters more than most designers expect. Consider a 5 MWp ground-mount array in Lincolnshire (Ng approximately 2.0) with a rear post height of 2.8 m and an overall footprint of 120,000 m². The equivalent collection area, calculated per IEC 62305-2 Annex A, expands the footprint by a border zone proportional to the structure height, increasing the effective area by roughly 30 to 40 percent. That increase alone can push the annual lightning strike frequency Nd above the tolerable loss threshold, triggering the requirement for an external lightning protection system (LPS) rather than SPD-only protection.

External LPS Versus SPD-Only: Making the Technically Defensible Decision

IEC 62305-1 establishes four Lightning Protection Levels (LPL I through IV), and the risk assessment output determines which level, if any, is required. For rooftop systems on commercial buildings, the host building’s existing LPS class must be factored in. A building already fitted with a Class I LPS (air termination network, down conductors, and earth termination system to BS EN 62305-3) provides significant shielding, and in many cases the PV system designer only needs to integrate the array’s mounting structure into the existing equipotential bonding network and specify coordinated SPDs at LPZ boundaries.

Ground-mount arrays are a different proposition. When no existing structure provides a shielding function, and when the Ng-based risk assessment confirms that Nd exceeds the acceptable risk threshold for LPL III or above, an external LPS becomes mandatory. In practice, this typically means installing shielded air termination masts at the perimeter of the array, bonded to a ring earth conductor encircling the installation. The cable routing strategy then becomes critical: DC string cables routed inside screened trunking or metal conduit, with the screen bonded at both ends, dramatically reduces the induction loop area and can reduce the calculated surge overvoltage at inverter terminals by a factor of four or more. This is the engineering argument for screened cable routing not cable protection from mechanical damage, but loop area reduction as a defined input into the SPD coordination calculation.

For a deeper look at how PV system design integrates these structural decisions from the earliest stages, visit our guide on how a PV solar system is designed.

SPD Type Selection Hierarchy for PV Systems

Type 1 SPDs: Where Direct Strike Energy Enters the System

The SPD specification solar discipline follows a defined hierarchy aligned with IEC 61643-11 (AC side) and IEC 61643-31 (DC side). Type 1 SPDs, sometimes referred to as Class I or B+C combined devices, are specified where direct lightning current may need to be diverted. On a solar PV site, this means two mandatory locations: the LV side of the MV/LV transformer serving the site (the service entrance point), and the main DC combiner box or string combiner where multiple string cables converge before reaching the central inverter.

A failure mode seen repeatedly on UK utility-scale projects is the omission of the coordinated Type 1 at the MV transformer LV side. The engineering argument offered for this omission is usually that the DNO’s own protection covers the incoming supply. This is incorrect from an IEC 62305 standpoint. The DNO’s equipment protects the network; the site’s own LV infrastructure, including the main LV distribution board, is outside that protection boundary. When a direct strike on the HV overhead line transmits a conducted surge through the transformer, it arrives at the LV busbar as a high-energy transient that a Type 2-only SPD cannot absorb without failing. The damage pattern is distinctive: the SPD thermal disconnect trips, the SPD is written off, and sometimes the inverter’s AC input filter is damaged in the same event.

Type 2 SPDs at Inverter DC Input Terminals

Type 2 DC surge protection devices are installed at the DC input terminals of string or central inverters. Their function is to clamp residual surges that have passed through the upstream Type 1 and any intermediate LPZ boundaries. The voltage protection level (Up) of the Type 2 device must be coordinated with both the upstream Type 1 Up value and the downstream inverter’s rated impulse withstand voltage (Uimp). This three-point coordination is where many specifications fail. Designers specify a Type 2 SPD with a competitive Up without checking whether the inverter’s Uimp is actually higher. If the SPD’s Up at nominal discharge current exceeds the inverter’s Uimp, the device provides no protective function it clamps above the equipment’s damage threshold.

Coordination margin in practice: IEC 62305-4 recommends a protective margin of at least 20 percent between the SPD’s Up and the protected equipment’s Uimp. For a central inverter with a DC input Uimp of 4 kV, the Type 2 DC SPD must achieve an Up of no more than 3.33 kV at the maximum discharge current expected at that LPZ boundary.

Type 3 SPDs for Monitoring and Communication Equipment

Modern utility-scale PV installations rely heavily on data acquisition systems, weather stations, string-level monitoring units, and remote SCADA communication interfaces. These devices typically carry impulse withstand voltages of 1.5 kV or below. Type 3 SPDs, specified per IEC 61643-11 for AC signal circuits and equivalent DC signal protection standards for monitoring cables, must be installed at every point where a signal cable crosses an LPZ boundary specifically where a cable enters a monitoring enclosure or inverter communication port from an outdoor environment.

The voltage protection level coordination chain now has four points: Type 1 at service entrance, Type 2 at inverter terminals, and Type 3 at communication ports. Each stage must show a reducing Up, with adequate energy coordination between stages to prevent the downstream SPD from seeing current it cannot handle because the upstream device failed to absorb its share. Coordination testing to IEC 62305-4 Annex C methodology should be requested from the SPD manufacturer for the specific combination of devices being used.

DC-Side SPD Sizing Parameters: The Numbers That Most Specifications Get Wrong

Maximum Continuous Operating Voltage for PV String Circuits

The most persistent and costly error in overvoltage protection PV array design is specifying SPDs using the system’s nominal operating voltage rather than the worst-case open-circuit voltage at minimum cell temperature. IEC 61643-31 is explicit: the maximum continuous operating voltage of a DC SPD (Ucpv) must exceed the maximum open-circuit string voltage under any expected operating condition. In UK practice, minimum cell temperature is typically taken as minus 10 degrees Celsius for sites in Scotland and northern England, with IEC 60364-7-712 and BS 7671 requiring Voc correction using the temperature coefficient of voltage provided in the module datasheet.

The sizing rule applied by competent EPC contractors is that Ucpv must be at least 1.2 times the corrected Voc at minimum cell temperature. For a string of 28 modules with a standard test condition Voc of 41.2 V and a temperature coefficient of minus 0.29 percent per degree Celsius, the corrected Voc at minus 10 degrees is approximately 1,250 V. Applying the 1.2 multiplier gives a required Ucpv of 1,500 V. Selecting a device rated at 1,000 V Ucpv a common default choice based on the system’s nominal 1,000 V maximum operating voltage results in a device that is permanently overstressed during cold-morning open-circuit conditions, triggering premature ageing of the metal oxide varistor (MOV) elements and eventual thermal runaway.

Short-Circuit Current Rating for Parallel String Configurations

The second critical sizing parameter for DC SPDs in PV applications is the maximum short-circuit current. This is where parallel string configurations create a non-obvious engineering requirement. IEC 61643-31 requires that the SPD’s discharge current rating accounts for the total fault current that can flow through the device during a surge event. In a parallel string array connected to a common DC combiner, a surge on one string can drive current contributions from all parallel strings through the SPD on the affected string.

The practical sizing rule is that the SPD’s rated short-circuit current (Iscpv) must be equal to or greater than (n minus 1) times the module’s short-circuit current (Isc), where n is the number of parallel strings. For a combiner with 12 parallel strings of 10 A Isc modules, the SPD on any given string position must be rated for at least 110 A Iscpv not 10 A. Specifying single-string Isc as the basis for SPD discharge current rating is a design error that appears regularly in UK EPC contractor documentation, and it creates a latent failure mode that only manifests during the first significant surge event on site.

Equipotential Bonding Under BS EN 62305-3 and G99 Grid Connection Requirements

Floating DC Systems and Their Interaction With AC Earth

Solar PV arrays are by nature floating DC systems the string circuit has no intentional DC reference to earth in a standard transformer-less inverter installation. This characteristic creates a specific challenge for lightning protection solar PV UK designs: the equipotential bonding network required under BS EN 62305-3 must connect all metalwork within the protection zone to a common reference potential, but this bonding must not inadvertently introduce a DC earth fault path that trips the inverter’s insulation monitoring function.

The resolution used in technically sound designs is to bond the array mounting structure (module frames, tracker rails, mounting posts) to the site earth ring, while maintaining isolation between the DC circuit conductors and that earth reference. Module frame bonding conductors are connected to the equipotential bonding bar in the inverter room, which in turn connects to the main earth terminal of the AC LV switchboard. The critical rule is that the DC circuit conductors positive and negative string cables are not bonded to earth directly. They are protected by the SPDs, which connect DC conductors to the equipotential bonding bar through their protective components only during surge events.

G99 and G98 Grid Connection Technical Schedules

The grid connection lightning protection requirements under DNO technical schedules G99 (for generators above 16 A per phase) and G98 (below that threshold) address surge protection implicitly through their requirements for equipment withstand levels and protection coordination. G99 Schedule 7 requires that the generating unit’s protection system maintains correct operation under the transient overvoltage conditions specified in IEC 60255-26. This directly links the inverter protection relay’s surge immunity to the site’s SPD coordination design.

A grid connection application that does not include a documented SPD coordination study as part of the protection philosophy document is increasingly being challenged at DNO technical review stage. The reviewers at some DNOs now ask for evidence that the SPD Up values across all stages are coordinated with the protection relay’s Uimp. EPC contractors who have not built this documentation into their standard deliverables face delays and redesign costs at the worst possible point in the project programme after physical build and before energisation. For guidance on navigating grid connection approval efficiently, see our resource on how to pass grid connection approval.

Common Design Failures on UK Utility-Scale Projects

Specifying SPDs Against AC Nominal Voltage Instead of DC Open-Circuit Voltage

This is the single most frequent error in EPC contractor solar UK SPD specifications, and it has a specific root cause: procurement teams using AC-side SPD datasheets as templates for DC-side specifications. An AC SPD rated at 275 V Uc is entirely appropriate for a 230 V AC circuit. A DC SPD marked “1000 V” without explicit confirmation that the 1,000 V is expressed as Ucpv for a PV application may be rated against the maximum system voltage at standard test conditions not the corrected Voc at minimum temperature. The distinction between Ucpv and Uc (AC) is defined in IEC 61643-31 versus IEC 61643-11, and mixing datasheets from the two standards families in a single specification document is an error that an independent protection engineer will identify immediately during a design review.

Omitting Lightning Protection Zone Boundary Definitions in the AutoCAD Package

IEC 62305-4 requires that LPZ boundary definitions be documented as part of the design deliverables. On UK utility-scale projects, this documentation should appear in the electrical schematic package as a dedicated LPZ boundary drawing showing which equipment sits in LPZ 0A (full exposure to direct strikes and full induced surges), LPZ 0B (protected from direct strikes but exposed to full induced surges), LPZ 1 (attenuated surges), and LPZ 2 (further attenuated surges for sensitive equipment). Without this drawing, the SPD type selection at each location cannot be technically justified, and the protection coordinator cannot verify that the installed system achieves the design intent.

The most common practical failure is placing monitoring and communication equipment physically inside an inverter room (nominally LPZ 1 or LPZ 2) but routing the signal cables from outdoor sensors without defining and protecting the LPZ boundary at the cable entry point. The cable entry point is the LPZ boundary not the building wall. If an outdoor temperature sensor cable enters a monitoring enclosure without a Type 3 SPD at the enclosure’s terminal block, the enclosure’s interior is effectively LPZ 0B at the cable terminal, regardless of what the drawing shows. Integrating LPZ boundary definitions into the AutoCAD electrical schematic package from the design stage prevents this failure mode. See how this is handled in professional solar planning packages at our AutoCAD solar planning service.

Neglecting Coordinated Type 1 at the MV Transformer LV Side

As discussed in the SPD type hierarchy section, the transformer LV side is a high-energy entry point for conducted surges from the HV network. In the UK, many grid-connected solar sites connect to the DNO network via an 11 kV or 33 kV overhead line, which has a significant collection area for direct strikes and induced surges. The transformer’s LV winding provides some surge attenuation, but the residual surge energy arriving at the LV busbar can still exceed the withstand capability of equipment protected only by Type 2 devices.

The correct specification is a Class I+II combined SPD (Type 1+2 combination device to IEC 61643-11) at the main LV distribution board, sized for an impulse current (Iimp) consistent with the site’s LPL determined from the risk assessment. For LPL III sites (the most common category for ground-mount solar in UK Ng conditions), Iimp of 25 kA per conductor (10/350 µs waveform) is the minimum acceptable specification for the Type 1 component. Devices marketed as “Type 1” that are rated only for the 8/20 µs waveform (Ipeak, not Iimp) do not meet the Class I requirement of IEC 61643-11 and will not survive a direct strike-derived impulse event.

Practical Checklist for EPC Contractors Specifying Lightning Protection on UK Solar PV Projects

The following items represent the minimum technically defensible deliverables for a lightning protection and SPD specification on a UK solar PV project:

1. Obtain site-specific Ng value from UK Met Office data and calculate the annual strike frequency Nd using the collection area methodology of IEC 62305-2 Annex A, accounting for array geometry and equivalent structure height.
2. Complete the full risk assessment per IEC 62305-2 to determine the required LPL and confirm whether an external LPS is required or whether SPD-only protection is sufficient.
3. Define LPZ boundaries on a dedicated drawing within the electrical schematic package, identifying all LPZ 0A, 0B, 1, and 2 zones and all cable crossings between zones.
4. Specify Type 1 SPDs (IEC 61643-11 Class I, rated Iimp at 10/350 µs) at the main LV distribution board and at each DC main combiner.
5. Specify Type 2 DC SPDs (IEC 61643-31) at all inverter DC input terminals with Ucpv greater than or equal to 1.2 times corrected Voc at minimum cell temperature, and Iscpv greater than or equal to (n minus 1) times module Isc.
6. Specify Type 3 SPDs at all LPZ boundary cable entries to monitoring, communication, and data acquisition equipment enclosures.
7. Verify Up coordination across all SPD stages against the Uimp of protected equipment, with a minimum 20 percent protective margin.
8. Document the equipotential bonding scheme per BS EN 62305-3, showing mounting structure earth connections that maintain DC circuit isolation.
9. Include the SPD coordination study as a named deliverable in the protection philosophy document submitted with the G99 grid connection application.

Conclusion: Building a Technically Defensible Lightning Protection Specification

The engineering discipline of lightning protection and surge arrester specification for UK solar PV systems is not a checklist exercise. It is a coordinated design process that begins with a site-specific risk assessment under IEC 62305-2, continues through LPZ boundary definition, SPD type selection, DC-side sizing based on corrected Voc and parallel string fault current, and concludes with documented coordination studies that support both BS EN 62305-3 compliance and G99 grid connection approval. Every one of the common failure modes described in this guide has a specific root cause: substituting nominal voltages for worst-case open-circuit voltages, using single-string Isc where combined parallel contribution is required, and omitting LPZ boundary documentation from the AutoCAD electrical package.

EPC contractors who integrate these disciplines into their standard design process from the earliest project stage not only produce safer, more reliable installations they also eliminate the programme risk associated with late-stage protection coordination failures at DNO technical review. The commercial argument for getting this right from the outset is as compelling as the technical one.

For support with grid connection approval documentation and full electrical schematic packages that include IEC 62305-compliant LPZ boundary drawings, explore our services at grid connection approval and AutoCAD solar planning.

Heval Arcan