Earthing and Bonding Design for UK Solar PV Systems

Earthing and Bonding Design for UK Solar PV Systems: The Critical Pre-Connection Specification Gap

Earthing and bonding design for UK solar PV systems is responsible for more delayed grid connections than almost any other single technical discipline on utility-scale projects. According to data compiled from UK Distribution Network Operator (DNO) application reviews, a significant proportion of G99 applications submitted by EPC contractors are returned with earthing-related deficiencies deficiencies that could have been resolved at the specification stage rather than discovered during the final commissioning sign-off. The cost of retrofitting an earthing design on an energised site is not simply financial; it introduces arc flash exposure, structural rework, and renegotiated connection dates that cascade into liquidated damages claims. Understanding exactly what must be specified, why, and in what sequence is not optional knowledge for any competent EPC contractor operating in the UK solar market today.

Why Earthing System Type Defines Every Other Protection Decision

TN-S, TT, and IT Configurations in Utility-Scale Solar PV

The choice of earthing system for the AC side of a solar PV installation in the UK is not purely an engineering preference. It is a DNO-driven decision that cascades directly into protection relay coordination, fault loop impedance calculations, and the selection of protective devices throughout the entire scheme. Most utility-scale solar PV projects connecting to the medium-voltage network in the UK operate on a TN-S earthing solar configuration on the high-voltage side, where the neutral and protective earth conductors are separated from the source transformer earthing point. However, the LV collection network serving string inverter combiner boxes frequently operates as a TT system where no direct metallic connection exists between the neutral and the main earthing terminal at each point of common coupling.

The practical consequence of this distinction emerged clearly on a 49.9 MW ground-mounted project in the East Midlands, where the EPC contractor had specified residual current devices (RCDs) calibrated on TN-S assumptions, only to discover during commissioning that the DNO supply transformer was earthed as a TT arrangement at the substation boundary. The entire RCD schedule required recalculation because trip thresholds set for TN-S fault loop impedances were wholly inadequate for the higher impedance TT fault current paths. That single specification error added six weeks to the energisation programme.

DNO Earthing Requirements and Protection Relay Coordination

DNO earthing requirements in the UK are expressed through Engineering Recommendation G99 (for generators above 16 A per phase) and Engineering Recommendation G98 (for smaller generators). Both documents impose specific obligations on the earthing arrangement at the point of connection. The DNO will specify whether neutral earthing resistors (NERs) are required at the grid-side transformer, what earth fault current levels the protection relay must detect, and what maximum earth fault clearance times are acceptable. These parameters are not advisory they form part of the connection offer and must be reflected in the earthing design documentation submitted with the protection coordination study.

What many EPC electrical designers overlook is that the solar PV earth fault protection coordination is a two-directional problem. The inverter’s internal earth fault detection must be coordinated with the upstream DNO protection, and the structural bonding earthing network must provide a fault current return path with sufficiently low impedance to ensure that the DNO’s earth fault relay actually operates within the declared clearance time. If the soil resistivity at the substation compound is high, and the earth electrode network is undersized, the earth fault current may be insufficient to trigger DNO protection within the required window leaving the site energised with a live earth fault condition that no commissioning test will reveal unless a deliberate fault current injection test is performed.

IEC 60364-7-712 and BS 7671 Requirements for Solar PV Earthing

What the Standards Actually Mandate Versus What Gets Specified

PV earthing IEC 60364 requirements relevant to the UK are primarily contained within IEC 60364-7-712, which is incorporated into the British standard framework through BS 7671 (the 18th Edition Wiring Regulations, including Amendment 2). Section 712 specifically addresses photovoltaic power supply systems and imposes requirements that are additional to, not a replacement of, the general wiring regulations. The critical requirements that EPC specifications most frequently underspecify are the equipotential bonding obligations for the DC side of the installation.

Under BS 7671 and IEC 60364-7-712, the following must be bonded together as part of a comprehensive equipotential bonding solar installation:

• All PV module mounting frames and racking systems
• All inverter enclosures and combiner box metalwork
• The armour of all DC and AC cables where metallic armour is present
• Metal cable containment systems including cable trays and conduit
• Any structural steelwork forming part of the mounting system support structure
• Fencing and security infrastructure within the lightning protection zones defined under BS EN 62305

The bonding conductors must be sized correctly and must be connected using proven low-resistance termination methods. A stainless steel grub screw through an anodised aluminium rail section does not constitute a reliable bonding connection unless the anodising has been removed at the contact point and an anti-corrosion compound appropriate for the bimetallic junction has been applied. This is an installer-level detail, but it must be specified in the EPC documentation to ensure it is executed correctly on site.

IEC 62446 As-Built Failures Caused by Bonding Omissions

Solar PV earthing UK EPC teams frequently encounter IEC 62446 sign-off failures that trace back to bonding omissions in the original AutoCAD electrical schematics. IEC 62446 defines the documentation and commissioning test requirements for grid-connected PV systems, and it requires measured earth continuity values rather than manufacturer’s assurances. When bonding conductors are absent from the electrical schematics, installers do not fit them. When measured continuity values are then taken during commissioning testing, the resistance values exceed acceptable limits and the system cannot be signed off.

The underlying cause is almost always a schematic that was drawn to show the DC string layout and the AC protective device schedule, but which omitted the structural bonding network as a separate drawing layer. This is correctable at the design stage at negligible cost. On a constructed site, it requires physical access to every module row, which on a 5,000-panel site may represent tens of thousands of pounds in remedial labour. For EPC contractors interested in ensuring AutoCAD electrical schematics include the full bonding schedule, the team at Lion Solar’s AutoCAD solar planning service provides compliant drawing packages that incorporate structural bonding as a native layer of the electrical design, not an afterthought.

AC and DC Earthing Separation: The IT System Problem on String Inverters

Floating DC Earthing and Insulation Monitoring Device Requirements

The DC side of most modern string inverter-based solar PV systems operates as an IT system an isolated or floating system where neither the positive nor the negative conductor is intentionally connected to earth. This is deliberate. The IT configuration allows the system to continue operating after a single earth fault, which is important for energy yield during intermittent fault conditions. However, it introduces a significant safety and compliance obligation that many EPC earthing specifications fail to address: the requirement for insulation monitoring devices (IMDs).

An IMD continuously monitors the insulation resistance between the DC conductors and earth. On a floating DC system, the first earth fault raises the voltage of the unfaulted conductor to full system voltage relative to earth. A second earth fault on the unfaulted conductor then creates a direct short-circuit condition that bypasses all overcurrent protection and can result in a serious arc fault. The IMD detects the first fault and triggers an alarm or disconnection before the dangerous second fault condition can develop. IEC 60364-7-712 requires IMDs where floating DC systems are used, and this requirement is absolute regardless of inverter size or system voltage.

The interaction between floating DC earthing and G98/G99 protection settings is a source of persistent confusion. G99 requires that the generating unit protection includes earth fault protection that can detect faults at the point of connection. On the DC side, the inverter’s internal IMD satisfies this requirement within the inverter enclosure boundary. However, the earth fault detection scheme must be documented in the Protection Settings Schedule submitted to the DNO, and the IMD alarm thresholds must be coordinated with the inverter’s trip settings so that a DC earth fault results in inverter disconnection before the DNO’s external protection operates. If the IMD threshold is set too high, this coordination fails silently the system operates normally under test conditions but may fail to disconnect under real fault conditions that the DNO’s protection scheme assumes will be cleared by the generator.

How Transformer Earthing Affects DC System Behaviour

An aspect of DC earthing that is almost never addressed in generic EPC specifications is the influence of the AC transformer earthing arrangement on DC system behaviour. Where a galvanically isolated transformer is present between the inverter AC output and the grid, the DC floating condition is maintained. Where transformerless inverters are used, or where the transformer earthing creates a conductive path between the AC neutral and the DC negative rail, the DC floating condition is compromised. In these configurations, the DC system behaves as a TN-C-S or TN-S arrangement on the DC side, changing the required protection approach entirely. The EPC specification must explicitly address which transformer configuration applies, document whether transformerless inverters are permitted, and specify the resulting DC earthing classification.

Earthing Conductor Sizing Methodology Under BS 7671 Amendment 2

Fault Current Duration, Soil Resistivity, and Hidden Protection Failures

The sizing of earthing conductors under BS 7671 Amendment 2 uses the adiabatic equation defined in Regulation 543.1.3, which relates conductor cross-sectional area to the let-through energy of the fault current (expressed as I²t). The critical variable in this equation is the assumed fault current duration specifically, the time taken for the protective device to clear the fault. On utility-scale solar PV sites connecting via G99, the upstream protection relay clearance times are DNO-specified and site-specific. They are not generic values. Using a standard 0.4-second or 5-second clearance time without confirming the actual DNO protection grading scheme is a specification error that can result in undersized earthing conductors that are thermally damaged by genuine fault events.

The utility-scale solar earthing conductor sizing exercise must therefore begin with the Protection Coordination Study, not conclude with it. The fault current magnitude and duration values from the coordination study are inputs to the earthing conductor sizing, not outputs. This sequencing requirement means that earthing conductor sizes cannot be finalised until the DNO has confirmed the protection relay settings which typically occurs late in the G99 application process. EPC contractors who specify earthing conductor sizes at the schematic design stage are almost always specifying without the correct input data.

Soil Resistivity and Earth Electrode Resistance

The solar PV grounding system performance is fundamentally constrained by soil resistivity at the installation site. Soil resistivity in the UK varies enormously from below 50 ohm-metres in clay-rich lowland soils to above 3,000 ohm-metres in granite-based highland areas. The earth electrode resistance required to ensure that earth fault current is sufficient to operate the DNO’s protection relay within the permitted clearance time depends directly on this value. An earth electrode network sized for clay soils will be catastrophically undersized if the same design is applied to a site with rocky, high-resistivity subsoil.

The failure mode created by an undersized earth electrode is particularly dangerous because it is invisible to standard commissioning tests. A Prospective Earth Fault Current (PEFC) measurement taken at the main earthing terminal may show acceptable values if the test is conducted under favourable soil moisture conditions. Six months later, in a dry summer, the same electrode network may present a significantly higher resistance, reducing the actual earth fault current below the DNO protection relay’s trip threshold. The system continues to operate normally, but the earth fault protection is functionally compromised. The only reliable way to identify this risk at specification stage is to conduct a seasonal worst-case soil resistivity survey and model the electrode resistance across the full range of expected soil conditions.

For a detailed overview of how soil surveys and site geotechnical data feed into the broader PV system design process, the PV solar system design guide at Lion Solar provides a useful framework for understanding how early-stage site data affects every downstream design decision, including earthing.

Common EPC Specification Errors That Create Earthing Failures

Structural Bonding Omitted from AutoCAD Electrical Schematics

The most pervasive earthing specification error observed across UK solar EPC projects is the omission of structural bonding from the electrical drawing package. The mounting structure bonding network is physically the largest earthing system on site it interconnects every module frame, every tracker drive unit (on single-axis tracker sites), every row end cap, and every intermediate mounting bracket. Yet it is routinely absent from AutoCAD electrical schematics because it is perceived as a civil or structural deliverable rather than an electrical one.

The consequence of this organisational gap is that the bonding network either does not get installed, or gets installed without any reference to the electrical specification, using whatever bonding conductors and termination methods the mounting structure manufacturer recommends. Manufacturer recommendations are not a substitute for a site-specific earthing design that considers the actual fault current magnitudes, the actual conductor lengths, and the actual soil resistivity conditions. When an IEC 62446 commissioning test then measures bonding resistance values that exceed the limits, no one can identify which portion of the bonding network is non-compliant because there was no compliant design to compare against.

Using Manufacturer Earth Continuity Claims in Place of Site Measurements

A related error is the substitution of manufacturer-certified earth continuity claims for site-measured earth electrode resistance values. Inverter manufacturers will often state in their technical documentation that the inverter enclosure provides a certain bonding conductance by virtue of its construction. Some EPC specifications cite these manufacturer claims in the earthing section and treat them as satisfying the BS 7671 and IEC 62446 measurement requirements. They do not.

BS 7671 and IEC 62446 both require measured values taken on the installed system at the specific installation location. Manufacturer continuity data is produced in controlled factory conditions on an uninstalled enclosure. It does not account for the quality of the cable termination at the installation point, the condition of the cable armouring after site handling, or the integrity of the bonding connection between the inverter enclosure and the main earthing terminal. Site measurements are mandatory, and specifying that they must be conducted and recorded in the as-built documentation is a non-negotiable element of a compliant EPC earthing specification.

Failure to Coordinate Earthing Design with Lightning Protection Zones

The final common EPC error is the failure to coordinate the earthing and bonding design with the lightning protection design under BS EN 62305. Lightning protection and earthing are deeply interdependent on solar PV sites. The earthing network serves both as the fault current return path for AC and DC earth faults and as the down-conductor termination point for lightning surge energy. These two functions impose conflicting requirements in some scenarios particularly regarding earth electrode placement, bonding conductor routing, and the separation distances between the lightning protection system and the metalwork of the PV structure itself.

Where the PV mounting structure falls within Lightning Protection Zone 0B (the zone where lightning strikes are possible but attenuated), BS EN 62305 requires specific bonding arrangements at the boundary of the protection zone that must be integrated into the earthing design. When the earthing designer and the lightning protection designer work from separate briefs without coordination, the resulting designs may be individually compliant but collectively create problems for example, a shared earth electrode that correctly serves both functions but has been sized only for fault current duty without considering the additional impedance requirements for lightning surge dissipation.

This type of cross-discipline coordination is exactly the kind of detail that needs to be resolved during the pre-design checklist phase, before any engineering hours are committed to detailed design. The solar design preparation checklist from Lion Solar provides a structured approach to capturing these inter-discipline dependencies before they become site-level problems.

Coordinating Earthing Design with Grid Connection Approval

What DNOs Expect in the Earthing Schedule

The earthing design documentation submitted as part of a G99 or G98 grid connection application must satisfy specific DNO expectations that vary between network operators but share a common core of required information. At minimum, the earthing schedule submitted to the DNO should include the following:

• The earthing system classification (TN-S, TT, or IT) for both the AC and DC sides of the installation
• The design earth electrode resistance value and the calculation method used to derive it
• The assumed soil resistivity value and the source of that value (measured survey or regional estimate)
• The earthing conductor sizes and the adiabatic calculation that justifies each size
• The bonding conductor schedule listing every bonded item and the conductor size used
• The protection relay earth fault settings and their coordination with the earthing network impedance
• The IMD specification and alarm thresholds for floating DC systems
• Reference to the lightning protection design and confirmation of coordination

DNOs will also frequently request a site-measured earth electrode resistance value before they confirm the final connection date. Specifying that this measurement will be conducted using the Fall of Potential method (per BS 7430) rather than a simple continuity test demonstrates to the DNO that the EPC contractor understands the difference between a meaningful measurement and a compliance formality. This distinction matters because it affects the DNO’s confidence in the submitted data and can influence whether the connection is approved on first review or returned for further information.

For EPC contractors navigating the grid connection approval process and seeking to understand what documentation is required at each stage, the resource at Lion Solar’s grid connection approval guide provides detailed guidance on preparing compliant technical submissions that satisfy DNO requirements the first time.

Pre-Construction Earthing Verification Steps

The earthing design should be verified before any civil or electrical construction begins through a structured pre-construction review process. This review should confirm that the soil resistivity data used in the electrode resistance calculation is site-specific and seasonally adjusted, that the protection relay settings used in the earthing conductor sizing calculation have been confirmed by the DNO and are not preliminary values, that the bonding schedule is present as a discrete section of the electrical drawing package, and that the lightning protection designer has reviewed and accepted the earthing electrode layout.

Completing these verification steps before construction removes the most common causes of IEC 62446 commissioning failures and DNO application rejections. It also establishes a clear audit trail that demonstrates due diligence in the event of a post-commissioning fault event a consideration that is increasingly relevant as solar PV sites age beyond their initial warranty periods and earthing degradation becomes a genuine operational risk.

Frequently Asked Questions

Conclusion: Earthing Design Is a First-Principles Specification Task, Not a Compliance Formality

The fundamental insight that separates a competent earthing design solar PV UK specification from a superficial one is the recognition that earthing design cannot be completed in isolation. It depends on soil resistivity data that must be measured on site, protection relay settings that must be confirmed by the DNO, lightning protection zone definitions that must be agreed with the BS EN 62305 designer, and transformer earthing configurations that must be verified against the DNO supply arrangement. None of these inputs are available at the time most EPC earthing specifications are written, which is why the specifications so frequently contain errors that only manifest at commissioning or grid connection review.

The solution is not to delay earthing design until all inputs are available that would simply push the problem later in the programme. The solution is to structure the EPC specification so that the earthing design is explicitly flagged as dependent on these inputs, with clearly defined hold points in the construction programme that prevent earthing conductor installation until the confirmed input values have been received and checked. This level of programme discipline is exactly what differentiates EPC contractors who achieve first-time grid connection approval from those who spend months in remedial design cycles.

For EPC teams working on UK solar PV projects who need structured support at the specification and design stage, the resources available through Lion Solar cover the full design and grid connection workflow. Starting with the solar design preparation checklist establishes the correct information-gathering sequence before any design work begins. The PV solar system design resource explains how site data feeds into design decisions across all engineering disciplines. The AutoCAD solar planning service ensures that structural bonding and earthing networks appear as explicit layers in the electrical drawing package. And the grid connection approval guide ensures that the documentation package submitted to the DNO is complete, accurate, and structured to achieve approval without avoidable iteration.

Earthing and bonding design for UK solar PV systems deserves the same level of engineering rigour as any other primary protection discipline. When it receives that rigour, it becomes an invisible enabler of safe, compliant, and profitable grid-connected generation. When it does not, it becomes the most expensive line item in the project’s remedial budget.

Heval Arcan