A Comprehensive Guide to the Installation of Photovoltaic Systems

Introduction

Solar PV installations have grown enormously in the UK, driven by feed-in tariffs and rising environmental awareness. The Microgeneration Certification Scheme (MCS) developed its Guide to the Installation of Photovoltaic Systems, with the Electrical Contractors Association, to set best-practice standards for quality, safety, and consumer confidence — primarily aimed at grid-connected systems up to 50kWp total combined DC output. This article summarises the key design, installation, and safety considerations from that guidance.

Designing the DC System

Modules and components. Every PV module must comply with the relevant international standard — IEC 61215 for crystalline types, IEC 61646 for thin film — plus IEC 61730 for safety qualification, and carry CE marking. For MCS-compliant installations, modules also need to appear on the MCS product database.

Voltage and current ratings. All DC components must be rated for the maximum voltage and current the system could produce. For mono- and multi-crystalline silicon modules, the minimum ratings are V_oc(stc) × 1.15 for voltage and I_sc(stc) × 1.25 for current. Other module types need specific worst-case calculations across a temperature range of -15°C to 80°C and irradiance up to 1,250 W/m².

String and array voltages. Where array voltage exceeds 120V, double or reinforced insulation is required on all parts of the DC circuit under BS 7671. Arrays with an open-circuit voltage above 1000V shouldn't be installed on buildings at all, given the added complexity and danger.

DC cables. Cables should be sized to BS 7671, keeping voltage drop between array and inverter under 3%. PV-specific cable complying with UL 4703 or TÜV 2 PfG 1169 08.2007 is required, and any cable routed behind the array must be rated for -15°C to 80°C. For a system of N parallel strings, each made up of M series-connected modules, string cables need to be rated above V_oc(stc) × M × 1.15 for voltage and above I_sc(stc) × (N−1) × 1.25 for current.

Connectors. DC plug and socket connectors must be the same type and manufacturer throughout, compliant with BS EN 50521, rated for UV, IP, and temperature exposure at their location, and of a locking type that needs a tool or two separate actions to disconnect.

String fuses. Reverse current through a string is calculated as I_R = (N − 1) × I_sc. Overcurrent protection is required wherever this exceeds the module's maximum series fuse rating. Where fitted, fuses go in both positive and negative string cables on every string, and must be gPV type per IEC 60269-6.

DC isolation and switching. String circuits don't need switching but do need a readily accessible means of isolation. Sub-array switching is optional, but isolation is still required. The array itself needs a readily accessible load-break switch-disconnector on the DC side of the inverter, capable of isolating all live conductors, rated for the system's maximum DC voltage and current, and clearly labelled.

Earthing, Bonding, and Lightning Protection

UK thunderstorm-day levels rarely require special lightning protection measures, though higher-risk buildings warrant a specialist consultation — and if a building already has a lightning protection system, a qualified person should be consulted before connecting the array frame to it.

Most PV systems have no exposed-conductive or extraneous-conductive parts, so protective equipotential bonding usually isn't required. To check whether an array frame counts as extraneous-conductive, test the resistance between the frame and the Main Earthing Terminal — a reading above 22kΩ means it can be treated as isolated from earth. Earthing a current-carrying DC conductor directly isn't recommended; where a functional earth is genuinely needed, high-impedance earthing is preferred, following the inverter manufacturer's instructions.

Designing the AC System

RCD protection is required when connecting an inverter to a TN-S or TN-C-S system, both to protect the AC cable and to provide earth fault protection — typically a 30mA double-pole RCD on the dedicated circuit. A readily accessible switch-disconnector is also required on the AC side, with a minimum of 2-pole switching for TN-S/TN-C-S connections and a 4-pole isolator for three-phase. The inverter itself must meet G83 or G59 requirements, be approved by the DNO for grid connection, and sit away from combustible materials and extreme temperatures.

Estimating System Performance

A few factors dominate real-world output. Orientation and tilt matter — due south at 30° to 40° tilt is optimal for the UK's latitude, though orientations within 45° of south are still viable. Shading is disproportionately damaging: because modules in a string are connected in series, even partial shading on one module can drag down the output of the whole string. Temperature also matters — output falls as temperature rises, with a typical V_oc temperature coefficient of around -0.4% per °C for crystalline modules.

The standard performance estimate is Annual Energy (kWh) = kWp × KK × SF, where kWp is the array's rated peak output, KK is a kWh/kWp figure derived from postcode zone, orientation, and inclination tables, and SF is the shade factor.

Installation Safety

PV systems bring hazards that aren't always familiar to general electrical installers: the supply from the modules can never be switched off, so accessible live parts need special precautions; modules are current-limiting devices requiring a different approach to fault protection than standard circuits; DC wiring itself is less familiar territory for many installers; and the combination of electric shock risk, working at height, and manual handling on a roof creates a distinct hazard profile.

Good practice follows from that: avoid live working wherever possible, cover the array if working live is genuinely unavoidable, use test instruments rated for the system's maximum voltage, and restrict DC-side work to competent persons only.

Testing, Commissioning, and Handover

DC-side testing, carried out to BS 7671 and BS EN 62446, covers polarity verification, string open-circuit voltage and short-circuit current, insulation resistance, and earth continuity. AC-side testing follows standard BS 7671 procedures.

At handover, the installer needs to provide an Electrical Installation Certificate, a PV Array Test Report, a schedule of inspections and test results, system design documentation, operation and maintenance instructions, and manufacturer warranties — plus a system schematic displayed at the consumer unit or meter position.

Conclusion

Installing PV systems well comes down to careful attention across design, safety, and regulatory compliance. Following recognised guidance, using quality components, and installing to a professional standard is what keeps systems safe, reliable, and performing as expected — and keeps consumer confidence high as the UK solar industry continues to grow.

This article summarises key concepts from the "Guide to the Installation of Photovoltaic Systems," originally published by the Microgeneration Certification Scheme (MCS). The current version of the standard is available on MCS's website.