Why do solar panel fires start, and what does RC62 ask of a farm roof?

Ask a farm insurer about a solar array and the first document mentioned will be RC62. This post explains what it is, what evidence it rests on and which of its clauses a farm roof cannot avoid. It is the research behind the guide to solar panel fire risk on farm buildings.

What RC62 is

RC62 is the Joint Code of Practice for fire safety with photovoltaic panel installations, with a focus on commercial rooftop systems. The 2023 second edition was developed through RISCAuthority, whose membership is a group of UK insurers, with the Microgeneration Certification Scheme and Solar Energy UK, and published by the Fire Protection Association. It is freely available, and it states its own purpose: a practical guide for insurers and their clients on the requirements for procurement, ownership, operation and maintenance of safe PV systems. Its drafting note matters when reading it: must identifies a compulsory procedure, shall a mandatory requirement, should recommended best practice.

Where the fires start

The code’s own summary is candid. The incidence of fires involving PV systems is very low. Issues associated with installation have been identified as the largest single cause of PV fires. A study by the Building Research Establishment in 2017 identified PV DC connectors, inverters and DC isolators as accounting for 84% of PV-system-related fires. Product failure is the second largest cause, and the IEC technical report on PV fire risk on buildings, IEC TR 63226:2021, found 33% of the fires it reviewed were caused by it.

That evidence points at one habit above all others. IEC 62548:2016 and IEC 60364-7-712:2017 explicitly do not allow DC connectors from different manufacturers to be connected together. RC62 asks for the connector the module manufacturer recommends, assembled with the correct crimp tool, the correct exposed cable length and the correct gland torque, and for compatible spares to be kept on site so that a repair five years later does not reach for a near-enough part. It also asks for connectors to lie horizontally, off the roof surface and on draining cable trays, and for interim inspections during installation to catch a cable pinched under a module, a fault it says can take several years to show.

The clauses a farm roof cannot avoid

• Roof construction. PV should go on non-combustible roofs meeting Class A1 or A2-s1,d0 to BS EN 13501-1; a new installation shall not lower the fire classification of the roof; where a combustible roof is unavoidable a fire-resistant covering should be applied.
• Compartment walls. Cables are not allowed to pass over a fire wall unless in fire-resistant ducts, and modules should keep 2.5 m clear on each side of the wall, or 1.2 m where the risk of spread is assessed as low.
• Grain stores. Ignition sources introduced by PV must be addressed under the DSEAR regulations where an explosive atmosphere may form, and the code names agricultural grain storage barns at harvest time as its example.
• Rodents and livestock. Cabling should be in secure sealed trunking where animals can reach it, with armoured anti-rodent PV cable where that is not possible.
• Birds and debris. Nests, droppings and foliage under panels are described as hot-spots with additional fuel in direct contact with the system, and insurers expect a regular cleaning programme for all commercial arrays. The options for keeping birds out, and the law on active nests, are in bird proofing solar panels on farm buildings.
• Inverters. Around 3% of conversion losses become heat; ventilation must not be obstructed; automatic fire detection should be fitted wherever inverters, isolators and distribution boards sit inside a building; separate DC isolators should be eliminated wherever the inverter integrates them.
• Firefighters. A clear access route, an information pack at the site entrance, a “PV on the roof” sign of at least 100 mm by 100 mm at the cut-out, and a fire-service switch where mandated or required by the insurer.
• Maintenance. Inspection at least annually by a competent person, maintenance to BS EN IEC 62446-2, insulation-resistance and string-resistance testing, thermography, and an inspection after any storm.

Why the insurer reads it before the installer does

RC62’s authors are the insurers’ own risk body, and its foreword says it was prepared to provide an updated practical guide for insurers and their clients. In practice that means the underwriter’s questions at renewal follow its checklist: roof class, cable routes, inverter siting, detection, contractor competence and the maintenance regime. The procurement section recommends that the installer hold MCS certification and ISO 9001, belong to a safety scheme in procurement such as CHAS or Acclaim, and carry appropriate insurance including professional indemnity. What the farm’s own insurer will ask on top of that, and the documents to hold, is on solar panel insurance for farm buildings.

The full set of clauses, with the figures and their sources, is on the guide to solar panel fire risk on farm buildings; the equipment choices that implement them are on solar inverters for farm buildings.