Brandschutz Photovoltaik: Risiken erkennen bevor es brennt

A PV system on the roof is a direct-current installation that is permanently live – at night too, even in overcast conditions, as long as light falls on the modules. This makes fire protection a serious task. Arc discharges at defective connectors, hotspots from damaged solar cells and overheated cables cause fires with considerable material damage every year. This guide explains the most common causes, why they are barely detectable with the naked eye – and how thermography as a preventive tool prevents fires before they occur.

PV fires: figures and causes

The number of building fires caused or co-caused by PV systems increases with the installed base. According to analyses by VdS and fire service associations PV systems account for an estimated 2–4% of all roof fires in commercial buildings – a proportion that continues to rise in view of the growing installed base and increasing system capacities.

The most common causes of fires in PV systems according to damage statistics are: arc discharges at DC connections (approx. 40%), overheating at inverters and string boxes (approx. 25%), hotspots in modules with critical temperature difference (approx. 20%) and external causes such as lightning or surge (approx. 15%). PV systems are particularly dangerous for fire service personnel in the event of a fire: DC cables are permanently live under solar irradiance and cannot be fully de-energised.

Arc discharges – the invisible hazard

Arc faults occur when direct current flows through an air gap between two conductors. This happens at loose or defective MC4 connectors, at damaged cables with insulation defects, at terminals with insufficient tightening torque and at corrosion points in junction boxes.

The special characteristic of DC arc faults compared with AC arc faults: direct current has no zero crossing. While an AC arc fault extinguishes itself at every zero crossing and can thereby be limited, a DC arc burns continuously. The temperatures generated exceed 3,000°C – hot enough to immediately ignite insulation materials, timber structures and bitumen membranes.

Hotspots with critical temperature difference

Hotspots in solar modules are local overheating of individual cells or cell areas that occur when these cells convert more energy into heat than into electricity. Causes are shading, microcracks, cell defects or contamination. Hotspots with a temperature difference of more than 30 K from the module surroundings are considered critical and a fire risk.

At extreme hotspots – temperature differences above 50–80 K – the local module temperature can reach 150–200°C. At these temperatures the encapsulation film (EVA) begins to degrade and can ignite. The fire typically spreads slowly because the module surface initially dissipates the heat – but under unfavourable conditions a single critical hotspot can lead to a fully developed fire within minutes.

Thermography classifies hotspots per IEC TS 62446-3 into priority levels: Low (ΔT 5–10 K), Medium (ΔT 10–30 K), High (ΔT 30–50 K) and Critical (ΔT >50 K). Modules in the High and Critical category should be taken out of operation and replaced promptly.

Überhitzte DC-Steckverbindungen

MC4 connectors are the most widely used DC connectors in PV systems. They are reliable for direct currents up to 30 A when correctly installed – but they are sensitive to incorrect assembly. Common installation errors are: incompletely seated connectors, use of connectors from different manufacturers (incompatibility), excessively wide cable bend radii at the connectors and missing or inadequate strain relief.

Each of these errors increases the contact resistance of the connection. By Ohm's Law increased resistance with the same current leads to more heat generation. This heat is visible in the thermogram – as a bright spot at the connector, clearly different from the ambient temperature. During hand thermography of BOS components junction boxes and all accessible connectors are inspected.

Fire risk: inverters and junction boxes

Inverters and DC junction boxes (string boxes) concentrate high electrical power in a small area. Overheating arises from defective contacts, overloaded fuses, capacitor failure (in older inverters after 10+ years) and inadequate ventilation following subsequent enclosure.

Inverters have internal thermal protection shutdowns – but these do not prevent a fire when the overheating originates not from the inverter itself but from external components. Hand thermography inspects all housing surfaces, cable entries and terminal strips for temperature anomalies. Anomalies above 60–70°C at connection points are warning signals requiring immediate inspection by a qualified electrical contractor.

Standards and regulations for fire protection in PV

Several standards and guidelines apply in parallel for the fire protection of PV systems. The VdS 3145 "Photovoltaic systems – planning and installation" is the most important guideline of the German insurance industry and contains specific requirements for fire protection measures, cable routing and inspection obligations. Insurers that reference VdS conditions can reduce payouts for non-compliance.

Die DIN VDE 0100-712 governs electrical systems in PV installations and prescribes among other things protective mechanisms against arc faults, insulation monitoring and the requirements for disconnection options. The DGUV Vorschrift 3 requires operators of commercial systems to arrange recurring inspection of electrical equipment by a qualified electrician.

Die IEC TS 62446-3 – the standard for thermographic inspection of PV systems – is not a fire protection standard in the strict sense but the methodological basis for identifying all heat-related risks. A standards-compliant thermography report to this standard is the robust document for insurers, expert witnesses and authorities.

Thermografie als aktive Brandschutzmaßnahme

The preventive effect of thermography lies in the temporal dimension: thermographically detectable anomalies typically develop weeks to months before they become a fire risk. A developing arc risk at a connector initially shows as a slight temperature increase of 10–15 K – long before the temperature reaches critical levels. This early detectability is the decisive advantage over reactive measures.

Drone thermography captures all modules and – in combination with hand thermography of BOS components in the Complete or Premium package – all electrical connections of the system. The result is a complete thermal condition picture that identifies, classifies and locates all active heat sources on a site plan. Operators thus receive a prioritised action list that clearly indicates which findings require immediate shutdown, which should be remedied at the next maintenance visit and which should be monitored.

Recommended inspection intervals per VdS and IEC

Frequently asked questions