Rooftop solar fire investigation, DC arc fault originating at a module junction box

What happened

A peer reviewed engineering investigation, reported in 2024, examined a fire at a rooftop photovoltaic installation in southern China. The investigators combined on site inspection of the fire scene, electroluminescence (EL) imaging of recovered modules and laboratory replication of suspected failure modes. They identified the ignition source as a single module junction box and attributed the fault mechanism to a DC arc at an internal contact within that box. No information on the extent of damage or the emergency response is recorded in the material reviewed.

The case is included here because the mechanisms described, junction box contact degradation, mechanical stress introduced during installation and DC arc escalation, are also relevant to ground mount and utility scale systems using similar module construction.

Contributing factors

As stated by the investigators: DC arc ignition originated from stress induced poor contact within a module junction box. EL testing of recovered modules across the site indicated a very high defect rate, with approximately 99% of inspected modules showing defects, mainly cell cracks. Inherent thermal failure modes, bypass diode heating and reverse current overload, were excluded as causes because laboratory replication of these mechanisms did not reproduce the observed ignition behaviour. Correlation analysis across the field tested modules associated the ignition with installation quality factors, specifically the gap between modules, cable tension and crimping quality. The investigators considered that mechanical tension arising from the installation method caused minute loosening of metallic contacts within the junction box, which then sustained a DC arc until ignition.

Lessons learned

1. Module junction boxes on rooftop solar can sustain DC arcs after mechanical stress loosens internal contacts, so operators should measure cable tension and crimp quality at installation rather than rely on visual checks.
2. High rates of cell cracks across a site can accompany junction box faults, so use EL imaging at commissioning to find defects that visual inspection misses.

More lessons on electrical protection and isolation, maintenance, inspection and ageing assets, from every incident on the register.

Not yet known

• Extent of damage to the installation
• Emergency response details and duration
• Module model and age
• Downtime or repair cost

This entry is updated when these are reported. Get an alert for the next one like it.

Source

sciencedirect.com