The 10th case occurred in March 2016 in Hiroshima Prefecture. The resident noticed smoke rising from the roof and contacted the fire department. Panels and part of the loft were burnt (Fig. 16). Some of the sheathing roof boards were burnt and fell down.
The fire department inferred that the roofing material installed on the sheathing roof boards was ignited by heating of a portion due to an unknown defect inside a panel. However, it is uncertain whether the defect in the panel is attributable to insufficient maintenance, according to the fire department.
According to the supposition of the NITE, the fire was caused by arc discharge due to a decline in insulation resistance between cells and the decline in insulation resistance was attributable to mixing of conductive foreign matter and moisture due to problems during production. However, foreign matter was not discovered in the collected parts and the cause of the accident was not identified.
The 11th case occurred in April 2016 in Kanagawa Prefecture. A neighbor noticed smoke coming from the roof and contacted the fire department. A few panels and sheathing roof boards under them were burnt, with a hole in the roof (Fig. 17 & 18). The panels were installed by the "method where panels are not integrated with steel plates."
The analysis by the fire department concluded that the fire was caused by shading of the northwestern corner of the panels integrated in the roof by a roof tile. The process leading to the fire based on that supposition as follows.
A hot spot was generated in the shaded cells due to the specification of the system, where reverse bias is applied to shaded cells, causing failure of the cells due to thermal denaturation, resulting in overheating due to constant energization of the bypass diode. Following the failure of the bypass diode, the negative terminals were overheated and melted and resin parts in the junction box were ignited over time, resulting in the fire.
The 12th case occurred in August in Fukuoka Prefecture. A neighbor noticed smoke coming from the roof and contacted the fire department. A few panels and sheathing roof boards under them were burnt, with a hole in the roof (Fig. 19). The panels were installed by the "method where panels are not integrated with steel plates."
The fire department inferred that the fire was caused by electrical leakage or a short circuit of an interconnector or a cable of one of the solar panels, according to the fire department, although the process is unknown.
The 13th case occurred in October 2017 in Aichi Prefecture. A person who passed by the house discovered white smoke coming from the roof. The person reported the fire to the fire department because the resident was not at home. The fire was extinguished by firefighters. The roof and parts of the roof were burnt (Fig. 20). The panels were installed by the "method where panels are not integrated with steel plates."
The fire department inferred the process leading to the fire to be as follows. In areas where positive cables extending from specific panels are connected to negative cables from other panels by connectors, a semi-disconnection state was generated due to a problem at the time of production or external force applied after production. Metal brackets used to fix the panels were overheated due to heat generated by proliferation of cuprous oxide in the areas. As a result, the sheathing roof boards were ignited, leading to extension of the fire.