Defective Solar Panels Can Cause Fires at Residential PV Facilities (2)
The fourth case was a fire that occurred in June 2013 in Niigata Prefecture. A neighbor of the house discovered smoke and fire coming from the roof and told the resident. The resident extinguished the fire by climbing up to the roof and using a hose connected to the water supply and called 119. Solar panels and the peripheral area were burnt in this case (Fig. 7 & 8). The panels were installed by the "method where panels are placed on roof."
According to the analysis of the fire department, sparks generated from an extension cable connected to a panel cable ignited dry cedar leaves on the roof, resulting in the spread of fire. A portion with continuous and wave-shaped gashes was discovered on a cable. The gashes were caused due to gnawing by small mammals such as flying squirrels. It was inferred that the cable coating was broken, causing a short circuit near the PF tube (plastic flexible tube) where extension cables are concentrated, generating a situation where sparks tend to fly easily.
The fifth case occurred in August 2013 in Nara Prefecture. A fire alarm was activated when no one was at home and an employee of a security company rushed to the house. Smoke was in the house, but the alarm stopped when the second floor windows were opened. The employee left the house after obtaining approval.
The resident came back later and checked the interior of the house. A trace of burning was discovered in the storage space in the loft, and the resident reported the accident to the fire department. Parts of cables and part of the loft were burnt (Fig. 9). The panels were installed by the "method where panels are integrated with steel plates."
The fire department inferred the cause of the fire to be as follows. A subsidiary of the company that sold the house was entrusted with reconstruction of the existing wiring for construction of the loft around January 2012. On this occasion, the cables for the solar power facility had to be passed above the ridge beam. The originally installed cables were cut and the cable cores were press fitted using ring sleeves, pasting vinyl insulation tape on the terminals. Then, plasterboard was installed on the rafters by a construction company before completing the work.
As a result, the cables were enclosed in a narrow space, with roofing material on top, plasterboard at the bottom and rafters to the left and right. The cables were heated every day in the daytime due to the generated solar power passing through the narrow space, with no measures to release the heat.
The cables were subjected to such environment continuously and heat accumulated inside the bundled cables, causing overheating and melting the vinyl tapes on the terminals. The connected copper wires were exposed due to press fitting by ring sleeves, which resulted in a short circuit due to contact between the copper wires. This generated sparks and burned the cable coating, causing the fire.
The sixth case occurred in August 2014 in Kanagawa Prefecture. A person passing by the house told the resident that "smoke is rising from the house," and the resident reported it to the fire department. Parts of solar panels, sheathing roof boards and the loft were burnt (Fig. 10 & 11). The solar panels were installed by the "method where panels are not integrated with steel plates."
The first department inferred that damage occurred in multiple locations in the covering of solar panel cables of different poles due to inadequate installation, which caused a short circuit via aluminum frames of the panels and metal brackets for fixing on the roof, causing electrical discharge and igniting the resin construction material for fixing the panels.
Before the fire, the circuit breaker tripping and the alarm indicating a DC ground fault of the PV inverter were observed. Responding to the circuit breaker tripping, two systems in which electrical leakage was detected were shut down, but the fire was not prevented. If appropriate actions such as shielding the panels were taken on this occasion, the fire could have been prevented, according to the fire department.
The seventh case occurred in August 2014 in Tokyo. The resident heard a crackling noise coming from the second floor. An unusual smell was detected when the window was opened, but the resident did not think the smell came from the house. Later, a neighbor told the resident that smoke was rising from the roof, and the resident contacted the fire department. Part of the loft and seven solar panels were burnt (Fig. 12 & 13). The panels were installed by the "method where panels are not integrated with steel plates."
Sparks generated from panel cables ignited the roofing materials and caused the fire, according to the analysis by the fire department.
The NITE inferred that a cable was pinched in the mounting system at the time of installation and load and vibration stress was applied to the cable due to changes in the weather after installation, which accelerated deterioration in insulation performance of the cable at the pinched portion, causing breakdown at the maximum power generation amount and forming a short circuit using the mounting system as the path, which led to the flow of excessive current and heating, resulting in the fire.
The eighth case occurred in February 2015 in Tochigi Prefecture. The residents were not at home, but children in the neighborhood discovered smoke coming from the roof and contacted the fire department. When a panel in the area where smoke was being generated was removed, a fire was about to start near the cables. Cables and one panel were burnt and roof tiles were cracked by the heat (Fig. 14).
The fire department inferred the cause of the fire to be as follows. When the conduit was replaced about 6 months before the fire, the cables of two systems were detached from the coupler joint and five cables including one ground wire were cut, followed by press fitting and coating of the cables. Defective press fitting existed in these areas, which generated heat over time and resulted in the fire.
The ninth case occurred in December 2015 in Kanagawa Prefecture. A neighbor discovered a fire in the area around the PV inverter installed on the outer wall. The fire was reported to the fire department by the resident. The fire was extinguished by the resident using a fire extinguisher and a firefighter confirmed that the fire had been controlled. Cables, part of the PF tube and part of the outer wall were burnt (Fig. 15).
The fire department inferred the cause of the fire to be as follows. Connectors of the cables that connect panels with the PV invertor were inside an outdoor cable protection tube made of resin, which is not supposed to house joints. The joints became loose due to deterioration of connectors, and so forth, causing defective contact and generating heat due to increase in contact resistance, resulting in melting of the insulating portion and a fire due to a short circuit between opposing poles.
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.