Defective Solar Panels Can Cause Fires at Residential PV Facilities

The Consumer Affairs Agency (CAA) released a report on fires generated from residential solar power generation systems.

The survey was conducted by the Consumer Safety Investigation Commission and the report is titled “Report on the survey of causes of accidents, etc., based on the specifications in Consumer Safety Act Article 23 Section 1.” The investigation commission tried to identify the causes of accidents and hazards based on the specifications in the abovementioned section and from the standpoint of consumers. The purpose of the survey was to prevent hazards that could affect the life or health of consumers and to prevent expansion of such hazards, not to accuse anyone of causing the accidents.

The survey was conducted because accidents including fires generated by residential solar power facilities could result in house fires, and as the number of residential solar power facilities exceeded 2 million as of the end of 2016, the possibility of avoiding such accidents seems to be low because it is difficult for consumers to stop these facilities generating power.

As for fires generated by solar panels and cables among the accidents involving residential solar power facilities, in many cases where cables are believed to be the ignition sources, the alleged cause is inadequate installation. On the other hand, when solar panels are believed to be the ignition sources, in many cases, the accidents are alleged to be caused by defective panels, not inadequate installation, according to the survey.

It is estimated that solar panels become ignition sources when defects in cable joints or bypass circuits deteriorate due to aging, based on survey reports of panel manufacturers.

The “process leading to panel ignition” was looked at in the report. Upon surveying residential solar power facilities in operation, problems that prove the validity of the process were actually discovered. Problems of this kind were observed in products of multiple manufacturers.

Among these problems, those in cable joints inside panels are caused by aging or problems in manufacturing. The duration after introduction is also an important factor because fires that are believed to have been ignited by panels occurred in residential solar power facilities that are older than about 10 years.

As for fires ignited by panels or cables, the degree of damage due to the spread of fires to roofing materials varies depending on how the panels and cables are installed on the roof.

Solar panels are usually installed on roofs by four methods (Fig. 1 & 2). In the “method where panels are placed on a roof,” panels are fixed on mounting systems that are installed on roof covering materials such as tiles, slates and metal sheets. In the “method where panels are placed on steel plates,” panels are integrated in roof covering materials or panels are fixed on the entire roof surface, with incombustible materials such as steel plates placed on roofing materials (waterproof materials for roofing) directly below the panels.

In the “method where panels are integrated with steel plates,” panels that have incombustible materials such as steel plates on the back are fixed directly to roofing materials. In the “method where panels are not integrated with steel plates,” panels without steel plates on the back are fixed directly to roofing materials.

The fire damage is serious when the fire extends to sheathing roof boards that constitute part of the roof. The damage was serious in seven fires where the fire extended to sheathing roof boards.

Sheathing roof boards are fixed on rafters and are generally made of wood. All of the seven fires that caused serious damage due to the spread of fire to sheathing roof boards occurred in the “method where panels are not integrated with steel plates,” in which noncombustible materials such as steel plates are not placed between solar panels and sheathing roof boards.

Fires are also caused by PV inverters and combiner boxes. The causes were also analyzed.

The Consumer Affairs Agency also disclosed the opinions submitted to the Minister of Economy, Trade and Industry and the Secretary of the Consumer Affairs Agency based on the survey results. The ministry and the agency will take action responding to the opinions and will report the details of the action to the Consumer Affairs Agency.

In the opinions to the Ministry of Economy, Trade and Industry (METI), the agency suggested that METI should request the manufacturers to take necessary measures to prevent recurrence of accidents caused by residential solar power facilities and at the same time promote such measures involving house builders in collaboration with the Ministry of Land, Infrastructure and Transport (MLIT).

The agency suggested that METI should take the necessary measures to ensure that owners of residential solar power facilities recognize that they are primarily responsible for maintenance and inspection of the facilities according to the current laws and regulations before taking the above action.

Owners of residential solar power facilities are consumers who purchase and use the equipment. In light of the fact that the roles of manufacturers are important from the viewpoint of consumer protection, related laws and regulations should be reviewed and a system that guarantees implementation of appropriate maintenance and inspection should be considered, while the contents of specific measures should be renewed as needed responding to future technological innovation, according to the agency.

Specifically, the agency requested risk assessments on fires and changes to solar panel installation methods, in respect to houses on which panels have already been installed by the “method where panels are not integrated with steel plates,” as measures to prevent recurrence of accidents due to ignition of solar panels or cables.

The agency requested METI to evaluate the results of risk assessment on fires in houses on which panels are installed by the “method where panels are not integrated with steel plates” and disclose the evaluation results. Because it is possible that fires are caused by problems while manufacturing the panels, the agency requested that instructions are given to ensure prompt and appropriate action as needed and risk assessment should be performed giving consideration to past problem-related information and the history of design changes related to the causes of problems.

However, it may be difficult to actually make corrections to panels that are installed by the “method where panels are not integrated with steel plates,” even if changes to other methods are encouraged.

The agency requests that manufacturers encourage users to carry out temporary inspections in such cases as alternative measures. The manufacturers are requested to explain to owners that panels installed by the “method where panels are not integrated with steel plates” pose a fire risk after expiration of the warranty period and encourage them to perform temporary inspections.

Specifically, the manufacturers encourage owners to check the energization state of the bypass circuit and check for disconnections. After the emergency inspection, periodic inspections are carried out according to the maintenance and inspection guideline to check for defects.

As for newly installed equipment, the agency requests that installation by the “method where panels are not integrated with steel plates” is avoided until the ignition risk of panels is confirmed to be low enough, that measures are taken for the “method where panels are integrated with steel plates,” in which cables tend to be pinched, that the connector design is reviewed to prevent defective contact due to loosening after installation and that measures against gnawing by small animals are prepared to ensure an appropriate installation method depending on the installation environment.

The agency also requests manufacturers of PV inverters and combiner boxes to provide measures against water intrusion into casing, defective contact of input terminals and condenser breakdown to prevent fires occurring in them.

As measures to prevent recurrence of fires during operation common to new and existing residential solar power facilities, the agency requested that products with ground fault detecting functions are used as standard products and renewal of devices in existing facilities is encouraged, as well as to establish and ensure appropriate measures responding to ground faults, including separation by disconnectors and shading of strings (units of solar panel connection) with ground faults.

The agency also requested that the inspection items for temporary inspection of the facilities incorporating “method where panels are not integrated with steel plates” are added to the maintenance and inspection guidelines of the Japan Electrical Manufacturers’ Association (JEMA) and the Japan Photovoltaic Energy Association (JPEA) and appropriate actions to be taken responding to ground faults are added to the guidelines.

The agency pointed out the importance of improving the reliability of residential solar power facilities by performing appropriate maintenance, positioning them as durables in future development of the facilities, ensuring durability of bypass circuits for long-term and constant energization aiming to reduce the risk of panel ignition, reviewing the related standards for this purpose, adding a function to detect abnormalities including constant energization and disconnection of bypass circuits and give warnings to the users, using noncombustible materials for sealing the panels, development of remote monitoring systems that contribute to the improvement of safety and reduction of inspection costs to METI.

In the “opinions” submitted to the Secretary of the Consumer Affairs Agency, the agency suggested that information on residential solar power facilities provided to consumers should be improved for easy understanding by consumers, referring to the report, to prevent occurrence and increase of problems for consumers.

The agency pointed out it is important to avoid using methods with a higher risk of fire expansion such as the “method where panels are not integrated with steel plates” and the “method where panels are integrated with steel plates” because the risk of fire varies depending on the method of solar panel installation on the roof, to use products with a ground fault detecting function and to ensure that the consumers are aware of their duty to perform inspections.

As of the time of the survey, 127 cases of fires, combustion, smoke generation, overheating, and so forth of residential solar power facilities were registered in the accident information databank operated by the Consumer Affairs Agency for the period from March 2008 to November 2017.

The targets of the survey were 72 of the 127 cases, including those that are under investigation by the National Institute of Technology and Evaluation (NITE) and those whose causes are unknown, excluding 72 cases not registered in NITE. Fires were caused by solar panels or cables in 13 cases of the survey targets, while fires were caused by PV inverters or combiner boxes in 59 cases.

The number of fires caused by panels or cables is comparatively small. However, the survey was conducted with a focus on these cases because they could lead to house fires, fatal accidents or injury to humans, because panels are close to roofing materials.

Details of the 13 cases where fires were caused by panels or cables are explained below in chronological order.

The first case occurred in Chiba Prefecture in September 2011. The resident of the house noticed an abnormal smell and a crackling noise when he/she tried to take in laundry from the veranda. The resident looked around and discovered smoke rising from under the eaves and called 119. Parts of the southern and northern sides of the roof, the ceiling and the panels were burnt.

The house incorporated the “method where panels are not integrated with steel plates.” While working to safeguard the house, two of the fire fighters complained of possible electrical shock, according to the fire department.

The fire department estimated that a contact failure occurred in a wire including panel connectors at a certain location for an unknown reason, generating heat and igniting over time, resulting in expansion of a fire to the roofing material. It was also recorded that it was possible that heating occurred due to a cable with melted coating, which nearly became disconnected due to pinching between a device and the construction material, or that irregular cable routing occurred during installation.

The second accident occurred in Saitama Prefecture in August 2012. When the resident returned home, the power supply to the solar power monitor was OFF and the circuit breaker was tripped. The resident smelt something burning in a room, but he/she did not check the outdoor areas.

The resident contacted the manufacturer the next day, and an employee of the manufacturer visited the house two days later and checked the situation. The resident reported the case to the fire department because an outdoor wiring was burnt. Six cables that extended from panels to a PV inverter and one ground wire were burnt. The panels were installed by the “method where panels are placed on a roof.”

The fire department estimated that the fire was caused by wires short circuiting due to a flaw in cable coating caused for some unknown reason. According to the manufacturer, one of the cables was connected in a direction reverse to the design direction and the cable was extended by being connected in the area near the burnt portion. This installation method is prohibited by the technical standards for electrical equipment.

The third case occurred in Kyoto Prefecture in March 2013. The power generation amount was insufficient and the resident asked about measures to solve the problem. Upon inspection by the manufacturer, a decline in the power generation amount and breakage of a cover glass for one of the panels were discovered.

The resident discovered a burnt portion during replacement of the panel whose cover glass was cracked and reported it to the fire department (Fig. 5). At the time of the discovery, the fire inside the panel had already burnt itself so no action was taken. The roof was not burnt. The panels were installed by the “method where panels are placed on a roof.”

According to the estimation by the fire department, the soldered connection of a solar battery cell (power generation element) electrode and interconnector was imperfect, causing contact failure following continued use, increasing the resistance value and causing localized heating due to energization during power generation, resulting in burning of EVA (ethylene-vinyl acetate copolymer resin), the sealing material. It is possible that a defective solder connection in panels caused generation of heat that reached 715°C momentarily in solder connection portions and caused flameless combustion of EVA, according to the fire department.

Based on the analysis of this case, the manufacturer estimated that the contact area decreased due to a decline in the soldering strength and the temperature increased due to a concentration of the current in the high resistance joint, causing burning of the sealing material and back sheet due to localized heat generation and reducing the contact area, which resulted in opening (disconnection) of the circuit or the high resistance state.

As a result, the current constantly flowed to the bypass diode, causing heat generation and resulting in damage to and failure of the bypass circuit, which was one of the possibilities that were pointed out. In the final stage, the voltage of the entire power generation system was applied to the area connected by solder, which led to overheating and resulted in cracking of the glass.

The solar panels also had two factors that could lead to accidents. Firstly, for products designed before June 2005, when the panels were manufactured, there was a possibility that the strength of the solder connection between electrodes on the back side of cells and interconnectors become insufficient due to a variation in manufacturing conditions.

Secondly, sealing material additives were not strictly controlled for the products manufactured during the period and foaming could have been caused due to variation in manufacturing conditions, and swelling of the panel back side due to foaming generated a force in the direction of separating the interconnector from cell electrodes. The two defects have been improved and have not been detected in products manufactured in and after July 2005, according to the manufacturer.

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