## 1 Introduction
EL inspection applies an electric field to the PV module and captures near-infrared light when excited electrons return to the ground state. Using this infrared light, it is possible to check for claps in the cells of the PV module and disconnection/connection failure of the intercollector. Similar to the EL inspection, the PL inspection is based on imaging near-infrared light when excited electrons return to their ground state. However, PL inspection uses light irradiation, rather than an electric field, to excite electrons. These methods evaluate PV cell defects, such as the clap and forbidden band of PV cells [16]. However, quantitative infrared thermography analysis remains controversial. In addition, EL/PL inspection requires reliable equipment. Furthermore, these methods have been established as performance measurement and evaluation methods for manufacturing silicon wafers and PV cells. However, their application in PV modules remains limited. The current and voltage of the PV cells and modules are essential indicators for evaluating their performance. In addition, if the current and voltage of the element cells in a PV module can be measured, a more detailed performance evaluation is possible. Previous studies have investigated non-contact potential measurements of PV modules. This study investigated a method for identifying faulty modules in PV arrays based on non-contact potential measurements of the PV modules. Because the purpose of this study was failure diagnosis, we focused only on the potential of the PV module. However, when performing performance measurements and evaluations, it is preferable to measure the current and voltage, which directly reflect the performance. In addition to performance, safety is also essential for PV systems. Several cases of fire caused by PV systems were reported and investigated [17; 18; 19]. A local temperature rise caused these fires, called hot spots, and their temperature rise was the concentration of the energy loss of the solar cells owing to current-mismatching loss [20; 21; 22; 23; 24]. Thermal images typically monitor and detect these local losses [25; 26; 27; 28; 29; 30; 31; 32; 33]. However, it is essential to detect early symptoms using non-contact and non-destructive test methods and to quantify the modeling and measurement of the degree of concentration of the mismatching loss, for example, xx W loss (xx may be obtained by measurement) in this particular solar cell. The fire risk is also known to organic photovoltaics [34]. In addition to these approaches, such diagnostic and monitoring functions can be implemented in the solar system using power optimizers. They detect it by the current balance and shut down the damaged or degraded strings. However, physical, non-destructive, and non-contact inspection approaches are demanded for detailed information about the risk. A standard Inspection method for the fire risk assessment is temperature imaging by IR camera. This inspection evaluates the status (temperature) in a specific environment (solar irradiance, ambient temperature, and wind). The criteria may be if the hot-spot temperature is substantially high (more than the firing point). Although it visually shows the risk of the temperature rise, the inspected value varies by measurement environment and is not reproducible. The inspection method proposed in this work evaluates the potential (magnitude of heat source that raises temperature). The advantage of this method is to anticipate the worst-case temperature and judge if it is higher than the firing points. The differences between the new measurement method and the standard IR camera inspection are summarized in Table 1. The proposed method detects the reverse-bias voltage of each cell [35] and calculates the dissipated heat by multiplication of the circuit current. The conversion to the temperature from the dissipated heat of the solar cell can be calculated by multiplication of the proportional coefficient [36].