Continued from Cause of Solar Panel Ignition, Spread of Fire (1)

The "disconnection of bypass circuit" in the third phase is understood by measurement using a bypass circuit inspection system or from the measured panel I-V values. Disconnection of bypass circuit was observed in two cases by the on-site survey.

"Disconnection or abnormal heating of soldered wiring joints" in the fourth phase is checked by measuring the open circuit voltage of a string or using a wiring route survey instrument. Abnormally hot areas inside panels can be clarified from temperature distribution images shot by infrared cameras.

Solar panels that are abnormally hot at soldered wiring joints were not detected by the inspections. However, panels with disconnected wiring joints were discovered in two of the investigated sites.

When the open circuit voltage of the defective panels was measured, the value was "0V." Based on the result, it was confirmed that both the bypass circuit and the soldered wiring joint were disconnected in the same cluster.

The estimated continuity in the ignition process was also validated for this phase. In the solar panel in which "disconnection of soldered wiring joint" in the fourth phase was observed, a black discolored portion, which seems to have been affected by the progress of "increase in resistance" in the first phase, was detected (Fig. 5).

Fig. 5: Discolored portions in soldered wiring joint (left) and bypass circuit (right). (source: CAA)

In the cluster where the resistance seems to be high, "disconnection of bypass circuit" in the third phase is observed. Discoloration to brown, which seems to have been caused by "constant energization" in the second phase, was observed in the resin filled in the junction box.

Base on the above and the facts that the defect in the first phase occurred in the wiring joint inside the cluster in which the fourth phase defect was generated and the second phase defect was confirmed in the junction box for the cluster where the third phase defect was generated, it is estimated that the fourth phase defect occurred via the first phase defect and the third phase defect occurred via the second phase defect.

Since all of these defects occurred in the same cluster, it is likely that the defects in the panel progressed from the first phase to the fourth phase in this order, causing disconnection of the soldered wiring joints (Fig. 6).

Fig. 6: Discolored bypass circuit (left) and swelling in back sheet (right) (source: CAA)