Hot spot

Definition

A hot spot forms when current is forced through a narrow region inside a cell. In this case only part of the cell heats up. Because the same energy is concentrated in a small area, the local temperature rises. According to IEC TS 62446-3, local heating on a panel is assessed by the size of its temperature difference.

Signs in the imagery

In the thermal image it shows as a bright point or patch with soft edges that does not follow the cell boundary, usually inside a single cell. The RGB image may show a cause such as a bird dropping or a leaf. If no cause is visible, the problem most likely lies within the cell itself.

Possible causes

• A bird dropping, leaf or heavy dirt covering part of a cell
• A local short (shunt) or partial crack inside the cell
• Rising resistance at a ribbon or solder joint
• Delamination or moisture ingress at the cell surface

Effect on production and safety

On its own the effect on production is limited, but the local temperature can rise quickly. A long-lasting hot spot ages the encapsulant and backsheet and can permanently damage the glass and the cell. An advanced hot spot counts among the anomalies that carry fire risk.

Recommended action

If the cause is visible in the RGB image, clean the panel and confirm at the next flight that the spot has gone. If no cause is visible, inspect the panel in the field and bring forward the replacement of panels with a high temperature difference. For panels that are replaced, attach the measurement history to the warranty claim.