Bypass diodes protect solar cells from the destructive effects of hot spotting. When a portion of a module is shaded by debris, soiling, or structures, those cells stop producing power and act as electrical resistors. A functioning diode routes the current around the obstructed string to prevent damage.
Joerg Althaus of Intertek CEA explained in a report why lost bypass diodes in PV modules are a hidden fire risk. Althaus said diode failures can pass standard factory and field inspections, yet create severe overheating when shading occurs, posing a hidden risk.
When a bypass diode is physically missing or disconnected during manufacturing, the module behaves completely normally under uniform, unshaded sunlight. Because factory flash tests use uniform artificial light, the diodes remain inactive during production sign-off.
Field inspections run into the same dead end. Standard drone or handheld infrared thermography performed under clear sky conditions will show a perfectly uniform thermal profile because no active shading is triggering the bypass mechanism.
The danger manifests when real-world shading inevitably occurs. Without a functional diode to reroute the electrical current, the obstructed solar cells are forced to dissipate the full power of the entire unshaded string.
This extreme, localized energy dissipation drives cell temperatures up rapidly. Overheating can quickly melt internal junctions, compromise the module’s backsheet, and escalate into a severe thermal event or full-scale fire. It exposes a critical detection gap in current operations and maintenance practices, where asset owners assume clear thermal scans mean a clean bill of health.
Catching missing diodes requires moving away from passive monitoring toward targeted diagnostics, said the report from Intertek CEA. Electroluminescence imaging can identify missing or disconnected circuitry within the junction box before modules leave the factory floor or get mounted on racks.
In the field, teams must utilize shaded thermography by intentionally creating or waiting for localized shading during an infrared scan, said Althaus. This forces the bypass mechanism to engage, instantly isolating the vulnerable string segments via extreme heat signatures.
A related topic focused on junction box failures in PV modules will be explored in a pv magazine webinar featuring Intertek CEA this August, stay tuned for the announcement, and find the full lineup of pv magazine webinars here.
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