Double bypassed (2 substrings)
Two of the module's three substrings are being carried by their bypass diodes, so two-thirds of the module is out of production. Persistent double bypassing usually means hardware failure rather than shading.
Thermal signature
Two-thirds of one module — two adjacent substring bands — reading uniformly warmer than the remaining operating third. The boundary between warm and normal follows the module's internal substring layout in clean straight lines.
Typical ΔT
2–6 °C between the bypassed substrings and the still-operating substring (typical industry interpretation aligned with IEC TS 62446-3).
What causes it
- Two shorted bypass diodes (a common surge/lightning failure pattern)
- Cell or interconnect faults in two substrings forcing both diodes to conduct
- Severe persistent shading or soiling covering two substring regions
- Junction-box damage affecting two circuits
Power impact
About two-thirds of the module's output is lost while both substrings are bypassed. If the cause is shorted diodes, the loss is continuous — it does not come back when shading moves — and the module also loses hotspot protection on those circuits.
Recommended action
Fix: within 30 days
Inspect within 30 days. Test all bypass diodes at the junction box, check the two affected substrings for cell damage, and replace shorted diodes or the module as warranted. Verify against shading and soiling first to rule out an external cause.
Frequently asked questions
Why do two substrings fail together so often?
Surge events are indiscriminate. A nearby lightning strike or grid transient can short two — or all three — bypass diodes in the same junction box at once, which is why double-bypassed modules cluster after storm activity.
Is a double-bypassed module worse than a hotspot?
In pure energy terms, usually yes: two-thirds of a module continuously lost outweighs the few percent a typical hotspot costs. A severe hotspot can still outrank it on safety grounds, which is why both the fault type and the measured ΔT feed the final severity grade.
Related anomalies
Suspect double bypassed (2 substrings) on your site?
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ΔT figures and abnormality classes on this page describe the typical industry interpretation aligned with IEC TS 62446-3 — indicative engineering guidance, not normative text from the specification. Temperature differentials are only meaningful when captured at ≥600 W/m² irradiance (per the TS) and verified on site before repair work.