String offline (open circuit)
An entire string of modules is electrically disconnected and producing nothing. It is usually the single largest energy loss visible on a thermal inspection.
Thermal signature
Every module in one string appears uniformly warmer than the modules in neighbouring, operating strings. The warmth is even across each module face rather than spotty, and the affected run of modules follows the site's string wiring layout exactly — a clean row or block that ends at a string boundary.
Typical ΔT
2–7 °C above comparable operating strings, uniform across every module in the string (typical industry interpretation aligned with IEC TS 62446-3).
What causes it
- Blown string fuse or tripped/failed breaker in the combiner box
- Unmated, corroded or burnt DC connector between modules or at the string ends
- Damaged or severed string cabling (rodents, machinery, thermal damage)
- Inverter MPPT input switched off, failed or de-rated
- Isolator left open after maintenance
Power impact
The string exports nothing until it is repaired — 100% of that string's capacity is lost for every daylight hour. On utility-scale sites a single offline string is routinely the largest recoverable loss in the whole report.
Recommended action
Fix: immediate
Act immediately. Isolate the string safely, then trace the open circuit from the combiner back through the connectors and cabling: check the string fuse first, then measure open-circuit voltage at accessible points to bracket the break. Restore and confirm production at the inverter.
Frequently asked questions
Why does an offline string look warmer, not colder?
An operating module converts part of the absorbed sunlight into exported electricity. When the string is open-circuit no current flows, so that energy stays in the modules as heat instead — every module in the string runs a few degrees warmer than its producing neighbours.
Wouldn't inverter monitoring already catch an offline string?
Only if the site has string-level monitoring and someone is watching it. Many sites monitor at the inverter or MPPT level, where one lost string inside a large input group is easy to miss. Aerial thermography finds it and shows exactly which physical string is affected.
How much energy does an offline string lose?
All of its output for as long as it is down. As a rough example, a 30 kW string in a sunny Australian location can lose in the order of 100–150 kWh per day, so an open circuit that goes unnoticed for a quarter is a substantial revenue loss.
Related anomalies
Suspect string offline (open circuit) on your site?
Fly the site, upload the thermal frames, and get every module graded against this taxonomy — severity-ranked, with flagged imagery and per-module locations. Quote your site size in seconds.
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.