String fault (3 or more consecutive modules)

Definition

When the current path in a series string is interrupted, none of its panels can deliver power. All of them run at open circuit and warmer than the neighbouring strings. When three or more consecutive panels show the same signature, the cause lies in the string circuit rather than in the individual panels. It is usually found in a connector, a cable, a fuse or the inverter input.

Signs in the imagery

In the thermal image a run of adjacent panels along a row is uniformly warm, without any cell or band pattern. The warm panels stay within the string's electrical boundaries while neighbouring strings are at normal temperature. Additional point heating at a connector or cable joint can indicate arcing.

Possible causes

• A blown string fuse or an open string switch
• A loose, badly crimped or burnt connector
• A cable cut by rodents, mowing equipment or heat damage
• A broken connection in the combiner box
• An inverter fault or a disabled inverter input

Effect on production and safety

All of the string's output is lost, every day until the string is repaired. If the interruption comes from a loose or burnt connector, arcing can occur, and arcing is among the anomalies that carry fire risk. String losses are much larger than single-panel losses and should be cleared first.

Recommended action

Check the string fuse and switch in the combiner box, then the connectors and cables along the string. If there are arc marks, melting or burns, isolate the string safely and do not re-energise it before the connector is replaced. The string current data in SCADA can be used to determine when the interruption began.