This from Bill Brook in 2015;
https://www.nrel.gov/docs/fy16osti/65050.pdf
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5. Finding Arc-Fault Failure Locations in a PV Array
Recently, many questions on field testing have been related to finding the location of arc faults after an arc-fault detector has activated. This is particularly difficult when the circuit is still intact. Few field technicians would have problems finding an arc-fault location in a three-string array where one of the three strings is open circuit. The open-circuit string is the likely fault location, and the technician can pick their favorite method to find which connection is bad. Some might use “divide and conquer,” where the voltage between the middle of the string to each of the two ends is tested to see which has voltage. Obviously, the end without voltage is the faulted section and the “divide and conquer” method can be continued (depending on how difficult it is to unplug modules).
Open circuits with arc faults are relatively easy to find, but what about when the circuit is still intact? The circuits may have good open circuit voltage, and even decent current, so what can be done with a case that may be intermittent? This has been the reason for many questions. In this case, the next tool to employ is an IR camera. Arc faults are the result of bad connections. Connections may have satisfactorily checked out at installation, or at the last O&M check, but possibly something may have happened since then. If an arc fault has truly occurred, there could be an intermittent connection that is getting worse. It could be a wire coming loose in a combiner box terminal, a connector coming loose that was not fully latched, an improperly crimped plug connector, or a connection coming loose in the module itself. In all of these cases, there will often be significant heat being generated at the connection. Combiner box problems or module defects are easy to find with an IR camera. The challenge comes when the connector is the location of the intermittent arc fault. Many rooftop systems do not have good access to the connectors, so getting a clean IR image of a connector can be difficult to view. Unfortunately, connectors are a common source of arc-fault fires, so they must be checked. Even without an IR camera, connectors, modules, and combiner boxes can be visually checked for melting or heat damage. Overheating in modules often causes browning of the encapsulants or backsheets. Overheating in combiner boxes may cause discoloration or deformation of insulating plastics. Overheating on connectors often causes distortion of the connector plastic.
Finding arc faults is currently more of an art rather than common practice.
Over the next few years it is envisioned new products will be developed to establish the location of arc faults. For example, there could be some type of signal generator used with a receiver identifying where the signal is attenuated, thus identifying that location as a bad connection. These types of troubleshooting devices are used throughout the electrical industry for different purposes and could probably be repurposed for this application. In the future, this type of concept could benefit troubleshooting PV arrays that have triggered an arc-fault detector."
