Recommendations for Isolation Monitor Ground Fault Detectors on Residential and Utility-Scale PV Systems

PV faults have caused rooftop fires in the U.S., Europe, and elsewhere in the world. One prominent cause of past electrical fires was the ground fault detection "blind spot" in fuse-based protection systems uncovered by the Solar America Board for Codes and Standards (SolarABCs) steering committee in 2011. Fortunately, a number of alternatives to ground fault fuses have been identified, but there has been limited adoption and historical use of these technologies in the United States. This paper investigates the efficacy of one of these devices known as isolation monitoring (or isolation resistance monitoring, R-iso) in small (similar to 3kW) and large (similar to 700 kW) arrays. Unfaulted and faulted PV arrays were monitored with R-iso technology and compared to SPICE simulations to recommend appropriate thresholds to the maximize the range of ground faults which could be detected while minimizing unwanted tripping. Based on analytical and computational models, it is impossible to determine a trip threshold that provides fire safety and negates unwanted tripping issues. This paper mathematically demonstrates that appropriate R-iso trip thresholds must be determined on an arrayby-array basis with sufficient leeway by system operators to adjust trip threshold settings for their particular usage cases.