Recommendations for CSM and Riso Ground Fault Detector Trip Thresholds

PV ground faults have caused many fires in the U.S. and around the world. One cause of these fires is a “blind spot” in the ground fault ground fault fuse.. As a result of this discovery, the Solar America Board for Codes and Standards identified a number of alternatives to ground fault fuses, but these technologies have limited historical use in the United States. This paper investigates the efficacy of two of these devices, isolation resistance monitoring (Riso) and current sense monitoring (CSM), in small (~3 kW) and large (>500 kW) arrays using both simulation and field data. The field data includes Riso and leakage current measurements of multiple PV systems, while the simulations include Riso and CSM measurements from various ground faults. From these results, it was found that the majority of leakage current is not from the modules, but from low inverter isolation-to-ground. Therefore appropriate thresholds to maximize detection area while minimizing nuisance tripping should be made based on the specific inverter isolation and switching noise rather than the configuration of the PV system. Index Terms — Riso, RCD, SPICE, ground faults