Insulation monitoring

Insulation monitoring checks the resistance between the PV positive or negative pole and ground, and the inverter disconnects from the grid if either DC conductor is shorted to ground, for example because of poorly insulated cables or faulty modules. Low insulation resistance can defeat direct and indirect contact protection, and the resulting ground fault current can cause fire, equipment damage or injury. Fronius says UL 1741 and the National Electrical Code require this safety function. Insulation monitoring or insulation resistance detection is listed for the Huawei SUN2000-25KTL-US, SUN2000-30KTL-US and SUN2000-45KTL-US-HV-D0, the Fronius Primo 208-240 models, and the CPS SCA36-60KTL-DO/US-480 inverters. SMA datasheets give an Riso limit for each model, for example Riso > 417 kΩ for the STP 12000TL-US-10 and Riso > 150 kΩ for the STP 33-US-41.