Abstract: Method and system for predictive maintenance of switch contactors, preferably of converters in wind turbine generators, comprising measuring the temperature of an element located outside a switch contactor enclosure in contact with the contactors inside the enclosure, and calculating said switch contactor temperature based on said element temperature and predetermined correction values, and measuring the current through the switch contactor, and calculating a switch contact resistance value based on said calculated switch contact temperature and said measured current through the contactor, and determining a maintenance parameter of said switch contact by comparing said calculated switch contact resistance value to a predetermined nominal contact resistance value.