The ISOMETER® IR123P monitors the insulation resistance RF of an unearthed AC system of 100…300 V against earth, that is supplied by a mobile generator according to DIN VDE 0100-551. The IR123 is suitable for AC systems with operating frequencies of 22…65 Hz as well as for AC systems including DC components. The maximum permissible system leakage capacitance Cemax is 5 μF.