The invention relates to an insulation monitoring device and to a method for conforming-to-standards monitoring of an insulation resistance of an ungrounded single-phase or multi-phase power supply system having at least two active conductors. According to the invention, the insulation monitoring device comprises a voltage monitoring circuit for permanently registering the conductor voltage to ground potential for at least one active conductor. If an excess voltage is detected on an active conductor, the voltage monitoring circuit generates a shut-down signal so as to shut down the power supply system.