An apparatus for monitoring and controlling electrical systems in a motor vehicle with electric or hybrid drive has a first and a second AC voltage source for an alternating voltage between a first or second terminal and a reference terminal. First and second voltmeters detect a first or second voltage between the first or second terminal and the reference terminal. A control unit controls the AC voltage sources, receives the voltages from the voltmeters and determines insulation resistance for subassemblies that are connected to the terminals. The insulation resistance is relative to a potential connected to the reference terminal. A precharge state of a capacitative load connected to the first terminal is determined. The load is precharged by a voltage supply unit connected to the second terminal. The switching state of at least one switch between two subassemblies is determined. The subassemblies are connected to the first or second terminal.