A device for determining insulation resistances of power networks (106, 108) with respect to an electrical earth in a motor vehicle, which power networks are coupled to one another by way of at least one power converter, wherein the at least one power converter comprises an inverter and/or a DC-DC converter, the device comprising: a detection device, which is designed, in at least two measuring intervals, to detect a respective operating state of the at least one power converter of the motor vehicle, which power converter is conductively connected to a traction energy store; a measuring device with a measuring terminal, which is conductively connected or connectable to at least one DC voltage pole of the electric traction energy store of the motor vehicle, and with an earth terminal, which is conductively connected or connectable to a reference potential of the motor vehicle, wherein the measuring device is designed, in the at least two measuring intervals, to respectively measure a conductance between the measuring terminal and the earth terminal; and a calculation device, which is designed to determine the insulation resistances as a function of the at least two measured conductances and the at least two operating states detected, wherein the operating state comprises a duty cycle of at least one switching element of the power converter, wherein the detection device is designed to detect the operating states of several power converters respectively in several measuring intervals, the measuring device is designed to measure the conductance in each of the measuring intervals, and the calculation device is designed to calculate for each of the power converters an insulation resistance based on the measured conductances, the operating states detected and a number of measuring intervals, which is greater than or equal to the number of power networks. a detection device, which is designed, in at least two measuring intervals, to detect a respective operating state of the at least one power converter of the motor vehicle, which power converter is conductively connected to a traction energy store; a measuring device with a measuring terminal, which is conductively connected or connectable to at least one DC voltage pole of the electric traction energy store of the motor vehicle, and with an earth terminal, which is conductively connected or connectable to a reference potential of the motor vehicle, wherein the measuring device is designed, in the at least two measuring intervals, to respectively measure a conductance between the measuring terminal and the earth terminal; and a calculation device, which is designed to determine the insulation resistances as a function of the at least two measured conductances and the at least two operating states detected, wherein the operating state comprises a duty cycle of at least one switching element of the power converter, wherein the detection device is designed to detect the operating states of several power converters respectively in several measuring intervals, the measuring device is designed to measure the conductance in each of the measuring intervals, and the calculation device is designed to calculate for each of the power converters an insulation resistance based on the measured conductances, the operating states detected and a number of measuring intervals, which is greater than or equal to the number of power networks.