Device for determining insulation resistances of power networks with respect to an electrical earth in a motor vehicle , wherein the 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 control device which is designed, in at least two measuring intervals, to control a respective operating state of the at least one power converter of the motor vehicle , which 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 ground 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 controlled, wherein the operating state comprises a duty cycle of at least one switching element of the power converter , wherein the duty factor controlled is less than 100% and greater than 10%, and wherein the function incorporates parameters for the weighting of a dependence of the insulation resistance on the operating state controlled, and wherein the calculation device incorporates a memory, in which motor vehicle-specific parameter values are saved, characterized in that the control device defines at least one measuring interval of the at least two measuring intervals in such a way that it precedes driving readiness being established and/or follows driving readiness of the motor vehicle being withdrawn. a control device which is designed, in at least two measuring intervals, to control a respective operating state of the at least one power converter of the motor vehicle , which 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 ground 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 controlled, wherein the function incorporates parameters for the weighting of a dependence of the insulation resistance on the operating state controlled, and wherein the calculation device incorporates a memory, in which motor vehicle-specific parameter values are saved, characterized in that the control device defines at least one measuring interval of the at least two measuring intervals in such a way that it precedes driving readiness being established and/or follows driving readiness of the motor vehicle being withdrawn. <EOS>