A method and device for monitoring insulation between a DC bus and protective earth, wherein the bus is connected with a DC source. Terminals of the source are connected to protective earth by a first electrical network during a first timespan, and transient first electrical values related to the first network at a plurality of time steps in the first timespan are measured. A first steady state value is calculated using a plurality of first measurement points at the plurality of first time steps. Further, the terminals of the source are connected to protective earth by a second electrical network, during a second timespan, and transient second electrical values related to the second network at a plurality of second time steps in the second timespan are measured. A second steady state value is calculated using a plurality of second measurement points at the plurality of second time steps. An indication of insulation resistances between the terminals of the source and protective earth is determined based on the calculated steady state values of the first electrical value and the second electrical value.