Method and device for insulation monitoring of non-grounded electrical DC and AC grids

The invention relates to a method for monitoring the insulation of non-grounded electrical DC grids and AC grids, comprising an unavoidable ohmic (Rf) and capacitive (Cf) insulation resistance between the grid and ground, particularly those having rectifiers and other converters, wherein an impulse alternative voltage (Uo) comprising alternating different impulse voltage values is applied between the grid and ground by means of an ohmic grid coupling (Rj), wherein the time curve of the instantaneous grid-dependent transient response of the measured current or a parameter derived therefrom is monitored as a measured voltage (Vm) for each impulse voltage value (Uo). The time curves of the transient responses of the measured voltage (Vm) before grid transient to each impulse voltage value (Uo) for two impulse voltage value applications of opposite polarity are used for determining the insulation resistance (Rf) between the grid and ground.