A method for quickly measuring an insulation resistance for the application in a method for determining the insulation resistance (R_F1, R_F2) according to a pulse measuring method in an ungrounded power supply system, a measuring voltage (U 0) compiled of temporally consecutive square measuring pulses being applied between an active conductor (HV+, HV-) of the ungrounded power supply system and ground by means of a measuring pulse generator via a coupling branch having a coupling resistance (R_C1, R_C2) and a measuring resistance (R_M1, R_M2) and a continuous-time curve of a voltage (U m) measured via the measuring resistance (R_M1, R_M2) being captured, the method comprising the following steps: forming and storing a sequence of time-discrete output-voltage measured values (V i) from the curve of the measured voltage (U m), calculating time constants (50, tau i) as negative quotient from a derivative of the first order (V' i) and a derivative of the second order (V" i) of time-discrete output-voltage measured values (V i = V1, V2, V3) stored at temporally directly consecutive points in time (T1, T2, T3) in a calculating period (T c) within a settling phase of the square measuring pulse, calculating an effective tau value (54, tau_eff) via time constant averaging, characterized by indicating a target charging state (r) of the network capacities (C_F1, C_F2) available in the ungrounded power supply system, calculating a target point in time (Tt) at which the network capacities (C_F1, C_F2) will have reached the target charging state (r) from the target charging state (r) and the effective tau value (54, tau-eff), calculating a first wait time (dTr) starting from the current point in time as the difference between the target point in time (Tt) and the current point in time, outputting a safe-to-start signal (ss) should the first wait time (dTr) be less than an indicated quick-start time (Ts), otherwise outputting a not-safe-to-start signal (ns). forming and storing a sequence of time-discrete output-voltage measured values (V i) from the curve of the measured voltage (U m), calculating time constants (50, tau i) as negative quotient from a derivative of the first order (V' i) and a derivative of the second order (V" i) of time-discrete output-voltage measured values (V i = V1, V2, V3) stored at temporally directly consecutive points in time (T1, T2, T3) in a calculating period (T c) within a settling phase of the square measuring pulse, calculating an effective tau value (54, tau_eff) via time constant averaging, characterized by indicating a target charging state (r) of the network capacities (C_F1, C_F2) available in the ungrounded power supply system, calculating a target point in time (Tt) at which the network capacities (C_F1, C_F2) will have reached the target charging state (r) from the target charging state (r) and the effective tau value (54, tau-eff), calculating a first wait time (dTr) starting from the current point in time as the difference between the target point in time (Tt) and the current point in time, outputting a safe-to-start signal (ss) should the first wait time (dTr) be less than an indicated quick-start time (Ts), otherwise outputting a not-safe-to-start signal (ns).