Method for protecting an electrical supply network in case of a fault, comprising the steps: a) measuring at least an electric time-periodic signal (i(t)) of the supply network, b) determining real parts and imaginary parts (Re, Im; Re, Im) of the first and the second harmonic (I1, I2) by transforming the time-periodic signal (i(t)) into the frequency domain, c) forming modified filter coefficients at least for a modified imaginary part (I1D) of the transformed time-periodic signal (i(t)) by combining imaginary part filter coefficients of the first and the second harmonic (I1, I2), wherein the imaginary part filter coefficients of the first and the second harmonic (I1, I2) are combined for forming the modified coefficients (I1D) in such a way that the imaginary part filter coefficients of the second harmonic are multiplied with a factor and subsequently are subtracted from the imaginary part filter coefficients of the first harmonic, subsequently either d1) verifying for each sample value if the modified imaginary part (I1D) has the same algebraic sign like the imaginary part of the first harmonic and if the value of the modified imaginary part (I1D) is greater than the value of the imaginary part of the first harmonic, e1) setting an output imaginary part (I1A) such that it is equal to the modified imaginary part (I1D) if the conditions of step d1) are satisfied, or setting the output imaginary part (I1A) equal to the imaginary part of the first harmonic if the conditions of step d1) are not satisfied, f1) determining a line impedance (Z L) with the output imaginary part (I1A), or d2) determining a first impedance by using the first harmonic I1), e2) determining a second impedance by using the modified imaginary part (I1D), f2) choosing from the first and the second impedance (Z1, Z2) the impedance having a smaller value or from the first and the second impedance (Z1, Z2) the impedance with a smaller value of the imaginary part than line impedance (Z L), and subsequently g) comparing the line impedance (Z L) with at least a given value, and h) issuing a control signal for an interruption component of the supply network if the line impedance (Z L) is smaller than the at least one given value. a) measuring at least an electric time-periodic signal (i(t)) of the supply network, b) determining real parts and imaginary parts (Re, Im; Re, Im) of the first and the second harmonic (I1, I2) by transforming the time-periodic signal (i(t)) into the frequency domain, c) forming modified filter coefficients at least for a modified imaginary part (I1D) of the transformed time-periodic signal (i(t)) by combining imaginary part filter coefficients of the first and the second harmonic (I1, I2), wherein the imaginary part filter coefficients of the first and the second harmonic (I1, I2) are combined for forming the modified coefficients (I1D) in such a way that the imaginary part filter coefficients of the second harmonic are multiplied with a factor and subsequently are subtracted from the imaginary part filter coefficients of the first harmonic, subsequently either d1) verifying for each sample value if the modified imaginary part (I1D) has the same algebraic sign like the imaginary part of the first harmonic and if the value of the modified imaginary part (I1D) is greater than the value of the imaginary part of the first harmonic, e1) setting an output imaginary part (I1A) such that it is equal to the modified imaginary part (I1D) if the conditions of step d1) are satisfied, or setting the output imaginary part (I1A) equal to the imaginary part of the first harmonic if the conditions of step d1) are not satisfied, f1) determining a line impedance (Z L) with the output imaginary part (I1A), or d2) determining a first impedance by using the first harmonic I1), e2) determining a second impedance by using the modified imaginary part (I1D), f2) choosing from the first and the second impedance (Z1, Z2) the impedance having a smaller value or from the first and the second impedance (Z1, Z2) the impedance with a smaller value of the imaginary part than line impedance (Z L), and subsequently g) comparing the line impedance (Z L) with at least a given value, and h) issuing a control signal for an interruption component of the supply network if the line impedance (Z L) is smaller than the at least one given value.