Method for determining a relationship between a voltage (a(t)) periodically changing substantially according to a sine curve and a current strength (b(t)) periodically changing substantially according to a sine curve, wherein the voltage (a(t)) is applied to a cable and the current strength (b(t)) is a current strength of a current flowing through the insulation of the cable, having the following steps: - measuring the voltage (a(t)) at a multitude of points in time t k, k =1,..., N, - measuring (S10') the current strength (b(t)) at the same points in time t k, - for each of the points in time t k, forming a 2-tuple from two values of the voltage measured at different points in time for specifying a respective complex number a [k], - for each of the points in time t k, forming (S12') a 2-tuple from two values of the current strength measured at different points in time for specifying a respective complex number b [k], wherein, for each 2-tuple for a point in time t k, in each case a measurement at the point in time t k is used, and a measurement at the point in time t k' is used which is spaced apart temporally from the point in time t k by 1 4 ± n, n ∈ Z period durations T of the sine curve, wherein the first entry of the 2-tuple is interpreted as the real part and the second entry of the 2-tuple as the imaginary part, characterised by - forming the ratio c _ k = a _ k b _ k for all k, - calculating, directly from all ratios c [k], an average c depicted as a 2-tuple according to a mathematical calculation specification, - calculating the phase as an argument of the average (φ = arg ( c)) and calculating the absolute value of the average | c |, wherein the first entry of the 2-tuple is interpreted as the real part and the second entry of the 2-tuple as the imaginary part, and - using the phase (φ) and the absolute value of the average | c | as an assertion for the relationship between voltage and current strength. - measuring the voltage (a(t)) at a multitude of points in time t k, k =1,..., N, - measuring (S10') the current strength (b(t)) at the same points in time t k, - for each of the points in time t k, forming a 2-tuple from two values of the voltage measured at different points in time for specifying a respective complex number a [k], - for each of the points in time t k, forming (S12') a 2-tuple from two values of the current strength measured at different points in time for specifying a respective complex number b [k], wherein, for each 2-tuple for a point in time t k, in each case a measurement at the point in time t k is used, and a measurement at the point in time t k' is used which is spaced apart temporally from the point in time t k by 1 4 ± n, n ∈ Z period durations T of the sine curve, wherein the first entry of the 2-tuple is interpreted as the real part and the second entry of the 2-tuple as the imaginary part, characterised by - forming the ratio c _ k = a _ k b _ k for all k, - calculating, directly from all ratios c [k], an average c depicted as a 2-tuple according to a mathematical calculation specification, - calculating the phase as an argument of the average (φ = arg ( c)) and calculating the absolute value of the average | c |, wherein the first entry of the 2-tuple is interpreted as the real part and the second entry of the 2-tuple as the imaginary part, and - using the phase (φ) and the absolute value of the average | c | as an assertion for the relationship between voltage and current strength.