A method of detecting electric variables of a three-phase AC network having a first, a second and a third phase (L1, L2, L3), including the steps: - measuring a respective voltage value (u 1, u 2, u 3) of the first, second and third phases (L1, L2, L3) in relation to a neutral conductor at a first moment in time (t 1), - transforming the three voltage values u 1, u 2, u 3 of the first moment in time (t 1) into polar co-ordinates with a voltage amplitude (U N) and a phase angle φ N so that transformation of the voltage values u 1, u 2, u 3 measured at the first moment in time in polar co-ordinates U N, φ N into a complex-value variable is effected in accordance with the equation: u → = u 1 + u 2 exp j 2 3 π + u 3 exp j 4 3 π so that an amount and a phase angle correspondingly occur in that case, - repeating measurement and transformation for at least one further moment in time, and - determining the currently prevailing frequency and the filtered voltage amplitude from the voltage values transformed into polar co-ordinates U N, φ N wherein the currently frequency (ω A) and the filtered voltage amplitude are output, wherein - in a digital filter the voltage amplitude (Un) is filtered and the filtered voltage is output as an estimate for the effective value U of the AC voltages, wherein - the digital filter comprises a holding member, a amplification factor and a first and a second summing point, and - at the first summing point, a difference is formed from the voltage amplitude (Un) and the filtered voltage given by the holding element, and the difference is multiplied by the amplification factor, and at the second summing point, the difference is added to the filtered voltage given by the holding element to form the filtered voltage to be output. - measuring a respective voltage value (u 1, u 2, u 3) of the first, second and third phases (L1, L2, L3) in relation to a neutral conductor at a first moment in time (t 1), - transforming the three voltage values u 1, u 2, u 3 of the first moment in time (t 1) into polar co-ordinates with a voltage amplitude (U N) and a phase angle φ N so that transformation of the voltage values u 1, u 2, u 3 measured at the first moment in time in polar co-ordinates U N, φ N into a complex-value variable is effected in accordance with the equation: u → = u 1 + u 2 exp j 2 3 π + u 3 exp j 4 3 π so that an amount and a phase angle correspondingly occur in that case, - repeating measurement and transformation for at least one further moment in time, and - determining the currently prevailing frequency and the filtered voltage amplitude from the voltage values transformed into polar co-ordinates U N, φ N wherein the currently frequency (ω A) and the filtered voltage amplitude are output, wherein - in a digital filter the voltage amplitude (Un) is filtered and the filtered voltage is output as an estimate for the effective value U of the AC voltages, wherein - the digital filter comprises a holding member, a amplification factor and a first and a second summing point, and - the digital filter comprises a holding member, a amplification factor and a first and a second summing point, and - at the first summing point, a difference is formed from the voltage amplitude (Un) and the filtered voltage given by the holding element, and the difference is multiplied by the amplification factor, and at the second summing point, the difference is added to the filtered voltage given by the holding element to form the filtered voltage to be output.