Abstract: Apparatus for determining the root-mean-square value of a pulsatile signal generated from a power line signal. Samples of the pulsatile signal are produced at predefined substantially equal sampling intervals beginning at substantially the onset of a pulse in said pulsatile signal and continuing thereafter for substantially the entire duration of a half period of the power line signal to generate corresponding sampled values. The sampled values are squared and accumulated, then divided by the total number of samples taken and accumulated during said half period to yield an intermediate value for which the square root is determined, resulting in the root-mean-square value of the pulsatile signal.