AbstractThe frequency distribution of the crest values of overvoltage waves in low voltage AC power circuits can be approximated by an exponential function. The flashover probability P of a clearance with an impulse withstand voltage Ǔst in an electrical equipment is given by P= exp (‐ α Ǔst). The annual number of flashovers of a single clearance in an unprotected house wiring system supplied by an unprotected overhead network depends upon the impulse withstand voltage, the latitude of the network, the annual number of thunderstorm days, the effective length and the height above ground of the overhead line. However, this does not mean, that every flashover triggers a dangerous power frequency follow current.