Abstract: Ferroresonance overvoltage is one of the most important factors to impact electric equipment inner insulation. In system with non-effectively earthed neutral, the equipment damage caused by ferroresonance overvoltage possesses great proportion. This paper derives the criterion of power frequency ferroresonance in system with non-effectively earthed neutral in detailed, in order to conveniently judge whether there is power frequency ferroresonance when system transferring working mode. This criterion derives the relation between equivalent circuit resistance and critical resistance using analytical method and can be used for determining the zero sequence resistance value in cascade with PT secondary side for eliminating ferroresonance. The method is verified by an example under two conditions. Finally, the effect of neutral connecting arc suppression coil on ferroresonance is analyzed.