ABSTRACTThe operating conditions of metal-oxide arresters under PT ferro-resonance was investigated with EMTP in this paper. A simplified method was proposed to estimate the energy absorbed by metal-oxide arreater during ferroresonant overvoltages. Existence regions of the typical ferroresonance were derived from an ungrounded overhead system. A few types of metal-oxide arresters were introduced into the existence regions. Both the effect of metal-oxide arresters on the ferroresonant overvoltages and the arreater energy absorption were examined to discuss the actual behaviour of metal-oxide arresters operating under PT ferroresonance.