Zero-sequence voltage will keep increasing when potential transformer ferroresonance occurs in an isolated neutral point system. Similar phenomenon happens during a single-phase-to-earth fault. An traditional line selection device starts when zero-sequence voltage and current exceed setting values， which is not effective for identification of ferroresonance and is apt to cause mis-operations. In the study， characteristics of fundamental frequency ferroresonance is emphatically analyzed and three-phase voltages and zero-sequence voltage in case of both ferroresonance and single-phase-to-earth fault are compared in detail. On this basis， ferroresonance identification technique is proposed based on comprehensive comparison of zero-sequence voltage and three-phase voltage. Verification by field data reveals that the technique can effectively increase the reliability of line selection devices and therefore satisfy the requirements for practical implementation.