There were twice lightning trip-out breakdowns occurred on 220 kV Yuetong line in Yingtan Electric Power Company in Jiangxi although the metal oxide surge arrester（MOA）is mounted on the fault phase. In order to verify the protection effects of the line MOA and to avoid similar incidents

the investigations are made which include the 50% discharge voltage test under lightning impulse and the power frequency withstand voltage test under wet condition are carried out to the series gap of the MOA. The co-ordination margin between the different gap clearances and the line MOA is obtained. Based on the above research

the gap distance of（840±10） mm for220 kV line MOA is recommended

which can satisfy the requirements both reliability of protection and safety of operation.