A surge arrester paralleled to the MRTB(Metallic Return Transfer Breaker) was damaged during the commutation from the ground return mode to the metal return mode in the Rated Load Test of Gezhouba to Nanqiao HVDC Transmission Project in 1998.By calculation using EMTP and test
it is concluded that the main reason of the damage of the surge arresters is that the surge arrester is not sealed tightly
and moisture penetrats into it. All the 16 new surge arresters are suggested to be installed. It is also suggested that the surge arresters and MRTB shall be tested preventatively once a year.