An unsuccessful conversion from metallic return of pole 1 to ground return in debugging phase after reform of Tianshengqiao-Guangzhou HVDC control and protection system is discussed.The waveform record and the line current at the fault moment are analyzed

and the metallic return switch（MRS）is tested by considering the protection configuration and principle of MRS.The result indicates that insulation defect of the arrester on the resonance circuit of MRS causes protective action of MRS in test.Correspondingly

preventive test for insulation of conversion bus equipment is suggested during annual outage for maintenance.