Non-conduction of bridge arms is the most common fault in a cycloconverter. Firing pulse failures and commutation failures can both cause such faults. The paper analyses the neutral point grounding mode of cycloconverter in FFTS and the voltages and currents of cycloconverter following a non-conduction fault in inverter mode. Compared with the same fault when converter works in rectifier mode， the consequence is more serious. Simulation results indicate the correctness of the above analysis. Finally， some suggestions for fault identification are proposed.