The objective of the present paper is to examine the behavior of a coal-fired supersonic diagonal channel which suffers from faults of external control circuits under the condition of electrical nonuniformities caused by coal slag polarization. The channel designed for a pilot-plant is controlled with external circuits, such as master/slave current controller and consolidation circuits, and is connected to the A.C. grid through double-bridge line-commutated inverters. The magneto hydrodynamic (MHD) channel and external circuits are simultaneously analyzed with a two-dimensional equivalent circuit method and time-dependent one-dimensional gasdynamic equations. The following results are obtained: (1) the short fault of switching device of both current control and consolidation circuits has only a small influence on performance, and no countermeasure is required; (2) when a switching device of the master/slave current consolidation circuit is assumed not to fire, the loading current leads to zero in about 20 ms, with the flow held of the MHD channel being largely influenced, and the designed supersonic flow is decelerated to subsonic how through the shock wave; (3) it is shown that, if the faulted switching device can be quickly shorted, the whole flow field recovers to the normal condition. Therefore, some fast protection systems must be installed.