Abstract In order to analysis and research the characteristics of series fault arc and detect the fault arc occurring in power line in a fast way as well as cut off the fault line immediately, the author puts forward a diagnostic approach which takes use of cycle analysis to detect the fault arcs in a high-speed. Depending on the random characteristics of current cycle when series fault arc appears in electrical equipment or in the power circuit, the faulting arcs can be estimated by calculating the difference value between adjacent current cycles and picking up the distortion signal. A large number of experiments under different load conditions demonstrate that method have good diagnostic performance, in addition, it is simple and easy to implement. In short, it is a fast and effective method for diagnosis of series arc.