Abstract: When the ring network appears in the substation AC system, the spontaneous combustion of the AC feeder wire and the burning of the series and parallel relay nodes may occur, which threatens the safe operation of the substation. This paper develops the ring network detection device, using power frequency intermittent input small current signal as the input signal, using CBB capacitor as load, silicon controlled cut capacitor load time interval can get the characteristic current, ring network loop handheld detection device will appear characteristic current waveform, through whether the characteristic current to locate the ac ring network. Through the resistance test of the substation AC system, the three-phase, single-phase and branch measurement of the station transformer line, and finally positioned to the ring network branch and ring network, so as to verify the correctness of the ring network detection of the device.