When a single-phase grounding fault occurs in a small current grounding system, the difficulty of line selection is increased due to the small fault information. The value of the capacitive current passing through the fault point generally does not exceed 20 ~ 30A. Especially in the resonant grounding system, because the arc suppression coil compensates the capacitive current at the fault point, the current change of the fault line is even less than that of the non fault line. The zero sequence current is converted to the secondary side through the current transformer, and the fault signal is very small compared with the load current.

        The small current grounding line selection device mainly judges whether the grounding fault occurs by setting the fixed value of zero sequence voltage. At present, the manufacturers of small current grounding line selection device will set the zero sequence voltage starting amplitude threshold of their own equipment to 30V, but it is difficult to meet the operation requirements in the actual situation. Because in normal operation, the actual zero sequence voltage may not reach 30V or 20V. In extreme cases, the zero sequence voltage may be 15V, but the fault also exists. In this case, the small current grounding line selection device cannot determine the fault starting device.

        However, there is a small current grounding line selection device of industrial control computer with adaptive function of operating conditions. The device can judge according to the changes of industrial control in field operation, such as the position changes of section switch and interconnection switch of ring network structure, so as to ensure the correct action of line selection device. In short, after the normal operation on site for a period of time and all amplitudes are stable, the small current grounding line selection device of industrial control computer will be debugged automatically according to the actual situation of on-site operation. For example, when a fault occurs, the zero sequence voltage rises to 25V, but the setting value of conventional small current grounding line selection is 30V, so the fault cannot be determined, The low current grounding line selection device of the industrial computer will adjust itself and set the fixed value to 24V. Once the fixed value is exceeded, the equipment will judge the fault and start the device.