The technology of automatic line selection for small current grounding is a difficult problem. Its main difficulties are as follows: the fault signal of single-phase grounding fault is small, not as large as that of other fault types such as two-phase short circuit and two-phase grounding short circuit; The type of single-phase grounding fault is complex and uncertain, including full grounding fault, intermittent arc grounding, grounding through unstable resistance, grounding through branches (high resistance grounding), etc; The neutral grounding mode is uncertain, including ungrounded and grounded via arc suppression coil. The main reasons why the principle of line selection device is not ideal are as follows:

Although there are as many as ten or more line selection methods, each method only uses the characteristics of one aspect of the fault. When the fault characteristics required by this method are not obvious, this method will make wrong judgment.

For example, the fifth harmonic line selection method takes advantage of the fifth harmonic characteristics of the zero-sequence current, but in the case of high resistance grounding, the fifth harmonic component is not obvious, and then the fifth harmonic line selection method fails. In the past, the small current grounding line selection device only uses one method, so the accuracy is low. There are also some devices that use a variety of line selection methods, but only a simple superposition can not solve the actual problem.