Disclosed in the present invention are a small-current grounding fault line selection system and method having a backup function; when a main protection malfunctions and circuit breakers of non-fault lines trip, a backup protection system calculates the integrals of absolute values of changes in failure quantity before and after the tripping of the circuit breakers of lines; and a backup protection tripping line is determined in accordance with the characteristics of the integrals, achieving rapid response of backup protection. In the present invention, the backup protection can trip a fault line quickly, improving the reliability of protection.
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