PURPOSE:To keep fault currents at the time of an incomplete short circuit within the interrupting characteristics of reverse power interrupting characteristics, and to protect a system reliably by obtaining the deformed S-shaped reverse power interrupting characteristics. CONSTITUTION:A spot network relay 10a is composed of an input converter 11, an A/D converter 12, a data memory 13, a constant memory 13a, a computing element 14 and an output section 15. When the region is the one below a fixed current or the one above the fixed current is decided from the data of voltage and current stored in the data memory 13 by the computing element 14. The sum of products of the same instantaneous voltage and currents stored in the data memory 13 is taken and compared with a constant set in the constant memory 13a when the region is the one below the fixed current, and the sum of products of the instantaneous value of currents stored in the data memory 13 and the instantaneous value of voltage sampled before one instantaneous value is taken and compared with the constant set in the constant memory 13a when the region has fixed currents or more, thus acquiring deformed S- shaped reverse power interrupting characteristics.
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