PURPOSE:To enable the judgement whether or no there is an error connection and specification of an error connection circuit by making a judgment of the error connection by instituting a comparison of difference between the sum of vector of each current introduced from a plurality of current transformers and the sum of vector obtained by inversing a polarity of one of optional current among each current. CONSTITUTION:If there is no error connection in a system starionary time, inflow current and outflow current are detected as an equal value, so that while the sum of vector becomes zero by the first means, the sum of vector reaches a specific value by the second means obtained by inversing a polarity of one of optional current. While, in case there is an error connection, inflow current and outflow current are not detected as an equal value, so that the sum of vector does not become zero by the first means. When a polarity of an applicable current having an error connection is inversed, the sum of vector becomes almost zero by the second means, and on the contrary, the sum of vector by the first means is larger than the sum of vector by the second means to clarify the error connection.
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