| 摘要 |
The circuit breaker includes disconnectable contacts provided in an a.c. power line. A current sensor (200) is provided for respective phases of a.c. power lines to detect the level of current. A phase selecting circuit (400) selects one largest output value amongst secondary output signals corresponding to the value of the current in each phase from the current sensor. This is done using a predetermined operation characteristic corresponding to a selected threshold value. A mean or effective value of the secondary output signals of the output of the phase selecting circuit is obtained from a signal converter (90) whose output is processed by a microcomputer to issue a releasing signal.
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