| 摘要 |
The protective circuit has a summation current transformer whose secondary winding has an auxiliary AC voltage applied thereto and is connected in series with a release, such as the operating coil of a circuit breaker. The auxiliary AC voltage has the same frequency as a main's phase and is generated by a voltage source having a low internal resistance. The secondary winding of the transformer is dimensioned so that its impedance, with the transformer core magnetically unsaturated, is at least of the same order as the impedance of the release, and the internal resistance of the auxiliary voltage source, with the transformer core magnetically saturated, is low relative to these impedances. The auxiliary voltage source may be part of a voltage divider connected between one phase and either a neutral conductor or another main, and the voltage divider may comprise a condenser and a resistance. The release may be designed as a relay which is pre-polarized by an electromagnet excited by rectification from the same phase as the auxiliary AC voltage. Various additional circuit modifications may be made.
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