| 摘要 |
In a protective system comprising a core-balance transformer 3 whose secondary winding 11 is connected to a tripping device 12 and a test circuit 10 for withdrawing a test current from the mains by way of a momentary-contact control switch 9, protection against saturation of the transformer 3 by direct current components is provided by an alternating current source 1, having a frequency which differs from the mains frequency, for producing alternating current premagnetisation of the transformer 3 by means of a winding 2, and a monitoring circuit 4-7 responsive only to the premagnetisation current frequency to control a semiconductor element 8 connected in parallel with the momentary-contact control switch 9. The monitoring circuit is responsive to the voltage across winding 2 and comprises a filter, rectifier and differential amplifier. <IMAGE> |
