Until now static switching signals have been used for triggering a switching device. This has the disadvantage that such static switching signals controlled by an exciter device can also occur in the event of a failure of the exciter device and then may effect faulty control of the switching device....http://www.google.de/patents/US5440539?utm_source=gb-gplus-sharePatent US5440539 - Method of controlling an electrical switching device in response to a signal configuration of a switching signal
