| 摘要 |
<p>A switching circuit for use with an electrical appliance, and comprising first and second normally open switch terminals and a high sensitivity impedance measuring circuit coupled to the first and second switch terminals for measuring an impedance therebetween, and producing at an output thereof a switching signal if the impendance is lower than a predetermined threshold of 500 MOMEGA. The appliance is coupled to the output of the impedance measuring circuit so as to be responsive to the switching signal. The circuit finds particular application for controlling and protecting electrical appliances operating from an a.c. electrical means supply, in which case one of the switch terminals is connected to a virtual ground connection which is electrically floating with respect to a ground feeder of an electrical mains supply. In the event of electrical contact between the virtual ground connection and the other switch contact, an apparent ground fault is produced, which gives raise to a small current which is detected by the impendance measuring circuit so as to produce a switching signal which may then be used to operate a suitable relay for controlling the appliance.</p> |
