| 摘要 |
A safety protection circuit for controlling the operation of electrically-powered tools including a switching device. In one application, the power tool is a drill which is utilized in drilling a hole through electrically non-conductive material such as a concrete floor and an electrically conductive material such as a grounded metallic conduit having electrical cables therein disposed directly below the floor. The switching device automatically discontinues the drilling operation when the drill bit contacts the metallic conduit after passing through the concrete floor thereby avoiding any damage to the cables within the metallic conduit or causing a safety hazard. Subsequently, the drill can be reactivated to penetrate the metallic conduit. The switching device will again automatically discontinue the operation of the drill after breaking contact with the metallic conduit.
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