| 摘要 |
witch 1 is located at an electric fence gate and has probes 2,3 which operate in a known manner to maintain circuit continuity. When the gate is opened a bolt (not shown) connected to probes 2,3 internally contacts bar 4 and when the gate is shut the probes are retracted against springs (not shown), so in both cases electrical continuity is maintained. The invention includes bolt 6 which when installed connects bar 4 to ground plate 8, and when removed disconnects bar 4 from ground plate 8. This facility allows for selective alarm actuation dependent on whether or not bolt 6 is installed.
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