| 摘要 |
A security installation has four, for example, normally-closed sensor switches which open in the presence of an intruder. Each switch is in series with a respective resistor and these series combinations are connected in parallel across a pair of bus lines which connect to a central monitoring point. There, normally open test switches are connected across the bus lines in series with respective resistors which have values corresponding to the resistors of the sensor switches. Constant current is applied to the bus lines, and a voltage monitor senses the rise in voltage when one of the sensors opens, and this initiates a scanning unit which closes the test switches in sequence. When the test switch corresponding to the opened sensor is closed, the line voltage reverts to the predetermined value, and the closed test switch identifies the operated sensor.
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