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How to Test an Alarm Circuit That Is Normally Closed

Nathan Fisher

Most alarm systems work on the principle of a normally closed circuit. Technicians install two contact points or sensors on an entry point, such as a door or window. When the door or window is closed, the sensors become aligned or "closed" to complete the circuit. If opening the door or window causes the sensors to separate, the broken circuit activates the alarm. You can test an alarm circuit that is normally closed by performing a simple continuity test.

Normally closed alarm circuits activate upon opening a door or window.
  1. Close all doors and windows where sensors have been installed.

  2. Perform a system check at the alarm's control panel, according to the manufacturer's instructions. This usually involves entering a test code into the control panel. Depending on the type of system, the panel may indicate which sensors are malfunctioning. If the alarm system does not identify a faulty sensor or confirm that the circuits are working correctly, check each sensor individually.

  3. Unplug the alarm from the power source at the control panel. If the unit is hard-wired, turn off the breaker that controls the alarm at the main service panel. Remove the batteries from the systems backup if the alarm has one.

  4. Disconnect the wires from the contact terminals on all the sensors, using a screwdriver.

  5. Set the multimeter to the continuity function. On some meters the continuity setting reads "1X."

  6. Begin at the alarmed entry point closest to the alarm's control panel. Touch one of the probes of the multimeter to the terminal on one of the sensors. Touch the second probe to the terminal on the opposing sensor.

  7. Verify that the needle on the meter moves from the far left to the far right on the meter's scale, indicating the circuit between the sensors is operating properly. If the meter's needle does not move, the sensor is faulty. Repeat the test on the remaining pairs of sensors at all alarmed entry points.

  8. Replace any sensors that do not pass the continuity test; sensors attach to the door or window frame with mounting screws.

  9. Reattach the sensor wires with the screwdriver. Turn on the power to the system and perform another system check at the control panel.