How is a pneumatic-type continuous-loop fire detection system checked for operation?

How is a pneumatic-type continuous-loop fire detection system checked for operation?

• A. Close the test switch and observe a cockpit indicator.
• B. Apply high voltage to the loop to trigger the alarm.
• C. When the test switch is closed, low-voltage AC flows through the tube that encloses the sensitive element. This current heats the element enough for it to release sufficient gas to activate the alarm switch.
• D. Physically bend the loop to test mechanical resistance.

Answer: (C)

Pneumatic continuous-loop fire detectors rely on a small heater inside the sensing loop. When the test is performed, a low-voltage AC current is allowed to flow through the loop, heating the sensitive element. The heat causes a gas inside the sensing capsule to be released or expands a diaphragm enough to activate the alarm switch, which produces the cockpit indication. This method directly tests that the loop can heat, release the gas, and drive the alarm circuit, confirming proper operation of the sensing element, the sealing/gas system, and the alarm output. Rationale for why the other ideas aren’t appropriate: simply closing a test switch to observe an indicator doesn’t ensure the heater and gas-reaction mechanism are functioning. Applying high voltage isn’t how this system is tested and could risk damage. Physically bending the loop tests mechanical integrity only, not the electrical/thermo-gas activation needed to trigger the alarm.

Pneumatic continuous-loop fire detectors rely on a small heater inside the sensing loop. When the test is performed, a low-voltage AC current is allowed to flow through the loop, heating the sensitive element. The heat causes a gas inside the sensing capsule to be released or expands a diaphragm enough to activate the alarm switch, which produces the cockpit indication. This method directly tests that the loop can heat, release the gas, and drive the alarm circuit, confirming proper operation of the sensing element, the sealing/gas system, and the alarm output.

Rationale for why the other ideas aren’t appropriate: simply closing a test switch to observe an indicator doesn’t ensure the heater and gas-reaction mechanism are functioning. Applying high voltage isn’t how this system is tested and could risk damage. Physically bending the loop tests mechanical integrity only, not the electrical/thermo-gas activation needed to trigger the alarm.