Detection and Alarm Systems Flashcards

Under NFPA 72, which wiring class allows a single open or ground fault to render an entire circuit inoperable?

Answer: Class B

Class B circuits are not supervised for opens or shorts at every point, so a single fault can disable the entire circuit.

A projected beam smoke detector measures which property to detect smoke?

Answer: Reduction in infrared light intensity across a path

Projected beam detectors measure attenuation of a light beam transmitted across a protected area; smoke reduces the received signal.

NFPA 72 defines the Temporal-3 evacuation signal as:

Answer: Three short pulses, pause, repeated

Temporal Pattern 3 is 0.5-s ON, 0.5-s OFF, 0.5-s ON, 0.5-s OFF, 0.5-s ON, then 1.5-s OFF, repeated continuously.

Which initiating device would be MOST appropriate to detect a slow smoldering fire in a large warehouse?

Answer: Air-sampling (aspirating) smoke detector

Air-sampling detectors actively draw air through a pipe network and can detect very low smoke concentrations produced by early-stage smoldering fires.

What is the purpose of a duct smoke detector installed in an HVAC return-air duct?

Answer: To prevent smoke from being recirculated through the HVAC system

Duct detectors shut down the air-handling unit and close dampers to prevent smoke from spreading through the duct system to other areas.

According to NFPA 72, a supervisory signal from a sprinkler control valve indicates:

Answer: An abnormal condition requiring attention but not immediate evacuation

Supervisory signals indicate off-normal conditions (such as a closed valve) that need correction but do not by themselves require evacuation.

A flame detector that responds to radiant energy in the 4.3-micron CO₂ absorption band is classified as:

Answer: Infrared (IR) detector

The 4.3-micron band is a characteristic infrared emission of hot CO₂ produced in hydrocarbon flames, making this an IR flame detector.