S95 Initiating Devices 2 — Questions and Answers
Question 1: Which type of smoke detector uses a small amount of radioactive material to ionize air and detect combustion particles?
- Photoelectric detector
- Ionization detector (Correct answer)
- Projected beam detector
- Aspirating smoke detector
Correct answer: Ionization detector
Ionization detectors use a small radioactive source (typically Americium-241) to ionize air between two charged plates; combustion particles disrupt the ion current and trigger an alarm.
Question 2: A photoelectric smoke detector is MOST effective at detecting which type of fire?
- Fast-flaming fires with little smoke
- Slow smoldering fires that produce large smoke particles (Correct answer)
- Fires producing invisible combustion gases
- Fires producing extreme radiant heat
Correct answer: Slow smoldering fires that produce large smoke particles
Photoelectric detectors use a light-scattering principle that responds most effectively to large smoke particles generated by slow, smoldering fires.
Question 3: What is the maximum spacing between smoke detectors on a smooth, flat ceiling as specified by NFPA 72?
- 20 feet
- 30 feet
- 50 feet (Correct answer)
- 70 feet
Correct answer: 50 feet
NFPA 72 establishes a default maximum spacing of 30 feet (900 sq ft area) between smoke detectors on smooth, flat ceilings, but allows up to 50 feet in some listing conditions.
Question 4: A projected beam smoke detector consists of which two primary components?
- Transmitter and receiver (Correct answer)
- Sensor and relay module
- Ionization chamber and sampling tube
- Aspirator and filter assembly
Correct answer: Transmitter and receiver
A projected beam detector uses a transmitter that emits an infrared or laser beam and a receiver that measures beam obscuration caused by smoke.
Question 5: Where are duct smoke detectors required to be installed according to NFPA 72 and NFPA 90A?
- Only at return air registers
- In HVAC ducts to detect smoke and shut down air handling units (Correct answer)
- Adjacent to sprinkler heads in mechanical rooms
- At every branch duct above 12 inches in diameter
Correct answer: In HVAC ducts to detect smoke and shut down air handling units
Duct smoke detectors are installed in HVAC supply and return air ducts primarily to detect smoke and initiate shutdown of the air handling unit to prevent smoke circulation.
Question 6: Which condition would cause a spot-type smoke detector to generate a nuisance alarm?
- High ambient temperature above 100°F
- Steam or high humidity near the detector (Correct answer)
- Low ambient light levels in the room
- Reduced building occupancy at night
Correct answer: Steam or high humidity near the detector
Steam and high humidity can cause water vapor condensation in the sensing chamber, mimicking smoke particles and triggering a false (nuisance) alarm.
Question 7: An aspirating smoke detection (ASD) system draws air samples through a pipe network to a central detection unit. What is a primary advantage of this design?
- Lower installation cost than spot detectors
- Ability to detect extremely low concentrations of smoke at very early stages (Correct answer)
- Simpler maintenance with fewer parts
- Higher alarm threshold to reduce nuisance alarms
Correct answer: Ability to detect extremely low concentrations of smoke at very early stages
ASD systems actively sample air and can detect very low concentrations of smoke particles, providing early warning long before a conventional spot detector would respond.
S95 Initiating Devices 2
Which type of smoke detector uses a small amount of radioactive material to ionize air and detect combustion particles?