FST Smoke Detection 4 — Questions and Answers
Question 1: When is a smoke detector NOT required in an elevator machine room under NFPA 72?
- When the building has a sprinkler system
- When the machine room has its own HVAC unit
- When the elevator does not serve floors above the fourth floor
- When the elevator machine room is in a fully sprinklered building and sprinkler activation initiates recall (Correct answer)
Correct answer: When the elevator machine room is in a fully sprinklered building and sprinkler activation initiates recall
NFPA 72 and ASME A17.1 allow sprinkler activation alone to initiate elevator recall when the machine room is fully sprinklered, eliminating the smoke detector requirement for that function.
Question 2: What does an obscuration level of 4% per foot indicate about a photoelectric smoke detector's listed sensitivity range?
- The detector alarms at very low smoke density — extremely sensitive
- The detector alarms at moderate smoke density within the typical listed range (Correct answer)
- The detector is at the desensitized end of its listed range
- The detector has failed and must be replaced
Correct answer: The detector alarms at moderate smoke density within the typical listed range
NFPA 72 specifies that smoke detectors must be listed within a sensitivity range of 0.5% to 4% per foot obscuration, so 4% per foot is at the upper (least sensitive) end but still within the listed range.
Question 3: Which spacing reduction factor applies to smoke detectors installed in a beam pocket ceiling where beams project more than 18 inches below the ceiling?
- No reduction — standard spacing applies
- Treat each beam pocket as a separate ceiling compartment and space detectors accordingly (Correct answer)
- Reduce spacing by 50%
- Use only heat detectors in beamed ceilings
Correct answer: Treat each beam pocket as a separate ceiling compartment and space detectors accordingly
When beams project more than 18 inches below the ceiling, NFPA 72 requires treating each beam pocket as a separate ceiling compartment for detector spacing purposes.
Question 4: What action should a fire safety technician take when a smoke detector consistently activates at low sensitivity but passes sensitivity testing?
- Replace the detector immediately
- Increase the alarm threshold in the panel programming
- Inspect and clean the detector, then retest (Correct answer)
- Move the detector to a new location
Correct answer: Inspect and clean the detector, then retest
Consistent nuisance alarms at low conditions often indicate contamination; the correct first step is inspection and cleaning per manufacturer instructions before further action.
Question 5: Which installation location is PROHIBITED for a standard spot-type smoke detector under NFPA 72?
- In a walk-in closet
- Within 4 inches of a wall-ceiling junction
- In a bathroom with a shower (Correct answer)
- Adjacent to a doorway
Correct answer: In a bathroom with a shower
Bathrooms with showers are high-humidity environments where steam can cause false alarms; NFPA 72 does not require and advises against standard smoke detectors in these locations.
Question 6: According to NFPA 72, what is the minimum number of smoke detectors required per sleeping room in a hotel or dormitory?
- One per room (Correct answer)
- Two per room
- One per 900 square feet of room area
- None if corridor detectors are present
Correct answer: One per room
NFPA 72 requires at least one smoke detector in each sleeping room of hotels and dormitories to provide occupant notification at the point of occupancy.
Question 7: What is the MAIN advantage of a dual-wavelength photoelectric smoke detector over a single-wavelength model?
- Lower cost per unit
- Ability to discriminate between smoke and steam/dust to reduce false alarms (Correct answer)
- Longer battery life
- Higher sensitivity to smoldering fires only
Correct answer: Ability to discriminate between smoke and steam/dust to reduce false alarms
Dual-wavelength detectors use two light sources at different wavelengths to distinguish actual smoke particles from non-fire aerosols like steam and dust, reducing nuisance alarms.
FST Smoke Detection 4
When is a smoke detector NOT required in an elevator machine room under NFPA 72?