FST Smoke Detection 3 — Questions and Answers
Question 1: In a duct smoke detector installation, where should the sampling tube inlet be positioned relative to airflow?
- Downstream of filters and coils, upstream of branch takeoffs (Correct answer)
- Upstream of filters, downstream of the fan
- Directly at the fan discharge
- At the return air grille in the occupied space
Correct answer: Downstream of filters and coils, upstream of branch takeoffs
Sampling tubes should be positioned downstream of filters and coils but upstream of branch takeoffs to sample representative air without filter interference.
Question 2: Which NFPA 72 category requires smoke detectors to be tested using 'canned smoke' or a listed aerosol during acceptance testing?
- Functional test only
- Sensitivity test
- Operational test using a listed test method (Correct answer)
- Inspection only — no functional test needed
Correct answer: Operational test using a listed test method
NFPA 72 requires an operational test using a listed smoke entry test method (such as canned smoke) during acceptance to verify detector response.
Question 3: What is the PRIMARY purpose of a smoke detector's 'alarm verification' feature?
- To increase detector sensitivity during nighttime hours
- To introduce a time delay before transmitting an alarm to reduce false alarms (Correct answer)
- To automatically silence a nuisance alarm after 60 seconds
- To notify the fire department before the local panel activates
Correct answer: To introduce a time delay before transmitting an alarm to reduce false alarms
Alarm verification introduces a programmed delay and re-sampling period before an alarm is transmitted, reducing false alarms from transient conditions.
Question 4: According to NFPA 72, how often must the sensitivity of system smoke detectors be tested after the first required test?
- Every 6 months
- Annually
- Every 2 years or per manufacturer recommendation (Correct answer)
- Every 5 years
Correct answer: Every 2 years or per manufacturer recommendation
NFPA 72 requires sensitivity testing within one year of installation and then at intervals not exceeding two years or per manufacturer recommendations.
Question 5: A smoke detector installed on a pitched ceiling with a slope greater than 1 foot in 8 feet should be placed how far from the peak?
- Within 36 inches of the peak (Correct answer)
- At least 36 inches from the peak
- At least 5 feet from the peak
- At the midpoint of the slope
Correct answer: Within 36 inches of the peak
NFPA 72 requires that on sloped ceilings steeper than 1:8, detectors be located within 36 inches of the peak measured horizontally, where hot gases accumulate.
Question 6: Which type of smoke detector would BEST minimize false alarms in a newly constructed building before finishing work is complete?
- Standard ionization detector
- Standard photoelectric detector
- Construction-mode detector with reduced sensitivity (Correct answer)
- Duct smoke detector
Correct answer: Construction-mode detector with reduced sensitivity
Construction-mode detectors or detectors with adjustable sensitivity can be set to a reduced threshold to prevent false alarms from dust and debris during construction.
Question 7: What is the listed maximum air velocity within a duct for which a duct smoke detector is typically rated to operate reliably?
- 500 FPM
- 1,000 FPM
- 4,000 FPM (Correct answer)
- 8,000 FPM
Correct answer: 4,000 FPM
Most listed duct smoke detectors are rated for duct air velocities up to 4,000 feet per minute (FPM), though manufacturers may list specific ranges.
FST Smoke Detection 3
In a duct smoke detector installation, where should the sampling tube inlet be positioned relative to airflow?