C7 Security System Design & Integration 3 — Questions and Answers
Question 1: A passive infrared (PIR) motion detector triggers false alarms repeatedly near a south-facing window. What is the most likely cause?
- Insufficient battery voltage
- Thermal gradients from sunlight heating the floor (Correct answer)
- Improper tamper switch installation
- Faulty end-of-line resistor value
Correct answer: Thermal gradients from sunlight heating the floor
PIR detectors sense infrared energy changes; sunlight warming surfaces near the detection zone creates thermal gradients that mimic body heat and cause false alarms.
Question 2: Which topology is recommended for a wired access control system serving 50 doors to minimize single-point-of-failure risk?
- Daisy-chain bus
- Star topology from a central controller
- Distributed controller architecture with peer-to-peer mesh (Correct answer)
- Single home-run loop
Correct answer: Distributed controller architecture with peer-to-peer mesh
A distributed controller architecture with peer-to-peer redundancy ensures that failure of one controller does not take down the entire access control system.
Question 3: What is the maximum allowable resistance for a supervised alarm loop using 1,000 Ω end-of-line resistors per typical burglar alarm panel specifications?
- 50 Ω
- 100 Ω
- 250 Ω (Correct answer)
- 500 Ω
Correct answer: 250 Ω
Most burglar alarm panels accept up to 250 Ω of loop resistance (wire resistance) when using 1,000 Ω EOL resistors before the panel flags a trouble condition.
Question 4: In IP video surveillance design, what does the term 'bandwidth aggregation' refer to?
- Combining multiple camera streams into a single high-speed fiber link
- The total network bandwidth consumed by all cameras simultaneously at full resolution (Correct answer)
- Compressing video streams using H.265 codec
- Using PoE switches to power multiple cameras from one source
Correct answer: The total network bandwidth consumed by all cameras simultaneously at full resolution
Bandwidth aggregation is the sum of all camera streams transmitting simultaneously, which determines the network infrastructure capacity needed.
Question 5: Which grounding method is required for a metal enclosure housing a low-voltage security control panel per NEC Article 250?
- The enclosure must be bonded to the equipment grounding conductor (Correct answer)
- The enclosure only requires an isolated ground
- The enclosure is exempt from grounding when operating below 50V
- The enclosure must be grounded to the telecommunications ground busbar only
Correct answer: The enclosure must be bonded to the equipment grounding conductor
NEC Article 250 requires metal enclosures to be bonded to the equipment grounding conductor to ensure safe fault-current paths.
Question 6: A request-to-exit (REX) device using a passive infrared sensor is installed on the secure side of a door. What is its primary function?
- To lock the door when someone approaches from inside
- To allow egress without generating an alarm when the door is opened from inside (Correct answer)
- To authenticate the identity of the person exiting
- To trigger a camera to record anyone leaving the area
Correct answer: To allow egress without generating an alarm when the door is opened from inside
A REX device shunts the door contact alarm when a person approaches to exit from the secure side, preventing nuisance alarms during authorized egress.
Question 7: Under NFPA 72, Class B wiring for initiating device circuits (IDC) differs from Class A primarily in that Class B:
- Uses shielded cable throughout
- Does not require a return path, making it more susceptible to open-circuit failures (Correct answer)
- Requires ground fault detection on every zone
- Uses a supervised wireless link as backup
Correct answer: Does not require a return path, making it more susceptible to open-circuit failures
Class B circuits use a single pathway with an end-of-line device; an open circuit disables detection on that zone, unlike Class A which provides a redundant return path.
C7 Security System Design & Integration 3
A passive infrared (PIR) motion detector triggers false alarms repeatedly near a south-facing window.
What is the most likely cause?