System Components and Devices
The System Components and Devices domain asks you to identify a device, state what it does, and - most importantly - classify the signal it produces. Many items are decided by the signal category alone.
Classify signals first
NFPA 72 separates three priorities:
- Alarm - fire condition (smoke detector, manual station, waterflow switch).
- Supervisory - a condition that could impair protection (valve tamper, low air pressure, low water level, pump running).
- Trouble - a circuit or device fault (open, ground, loss of primary power). Supervisory signals are not alarm signals. A tamper switch never reports as fire.
Component map
- Control unit (FACP) - receives initiating signals, executes the sequence of operations, drives notification, transmits off-premises.
- Manual pull station - at required exits, within 5 ft of the door, 42 to 48 in above finished floor, unobstructed.
- Spot smoke detectors - photoelectric or ionization; area devices, never spacing devices for other hazards.
- Duct detectors - sample HVAC air, typically to initiate fan shutdown and damper control; not a substitute for area detection in the room.
- Heat detectors - fixed temperature, rate-of-rise, or combination; listed spacing governs layout.
- Waterflow switch - alarm initiating, with a 10 to 90 second retard to ride through pressure surges; alarm annunciation required within 90 seconds.
- Monitor module - converts a dry contact into an address. Control/relay module - switches fan shutdown, door hold-opens, elevators. Isolator module - keeps a loop short from taking out the whole circuit.
Worked example
A riser has a flow switch, a post indicator valve with a tamper switch, and a low-pressure switch. How does each report? Reasoning: the flow switch is an alarm device and must annunciate at the panel within 90 seconds. The tamper switch and the low-pressure switch are supervisory devices. All three are dry contacts, so each needs an address through a monitor module (or a conventional zone) and a clear, distinguishable description at the panel and annunciator. The flow switch must never be programmed into a supervisory zone.
Traps that cost marks
- Reporting a tamper switch as an alarm.
- Assuming a waterflow switch is instantaneous - the retard period is intentional.
- Using duct detectors as the only detection in a space.
- Forgetting isolators on a large addressable loop.
- Mixing up monitor (input) and control (output) modules.