Fire detection, alarm and suppression systems
Domain 3 tests whether you can select, size and sequence detection and suppression systems, and set out the control philosophy that links them into one responding building.
Detection
Selection follows the expected fire signature: optical smoke for smouldering fires, heat for dirty or cooking environments, beam and aspirating detectors for atria and cable voids, flame detectors for high-hazard liquid risks. Spacing follows the applicable standard (BS 5839-1 gives smoke detectors 10.5 m spacing and 5.3 m from walls; heat detectors 5.3 m and 2.6 m). Manual call points sit within 30 m travel and at exits.
Sprinklers
Design rests on three numbers: design density (mm/min), area of operation (m²) and water supply duration (min). The hazard class fixes all three. Head spacing follows coverage area, and head flow comes from the K-factor: Q = K√P. Pumps and tanks are sized on the worst credible simultaneous demand of sprinklers, hydrants and hose reels, not on every system at full flow.
Gaseous and water mist
Clean agents (HFC-227ea, FK-5-1-12, IG-541) work by heat absorption and oxygen displacement. The design concentration must exceed the cup-burner value with a safety margin, discharge must complete within the time the standard requires (fast in unoccupied spaces, slower where people may be present), and the enclosure must pass a door-fan integrity test to hold concentration for the specified hold time. CO2 is unacceptable in occupied spaces. Water mist (NFPA 750) relies on cooling, oxygen displacement and radiative blockage, and needs manufacturer-specific fire test data; it is not interchangeable with sprinkler design.
Control philosophy
Deliver it as a cause-and-effect matrix: detection zone, alarm stage, plant action, and interfaces with doors, dampers, lifts, smoke extract and the fire services. Cross-zoning reduces unwanted discharge; pre-action systems protect water-sensitive spaces; any alarm delay must be justified against egress needs.
Worked example
An ordinary hazard storage: design density 5 mm/min over a 72 m² area of operation for 60 min. Pump duty = 5 × 72 = 360 L/min; tank volume = 360 × 60 = 21,600 L before adding hydrant or hose reel demand where the hydraulics show simultaneous operation. That calculation, not the building floor area, sizes the tank room.
Traps
Sizing tanks on building floor area; using discharge density at the head instead of design density over the area of operation; confusing discharge time with hold time in clean agent designs; forgetting that spacing rules change with ceiling height and obstructions.