A detector detects what reaches it, so placement is a physical prediction about a plume. Hot smoke rises in a buoyant plume, strikes the ceiling, and spreads outward as a thin, fast ceiling jet a few percent of the ceiling height deep; a spot detector must sit inside that layer, which is why devices go on the ceiling or within a stated distance of it, and why the corner where a wall meets a ceiling is a dead air pocket that the jet skips. Spacing follows from the same picture: coverage is set by how far the jet travels before it cools and thins below the alarm threshold. Three situations break the rule and are worth memorising. A very high ceiling lets the plume cool and lose buoyancy before it arrives, so smoke stratifies in a layer below the detector and never reaches it. Beams and joists dam the jet into pockets. Mechanical ventilation drags smoke to a return grille, which is where detection should be. After this Concept you can place or challenge a detector position by reasoning about where the smoke will go.
Fire and smoke detectors are point detectors that must have combustion products physically reach their sensing element to activate, so their effective coverage is bounded by a detection radius determ…