Fire classes A–K, extinguisher types, and detector comparison

One-line orientation

Fire classes define the fuel type burning, which determines the correct extinguisher; detectors differ in whether they catch fast-flaming or slow-smoldering fires — both distinctions are tested in exam selection scenarios.

Key points

Fire classes and extinguishers:

• Class A — ordinary combustibles: wood, paper, fabric, trash. Extinguishers: water, foam, dry chemical (ABC powder).
• Class B — flammable liquids and gases: oil, gasoline, paint, solvents. Extinguishers: foam, CO₂, dry chemical.
• Class C — energized electrical equipment: wiring, motors, appliances. Extinguishers: CO₂, dry chemical (both non-conductive — critical; water or foam would cause electrocution or spread).
• Class D — combustible metals: magnesium, sodium, titanium. Extinguishers: dry powder (specific to the metal type; standard dry chemical does not work).
• Class K — cooking oils and fats: deep fryers, commercial kitchens. Extinguishers: wet chemical (potassium acetate/bicarbonate; saponification reaction suppresses re-ignition).

Detection devices:

• Heat detector: responds to temperature rise, not smoke. Less sensitive than smoke detectors. Use in spaces where smoke is normal (kitchens, garages, dusty industrial areas).
• Ionization smoke detector: contains a small radioactive source that ionizes air in a chamber; combustion particles disrupt the current. Best at detecting fast-flaming fires with small particles.
• Photoelectric smoke detector: a light beam is scattered by smoke particles onto a sensor. Best at detecting slow, smoldering fires with larger particles; fewer false alarms from steam or cooking.
• Combination detector: incorporates both ionization and photoelectric technologies for comprehensive detection across fire types.

Automatic ventilating hatches (related fire protection component):

• Open automatically during a fire to release smoke and hot gases, improving visibility and firefighter access. Triggered by heat, smoke detection, or alarm system. Common in warehouses, industrial buildings, and atriums.

Fire class → fuel → extinguisher

Start with what is burning. The blue column is the matched agent; the detector shortcut is a separate selection question.

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Class C: use a nonconductive agent while equipment is energized. Class K: use wet chemical for hot cooking oils and fats—not ordinary dry chemical.

Confusions / comparison

Class	Fuel	Correct extinguisher	Do NOT use	Memory hook
A	Wood, paper, fabric	Water, foam, dry chemical	—	“Ash” = ordinary solids
B	Oil, gas, paint	Foam, CO₂, dry chemical	Water (spreads liquid)	“Barrel” = liquids
C	Electrical equipment	CO₂, dry chemical	Water or foam (conductive)	“Current” = electricity
D	Combustible metals	Dry powder (metal-specific)	Water (explosive reaction)	“Dross” = metals
K	Cooking oils/fats	Wet chemical	Dry chemical (insufficient)	“Kitchen” = cooking

Detector	Detects best	Mechanism	Avoid where
Heat	Temperature rise	Fixed temp or rate-of-rise	Anywhere smoke appears first
Ionization	Fast-flaming fires	Small particle current disruption	Near kitchens/steam (false alarms)
Photoelectric	Slow-smoldering fires	Light scatter from large particles	Less of an issue for false alarms
Combination	Both fire types	Dual technology	—

Related

→ Fire Protection: sprinkler system types (wet/dry/pre-action/deluge) · Code: fire-rated assemblies, occupancy separation · Site: means of egress and exit signage.