Question 3 of 8: Fire Protection

Question 3: Fire Protection (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) The Four Classes of Fire

• Class A — ordinary combustible solids: wood, paper, cloth, rubber, many plastics.
• Class B — flammable liquids and gases: gasoline, oil, solvents, paint, propane.
• Class C — fires in or near energized electrical equipment.
• Class D — combustible metals: magnesium, sodium, potassium, titanium.
• (Some systems add Class K/F for cooking oils and fats; the question specifies four classes.)

(b) Suitable and Prohibited Agents (with reasons)

• Class A — use water (cooling), foam, or ABC dry chemical. Water is effective and cheap because it removes heat.
• Class B — use foam, CO₂, or dry chemical (smother/interrupt reaction). Do not use a water jet: it spreads the burning liquid and can cause flare-up.
• Class C — use CO₂ or dry chemical (non-conductive). Do not use water or foam: they conduct electricity, risking electrocution of the operator and equipment damage. (De-energize, then it becomes a Class A/B fire.)
• Class D — use special dry-powder agents designed for the specific metal. Do not use water: burning metals react violently with water (releasing hydrogen/explosion); CO₂ and standard agents are also ineffective or dangerous.

(c) Suppression Agents and How They Work

Fire needs fuel, oxygen, heat and a sustaining chain reaction (the fire tetrahedron); each agent removes one or more legs:

• Water — cooling: absorbs heat, dropping the fuel below its ignition temperature.
• Foam — smothering + cooling: forms a blanket over a liquid surface, excluding oxygen and suppressing vapour.
• Carbon dioxide — smothering: displaces oxygen (and provides some cooling); leaves no residue, good for electrical/electronics.
• Dry chemical powders — chemically interrupt the chain reaction of combustion.
• Clean (gaseous) agents — inert gases or halocarbons that smother or interrupt the reaction without residue.
• Wet chemical — for cooking oils: saponifies the oil to form a cooling, smothering layer.