Fire Triangle, Classes of Fire and Portable Extinguishers

What a fire needs: the triangle and the tetrahedron

In short: Read first: the ship’s emergency organisation (alarms, musters, fire drills). Next: Fire Prevention, Detection and Fixed Fire-Fighting Systems. A fire needs three things at once: fuel, heat and oxygen — the fire triangle. Take one away and the fire goes out.

• Fuel: anything that burns — wood, paper, fabric, diesel, lube oil, paint, cooking fat, gas, some metals.
• Heat: enough to bring the fuel to its ignition temperature — a hot exhaust, a spark, a short circuit, a cigarette, a hot plate.
• Oxygen: normal air holds about 21% oxygen. As the oxygen level falls, burning slows; most flaming fires die down at roughly 15–16%.
• Fire tetrahedron: modern fire science adds a fourth side, the chemical chain reaction. The flame keeps itself going by producing free radicals that feed the next stage of burning. Break the chain and the flame stops, even with fuel, heat and oxygen still present.
• Why it matters: every extinguisher works by attacking one side (or more) of the triangle or tetrahedron. Know which side, and you know where it works and where it fails.

On board: where the three sides meet

• Engine room: fuel or lube oil spray (fuel) on a hot exhaust or turbocharger (heat), with plenty of air (oxygen).
• Galley: overheated cooking oil in a fryer or pan.
• Accommodation: bedding, curtains and furniture lit by a cigarette or a faulty electrical item.
• Paint locker and stores: flammable liquids and their vapours, plus a spark.

Four ways to put a fire out

In short: Remove one side of the tetrahedron: starvation (take away the fuel), smothering (take away the oxygen), cooling (take away the heat) and interference (break the chain reaction).

• Most media do more than one job: foam smothers and cools; wet chemical cools and seals the oil surface; CO2 smothers with a little cooling.
• Cooling is the only method that deals with deep heat in solid fuel. That is why water is best on class A fires and why fires put out by smothering or interference can re-ignite once air returns.

Classes of fire A, B, C, D, F — and “E”

In short: Fires are classed by what is burning. The European standard EN 2 and ISO 3941 use A solids, B liquids, C gases, D metals, F cooking oils and fats (IMO A.951(23), 5.1). Electrical fires are often called class E, but EN 2 has no class E.

• The “E” question: many charts and exam answers list E = electrical. EN 2 and ISO 3941 do not: electricity is not a fuel but a hazard that can sit behind a fire of any class. Australia and New Zealand do mark extinguishers “(E)” for live electrical fires. The practical rule is the same: use a non-conducting medium (CO2 or dry powder) while the power is on, and isolate the power as soon as you can — the fire then becomes an ordinary A or B fire.
• US system (NFPA 10), still seen on some yachts and in US-built kit: class C = live electrical equipment and class K = cooking oils (the European F). Gases have no separate US class (IMO A.951(23), 5.1).
• Class F exists because cooking oil burns at very high temperature: water makes it erupt, and ordinary foam or powder may not cool it enough to stop it re-igniting.

Extinguishing media: what works on what

In short: Five types to know: water, foam, dry powder, CO2, wet chemical. Match the medium to the class of fire. The IMO table lists what each is recommended for (IMO A.951(23), 3.1).

Water

• The best cooling agent. First choice for class A: wood, paper, textiles (IMO A.951(23), 3.1).
• Never on live electrics — water conducts and the operator can get a shock.
• Never on burning oil or fat — water sinks, boils into steam and throws burning liquid out, spreading the fire.
• Never on metal fires — it can react violently.

Foam

• A water-based solution that forms a blanket on the surface: it smothers and cools. Good on A and B (IMO A.951(23), 3.1). Most marine foam is AFFF (aqueous film-forming foam).
• On a pool of burning liquid, do not fire the jet straight into it: aim at a bulkhead or the edge and let the foam flow across the surface.
• Not for live electrics (it contains water) or for gas fires.

Dry powder

• Multi-purpose (ABC) powder: wood, paper, textiles, flammable liquids, electrical equipment and flammable gases. Standard (BC) powder: liquids, electrical and gases (IMO A.951(23), 3.1).
• Fast knock-down by interference with the chain reaction. It does little cooling, so watch for re-ignition.
• Downsides: a cloud that cuts visibility and irritates breathing, and a residue that damages electronics and machinery.
• Metal fires need special class D powder, not the ordinary ABC type (IMO A.951(23), 3.1).

Carbon dioxide (CO2)

• Flammable liquids and electrical equipment (IMO A.951(23), 3.1). A gas, so it leaves no residue — ideal for switchboards and electronics.
• Works by smothering: CO2 is heavier than air and pushes the oxygen away. Little cooling, so the fire can re-ignite when the gas drifts off.
• Poor on class A: the gas does not reach into deep embers.
• Danger to people: in a small space it lowers the oxygen you breathe (IMO A.951(23), 7.2). That is why CO2 extinguishers are not placed in accommodation spaces (SOLAS II-2/10.3.2.3).
• The horn gets extremely cold on discharge — hold the handle, never the horn, or you risk frost burns.

Wet chemical

• Made for class F: cooking grease, fats and oils (IMO A.951(23), 3.1). Also works on class A.
• Sprayed gently, it cools the oil and reacts with it to form a soapy layer on the surface (saponification) that seals off the oxygen and stops re-ignition.
• Galleys with deep fat fryers carry one class B and one extra class F (or K) extinguisher (IMO MSC.1/Circ.1275). The galley also carries a fire blanket.

Portable extinguishers: marking, colours and sizes

In short: A portable extinguisher weighs not more than 23 kg in working order (IMO A.951(23), 2.2). Each must match at least a 9 L water or foam extinguisher rated 2A (FSS Code ch. 4; IMO A.951(23), 7.1). In the UK and Europe the body is red with a coloured band showing the medium.

• Charge: water and foam are measured in litres, all other types in kilograms (IMO A.951(23), 2.4). Typical marine sizes: 9 L water or foam, 5 kg CO2, 6 kg dry powder.
• Driven by: stored pressure, or a gas cartridge opened when you operate it; a CO2 extinguisher simply releases its own pressurised gas (IMO A.951(23), Appendix).
• Visual indication of discharge: every extinguisher shows if it has been used — safety pin and tamper seal, and on stored-pressure types a gauge (IMO A.951(23), 9.3).
• Marking (IMO A.951(23), 8.1): manufacturer · classes of fire and rating · type and quantity of medium · approval details · instructions for use and recharge (pictures recommended) · year of manufacture · operating temperature range · test pressure.

Colour code (UK and Europe)

• Since BS EN 3 the whole body is red; only the band or label is coloured. UK colour coding follows BS 7863.
• Extinguishers made before about 1997 may still be fully cream, blue or black.
• Colours are not the same worldwide. On a foreign-built yacht, read the label: the class letters and pictograms are what count.

Using an extinguisher and a fire blanket

In short: Raise the alarm first. Fight a fire only if it is small, you have the right extinguisher and your escape route is behind you. Then PASS: Pull the pin, Aim at the base, Squeeze the handle, Sweep side to side.

• Before you go in: check the type and the gauge or seal; give a short test burst so you know it works.
• Approach low, keep the exit behind you; on deck, keep the wind behind you.
• Aim at the base of the flames, not the smoke. Sweep until the fire is out, then watch for re-ignition.
• Portable extinguishers empty in seconds. If the fire is not clearly shrinking, get out, close the door and report.
• Never turn your back on a fire you think is out. Back away facing it.
• After use: report it, and have the extinguisher recharged or replaced at once.
• In every fire drill at least one extinguisher is discharged by a different crew member each time (MGN 71 (M+F) Amd 1, 7.8).

Fire blanket

• For a small pan fire in the galley, or a person whose clothes are on fire. Fire blankets are made to BS EN 1869.
• Turn off the heat if you can do it safely.
• Pull the tapes sharply; wrap the top edge around your hands to shield them.
• Lay the blanket away from you over the pan, covering it completely — this smothers the fire.
• Leave it in place until the pan is completely cold. Lifting it early lets air in and the oil re-ignites.
• A person on fire: stop, drop and roll, or wrap them in the blanket from the neck down and lay them on the deck. Then cool the burns.
• Never water on a pan fire, and never try to carry a burning pan outside.

On board: carriage, spares and maintenance

In short: One extinguisher for each space is kept near its entrance (SOLAS II-2/10.3.2.2). Extinguishers are inspected and serviced at least once a year, some are test-discharged every 5 years and all are pressure-tested within 10 years (IMO A.951(23), 9.1; MGN 276 (M+F) Amd 1, Table 1).

• Where: one of the portable extinguishers for a space is stowed near the entrance (SOLAS II-2/10.3.2.2). No CO2 in accommodation spaces (SOLAS II-2/10.3.2.3).
• Examples (IMO MSC.1/Circ.1275): wheelhouse 2 (only 1 if under 50 m²), class A or C; corridors — no more than 25 m walk to an extinguisher; near the main switchboards 2 class C; galley with deep fat fryer, class B + class F.
• Spare charges (SOLAS II-2/10.3.3): 100% for the first 10 extinguishers that can be recharged on board, 50% for the rest, 60 at most. Extinguishers that cannot be recharged on board need the same number of extra extinguishers instead.
• Annual: inspection and service — safety clip and seal, pressure gauge, corrosion and dents, weight, hose and nozzle, instructions legible (IMO A.951(23), 9.1 and Table 9.1.3).
• Every 5 years: at least one extinguisher of each type and year of manufacture is test-discharged, as part of a fire drill (IMO A.951(23), 9.1.1).
• Within 10 years: all extinguishers and their gas cartridges are hydraulically tested (IMO A.951(23), 9.1.2).
• MGN 276 (M+F) Amd 1, Table 1 (UK): basic service yearly; extended service (test discharge) every 5 years (10 years for primary-sealed powder and composite bodies); overhaul with hydraulic test every 10 years. CO2 extended service applies only to cylinders already hydraulically tested.
• Who: a competent person. On board, basic and extended service may be done under the supervision of an officer with an STCW II/2 or III/2 unlimited certificate and Advanced Fire Fighting (MGN 276 (M+F) Amd 1, 1.5–1.6).
• Records: date, type of maintenance and whether a pressure test was done (IMO A.951(23), 9.2); a maintenance label on each extinguisher (MGN 276 (M+F) Amd 1, 1.8).