Fire, Fuels and Explosion
The fire triangle, extinguisher classes, flammable storage, oily rags, combustible dust, hot work and lithium batteries.
On this page
The fire triangle and extinguisher classes
Fire needs heat, fuel and oxygen. Take away any one and it stops; prevention means keeping the three from meeting, and fighting means removing one of them. Extinguishers are classed by the fuel they are made for, and using the wrong one can make a fire worse: water on burning oil spreads it, and water on a live electrical fire can electrocute.
| Class (US / EU) | Fuel | Suitable extinguisher | Not suitable |
|---|---|---|---|
| A | Wood, paper, cloth, plastics | Water, foam, ABC dry powder | — |
| B | Petrol, solvents, oils, paint | Foam, CO₂, ABC dry powder | Water jet (spreads liquid) |
| C (gas; US: electrical) | Flammable gas | Dry powder; stop the gas supply first | Extinguishing a gas flame with the leak unfixed can make an explosive cloud |
| Electrical (US class C) | Live equipment | CO₂, dry powder | Water or foam while live |
| D | Metals: magnesium, titanium, aluminum dust | Special dry powder (sand, graphite) | Water, CO₂, ABC (reacts) |
| F / K | Cooking oils and fats | Wet chemical | Water |
How fast a fire grows
Many fires grow with the square of time: the heat release rate is Q = α t² (kW, with t in seconds), where α depends on the fuel (slow, medium, fast, ultra-fast). A portable extinguisher can control a fire only while it is small, in the first minute or two.
| Growth class | α (kW/s²) | Typical fuel | Time to 100 kW | Time to 1 MW (flashover range) |
|---|---|---|---|---|
| Slow | 0.003 | Densely packed wood, solid timber | 185 s | 584 s |
| Medium | 0.012 | Furniture, paper stacks | 92 s | 292 s |
| Fast | 0.047 | Wooden pallets, foam, plastic storage | 46 s | 146 s |
| Ultra-fast | 0.188 | Flammable liquids, light foam | 23 s | 73 s |
Worked example: how long do you have?
- A medium fire reaches 42 kW after 1 minute (α t² = 0.012 × 60²) and 169 kW after 2 minutes: four times bigger in twice the time.
- A portable extinguisher can deal with perhaps 100 kW. A medium fire gets there in 92 s, a fast one (pallets, foam) in 46 s and a flammable-liquid fire in only 23 s.
- The fire is out of your control long before it is large: a fast fire reaches 1 MW (the whole room alight) in 146 s, about 2.4 minutes.
- So: tackle a fire at once only if it is small and you have an escape route behind you; otherwise get out, close the door and call the fire service. A shop is not worth a life.
Using an extinguisher
- PASS: Pull the pin, Aim at the base of the fire, Squeeze the handle, Sweep from side to side.
- Stand 2 to 3 m back with your back to the exit, and keep the exit clear.
- Know the type printed on the label and which classes it covers; keep one near each exit and each hot-work area.
- Read the gauge monthly; have it serviced yearly; replace after use, even if only partly discharged.
- Do not fight a fire that is bigger than a waste bin, or that blocks your exit, or that produces thick smoke.
- After the fire is out, watch it for an hour and call the fire service: it can restart.
Worked example: how long does it last?
- A 2 kg dry-powder extinguisher empties in about 10 s: a discharge rate of 0.2 kg/s. Do not waste it: aim at the base, short bursts.
- With a throw of about 3 m you must be within that distance; the powder cloud blinds you and does not cool, so a fire may re-ignite.
- If you use 85 % on the first pass, 0.3 kg (2 s) is all that is left for any flare-up: keep a second extinguisher.
Flammable liquids and vapor
It is the vapor, not the liquid, that burns. A liquid’s flash point is the lowest temperature at which it gives off enough vapor to ignite: petrol is below −40 °C, acetone about −20 °C, mineral spirits about 38 °C. Vapors of most solvents are heavier than air and collect at floor level, where a pilot light or a spark finds them. A mixture burns only between the lower and upper explosive limits (LEL and UEL).
Worked example: a spill in a small shop
- Petrol has a lower explosive limit of about 1.4 % by volume. In a 50 m³ shop the LEL is 1.4 % × 50 m³ = 0.70 m³ of vapor.
- At 25 °C one mole of gas occupies 24.5 L, so that is 29 mol; with a molar mass of about 95 g/mol, 2.7 kg of petrol: 3.7 liters evaporating in the room would make the whole air explosive.
- Even 0.37 liters (a tenth of that) makes a tenth of the room explosive: near the floor, where it collects, an explosive layer is easy to form.
- Acetone (LEL 2.5 %, molar mass 58): about 3.8 liters in the same room, about as much as petrol.
- Never use petrol or flammable solvent to clean parts or hands. Do not store it in the shop: keep only what you need, in approved cans or a flammable-liquids cabinet, away from heaters, pilot lights and sparks.
- Ventilate when using solvents, finishes and adhesives, and keep the quantity small.
- Remove ignition sources: pilot lights, heaters, grinders, switches that spark, static.
- Bond and earth metal containers when pouring flammable liquid; static can ignite the vapor.
- Keep lids closed and wipe up spills at once; put the wipes in a metal can with a lid.
- Spray finishing needs extraction rated for flammable vapor (see Dust, Fumes and Ventilation and Chemicals, Gases and Welding).
Oily rags and spontaneous heating
Rags soaked with linseed oil, tung oil, some stains and varnishes heat by themselves. The oil oxidizes and gives off heat; in a pile the heat cannot escape, the temperature rises, the reaction speeds up, and after a few hours the pile can ignite with no flame or spark. Many shop fires start this way hours after the work has stopped.
- Spread the rags flat outdoors or hang them to dry, away from combustibles, until they are stiff; or soak them in water in a sealed metal can.
- Never ball them up or put them in a bin or a pile.
- Use an approved oily-waste can with a self-closing lid and empty it each day.
- Wood-working finishes (Danish oil, boiled linseed oil) are the usual culprits; mineral oils and most cutting oils are not self-heating but do burn.
- Sawdust with finish from sanding can heat in the same way: keep it in a metal can.
Combustible dust and explosions
Fine dust of wood, flour, plastic and many metals can burn explosively when it is suspended in air. A fire needs a fuel, an ignition source and oxygen; a dust explosion needs those plus the fuel as a fine cloud in a confined space, and a primary explosion that shakes settled dust into a cloud can trigger a bigger secondary one.
Worked example: how little dust is needed
- The minimum explosible concentration of fine wood dust is about 40 g/m³ (typical; a thick fog in the air).
- A layer of dust only 0.8 mm thick (the 1/32 inch of fire codes) with a bulk density of 200 kg/m³ on 20 m² of floor weighs 200 × 0.0008 × 20 = 3.2 kg. If that were thrown into the 50 m³ of the shop it would make 64 g/m³: 1.6 times the minimum explosible level.
- A dust collector holding 1 kg of fine dust in a 5 m³ bag house, shaken, makes 200 g/m³ in the bag.
- Even 5 % of the floor covered (1.0 m²) with that layer is 0.2 kg of fuel.
- Housekeeping is the control: clean surfaces with a vacuum rated for dust (not a broom or compressed air), keep the collector outside the building, and avoid ignition sources (sparks from a grinder in a sanding area).
- Metal dusts (aluminum, magnesium, titanium) are worse than wood: they burn hot and react with water. Collect them in a separate system and never use a normal wood-dust collector.
- Put the collector outdoors or in a separate room, with an explosion vent and a spark trap on the duct.
- Bond and earth metal ducts to bleed static.
- Do not mix grinding sparks with a dust collector inlet: steel sparks from a grinder on a bench can ignite wood dust in a hose.
Hot work: welding, cutting and grinding
Worked example: the radius of risk
- Sparks and spatter from a grinder or a welding arc can travel up to 10 m horizontally and fall to the floor, where they land in sawdust, cracks and rags. The usual rule is to clear or cover combustibles within 11 m (35 ft) of the work.
- Fire watch: stay and watch for 60 minutes after the work stops (some codes require up to 3 hours); a smoldering spark can take that long to show.
- A fire blanket or a bucket of sand near the work, and an extinguisher in reach.
- Never weld or grind on a container that held fuel, even empty and “cleaned”: the vapor in seams can explode.
- Move or cover flammable material, sweep the floor, wet down sawdust, close gaps in walls and floors.
- Wear non-flammable clothing (cotton or leather; no synthetics; no turn-ups).
- Keep gas cylinders upright and secured and the hoses and regulators in good order (see Chemicals, Gases and Welding).
- Check the area under and behind the work, including the other side of a wall.
Lithium batteries
Lithium-ion cells store a lot of energy in a small package. A damaged, overcharged or shorted cell can go into thermal runaway: it heats itself, vents flammable gas and burns fiercely, and sets off the neighboring cells.
Worked example: the energy in a tool battery
- An 18 V 5 Ah pack holds 90 Wh = 324 kJ. If that is released as heat in 5 minutes it is an average of 1 080 W: about a one-kilowatt heater inside a plastic case.
- A short circuit through a 0.03 Ω pack internal resistance gives 600 A and a power of 10.8 kW: a metal object across the terminals (keys in a pocket, a loose battery in a toolbox with screws) becomes hot in seconds.
- Each of the 5 cells holds about 10 Wh (36 kJ), and a cell that fails dumps that heat into its neighbors.
- Do not charge unattended or overnight on a combustible surface; use the maker’s charger; stop using a swollen, hot or damaged pack; cover the terminals of loose packs.
- Store at room temperature, at 30 to 60 % charge, in a fire-resistant box or a metal cabinet, away from combustibles. A lithium fire needs a lot of water or an extinguisher rated for it, and the fire can restart.
Detection, escape and prevention
Worked example: time to get out
- Detection takes about 30 s for a smoke alarm to react; you may take 30 s to decide; walking 8 m at 1.2 m/s takes 7 s: 67 s in all.
- Compare this with the fast-fire time of 146 s to 1 MW: the margin is thin. Keep every route to the exit clear and unlocked, and fit smoke or heat alarms (and a CO alarm with any fuel-burning heater).
- A fire in a workshop with stacked foam, pallets and finishes reaches 100 kW in about 50 % of the time of a medium one; keep fuel away from the work area.
- Alarms: smoke alarms in the workshop (heat alarms where dust or fumes would set smoke ones off), tested monthly.
- Exits: two ways out if possible; no storage in front of the door; doors that open outward.
- Extinguishers at the exits, a fire blanket near the heat, and a phone to call the fire service.
- Housekeeping: remove waste, keep the bench clear, sweep dust.
- Electrical: do not overload sockets or run cords under rugs (see Electrical Safety).
- Tell the fire service about hazards such as gas cylinders and flammable stock in a building.
Troubleshooting
| Problem | Likely cause | Try |
|---|---|---|
| Small fire in the bin | Oily rags, hot swarf, a hot grinder spark | Smother with a lid or a blanket; use a class A or ABC extinguisher; empty and check the bin |
| Smoke alarm goes off from the grinder | Dust and smoke | Use a heat alarm near the work; ventilate; do not remove the alarm |
| Smell of burning plastic from a tool | Overloaded motor, blocked vent | Switch off and unplug; let it cool; check the vents and the load |
| Battery gets hot in the charger | Fault or damage | Unplug; move it to a non-flammable place; do not use it again |
| Cannot reach the extinguisher | Stored behind clutter | Mount it by the exit on the wall at a known place |
| A fire starts and keeps going | Too big for the extinguisher | Leave; close the door; call the fire service; do not go back |
Related
- Workshop Safety: the overview of this guide.
- Dust, Fumes and Ventilation: dust collection and housekeeping.
- Chemicals, Gases and Welding: solvents, gas cylinders and welding.
- Laser Safety, Fire and Fumes (the fire hazards of lasers) and Batteries and Power Supplies.
FAQ
What type of fire extinguisher do I need in a workshop?
A multi-purpose ABC dry-powder or a CO₂ extinguisher at each exit and near hot work, plus a fire blanket; for metal dust, a class D extinguisher.
How do oily rags start fires?
The drying oil oxidizes and heats the pile; in a heap the heat cannot escape and the rags ignite hours later. Hang them flat to dry or keep them in water in a sealed metal can.
Can sawdust explode?
Yes: fine dust in a cloud in a confined space (a collector, a duct, a room) can explode. Keep dust cleaned up, the collector outdoors and ignition sources away.
Should I fight a fire?
Only if it is small, you have the right extinguisher, you have an exit behind you and the smoke is not thick. Otherwise get out, close the door and call the fire service.
References
- NFPA 10, Standard for Portable Fire Extinguishers; NFPA 51B, Fire Prevention During Welding, Cutting and Other Hot Work; NFPA 652 and 654 (combustible dust).
- NFPA 704 and the SFPE Handbook of Fire Protection Engineering (t-squared fire growth; flash points and explosive limits).
- US CPSC and fire-service guidance on spontaneous combustion of oily rags; NFPA 855 and UL 1642 (lithium batteries).
Last reviewed: October 2026. Fire growth rates, limits and times are typical values: real fires vary widely. When in doubt, get out and call for help.