Understanding fire

One of the ways you can protect yourself, your family, your home and your workplace from the risk of fire is to have some understanding of how fires start, how they burn, the risks they pose and how they can be extinguished.

In this course we cover the basic characteristics of fire. We look at the Fire Tetrahedron, how to extinguish a fire and the different types of fires. We also look at the risks posed by fire including the dangers of smoke.

Introduction

Introduction

How to complete this course

Topics

This course is broken into the following topics:

  • What is fire?
  • Types of fire
  • The risks of fire

Each topic is a self-contained module of information and activities.

The final module consists of questions you can use to check you have understood the material correctly.

What is fire?

What is fire?

A fire is a rapid chemical reaction between oxygen and a combustible substance.

Fires produce heat, light and smoke.

The Fire Tetrahedron

The Fire Tetrahedron helps us understand how fires start, how they are sustained and how they can be extinguished. The four faces of the Fire Tetrahedron are fuel, oxygen, heat, and a chain reaction.

For a fire to ignite and continue to burn, all four faces of the tetrahedron must be present.

Fuel

Without a supply of suitable fuel, a fire cannot start or continue to burn.

To build a campfire, you need a supply of fuel such as dry twigs and small branches.

In a bushfire, the main sources of fuel are grass, ground litter, bushes and trees.

In a building fire, sources of fuel include construction materials, paint, furniture, floor coverings, fittings and materials stored in the building.

Oxygen

Fires need an oxidising agent. In most fires the oxidising agent is oxygen. Air is the main source of oxygen for fires. The Earth’s atmosphere is approximately 21% oxygen.

Heat

Without enough heat, a fire cannot start or continue to burn.

When starting a campfire you might use a lit match as a source of heat.

Bushfires are often started by lightning strikes which provide the required heat for ignition.

Building fires are often caused by electrical faults that generate a lot of heat.

Chain reaction

Combustion is a complex set of chemical reactions.

In a fire flame, fuel molecules are broken down by heat, producing very reactive free radicals. These free radicals react with oxygen, producing more heat and more free radicals, which then react producing more heat and more free radicals, and so on.

This self-sustaining process is the chemical chain reaction that keeps a fire burning, as long as there’s an adequate supply of fuel and oxygen. The fire cannot continue to burn if this chain reaction is interrupted.

Extinguishing a fire

A fire needs all four faces of the Fire Tetrahedron to continue to burn - fuel, oxygen, heat and a chain reaction.

By removing one or more of these, we can suppress or extinguish a fire.

Smothering – removing oxygen

Stopping or restricting the supply of oxygen to a fire will suppress or extinguish it.

A campfire can be extinguished by covering it in a layer of dirt or sand. The blanket of dirt will restrict or stop the flow of oxygen to the fire.

A fire blanket placed over a burning pan of oil separates the fire from its oxygen supply.

Carbon dioxide fire extinguishers work by displacing the oxygen around the fire.

Cooling – removing heat

Firefighters often fight fires using water.

Water is good at absorbing heat. When firefighters put water on a fire, the water absorbs heat from the fire as the liquid water converts into steam.

Water extinguishes a fire by removing heat.

Starving – removing fuel

Removing fuel will starve a fire.

When you have finished cooking your steak on your LPG fuelled BBQ, you extinguish the fire by turning off the supply of fuel.

One of the main methods of battling wildfires is to starve the fire of fuel. Back-burning or clearing a fire break are methods of removing potential fuel for the fire.

Inhibiting – breaking the chemical chain reaction

A fire cannot sustain itself if the chemical chain reaction is interrupted.

Some fire suppression agents react with or absorb free radicals when applied to a flaming fire. This slows or stops the chemical chain reaction that sustains the fire.

Fill in the missing word.

Fires produce heat, light and .

Fill in the missing word.

Air is the main source of for most fires.

Fill in the missing word.

In a wildfire, grass, ground litter, bushes and trees are the fire's main source of .

Fill in the missing word.

When firefighters put water on a fire, the water absorbs  from the fire as the liquid water converts into steam.

Types of fire

Types of fire

Different fuels create different types of fires.

For example, a pot of burning cooking oil is a different type of fire to burning paper or cardboard. The burning oil can be a more dangerous type of fire and needs to be controlled or extinguished in a different way to the burning cardboard.


Before fighting a fire it’s important to consider what type of fuel is involved. The type of fuel or fuels involved in the fire will determine how you fight the fire and what sort of fire extinguisher you need to use. We classify different fires into “Classes” designated as A, B, C, D, E or F.

Class A fires

Class A fires are fueled by combustible solids such as wood, paper, fabric and plastic.

In Australia, this symbol represents Class A fires.

These symbols appear on portable fire extinguishers. They indicate the types of fires the extinguisher is safe for use on, or is unsafe for use on.

Class B fires

Class B fires are fueled by flammable and combustible liquids such as petrol, paint, diesel or kerosene.

In Australia, this symbol represents Class B fires.

These symbols appear on portable fire extinguishers. They indicate the types of fires the extinguisher is safe for use on, or is unsafe for use on.

Class C fires

Class C fires are fueled by flammable gases such as LPG, natural gas or butane.

In Australia, this symbol represents Class C fires.

These symbols appear on portable fire extinguishers. They indicate the types of fires the extinguisher is safe for use on, or is unsafe for use on.

Class D fires

Class D fires are fueled by combustible metals. Under the right conditions metals such as magnesium and titanium will burn.

Class E fires

Class E are fires involving energised electrical equipment such as a clothes dryer, TV or electrical switchboard.

These fires can be especially dangerous to extinguish due to the risk of electrocution.

In Australia, this symbol represents Class E fires.

These symbols appear on portable fire extinguishers. They indicate the types of fires the extinguisher is safe for use on, or is unsafe for use on.

Class F fires

Class F fires are fueled by cooking oil or fat.

These types of fires can be especially dangerous if you try to extinguish one using the wrong method.

In Australia, this symbol represents Class F fires.

These symbols appear on portable fire extinguishers. They indicate the types of fires the extinguisher is safe for use on, or is unsafe for use on.

Which of these images shows a Class A fire?

Which of these images shows a Class E fire?

Which of these symbols represents a Class B fire?

Which of these symbols represents a Class F fire?

The risks of fire

Fire is a significant risk to life and property

Fire can cause death and serious injury. A fire can quickly destroy a home and its contents.

Fires can start and grow very quickly. A small flame can grow into an out-of-control fire in as little as 30 seconds.

In Australia, accidental house fires result in more than 50 deaths a year. It is estimated that 10,000 residential fires occur every year.

While fire produce a lot of heat, the main cause of death in fires is smoke inhalation.

The dangers of smoke

The main cause of death in fires is smoke inhalation. Smoke is a collection of heated solid and liquid particles, and toxic gases such as carbon monoxide.

Fires in buildings are fueled by synthetic furniture, floor coverings and fittings. These types of fuels can produce very toxic gases such as hydrogen cyanide.

Low levels of exposure to smoke can impair your vision, irritate your eyes and lead to disorientation and confusion.

Breathing in super-heated smoke can cause severe burns to your respiratory system.

The toxic gases in smoke can irritate and damage the linings of your respiratory system and cause asphyxiation.

Get down low

In a fire emergency, you should avoid moving through smoke.

You may have attended a fire safety lesson in school where you were told to "Get down low and go, go, go!” when moving in smoke.

In a fire you will find less smoke, cooler air and better visibility down low to the floor.

Smoke can kill by:

  • asphyxiation
  • burning and damaging the respiratory system
  • irritating and damaging the linings of the respiratory system
  • all of the above

Check your understanding

Fill in the missing words in the statement below.

A fire is a rapid chemical between and a combustible substance.

The four sides of the Fire Tetrahedron are:

  • Heat; Light; Free radicals; Oxygen
  • Oxygen; Heat; Fuel, Chain reaction
  • Reactants, Fuel; Flames; Oxygen
  • Heat; Light; Fuel, Smoke

A Class B fire is a fire fueled by:

  • combustible solids such as wood, paper, fabric and plastic
  • flammable and combustible liquids such as petrol, paint, diesel or kerosene
  • flammable gases such as LPG, natural gas or butane
  • cooking oil or fat

A Class F fire is a fire fuelled by:

  • combustible solids such as wood, paper, fabric and plastic
  • flammable and combustible liquids such as petrol, paint, diesel or kerosene
  • flammable gases such as LPG, natural gas or butane
  • cooking oil or fat

Drag each description on the right to the correct extinguishing method on the left.

  • Smother
    remove oxygen
  • Cool
    remove heat
  • Starve
    remove fuel
  • Inhibit
    break the chemical chain reaction

Which of the following is an example of how you might smother a fire?

  • Pour water on a fire
  • Place a fire blanket over a saucepan containing burning cooking oil
  • Turn the gas supply off to a BBQ
  • All of the above

Identify the types of fires show below.

Instructions

Drag the text label containing the correct class of fire onto each image.

  • Class B
  • Class C
  • Class E
  • Class F
  • Class A

Which of these symbols represents a Class A fire?

Is the following statement true or false?

  • The main cause of death in fires is smoke inhalation.