Fire Types and Classes

Table of Contents

At its simplest, fire can be defined as a chemical reaction that happens when fuel, such as paper, wood, gasoline, etc., ignite and combine with oxygen (combustion), resulting in light, heat, flames and smoke.

Fire’s destructive power is staggering. A fire can spread, often to twice its original size in only seconds. And it takes less than 30 seconds for this fire to become a large-scale conflagration. In just five minutes, an entire home can become engulfed in flames. And the materials used in modern residences and furnishings cause fires to burn faster than ever.

In 2022, the U.S. Fire Administration (USFA) estimated that there were 374,300 residential fires and 129,500 nonresidential building fires. The total death toll from those fires for civilians was estimated at 2,860. In 2022, 94 firefighters lost their lives while on duty, though not all were fire response related.

Given these statistics, at least seven people a day lose their lives in residential fires.

The total cost of these fires for residential buildings alone was $10,821,300,000. The cost to nonresidential properties, while much lower, was still huge at $3,741,200,000. These losses don’t even account for vehicle fires (including farming and construction equipment), and other outside fires.

As you can see, the cost of fires in lives and dollars is enormous.

Origins of Fire Classification

The practice of fire protection engineering—protecting people and their environment from fire using engineering and science principles, was likely first recorded after the burning of Rome in 64 AD. This practice led to the enacting of codes, regulations and standards over time in response to a number of historic fires.

These deadly fires include the Great London Fire of 1666, where some estimate that 85% of the city was destroyed. Also among them is the Iroquois Theatre fire of 1903 which killed 602 people due to a combination of factors, including locked and blocked exits, and the Triangle Shirtwaist fire of 1911 where 146 people perished due to the lack of and locked exits. 

Prompted by these and other terrible fires, the automatic fire sprinkler and other fire suppression systems were invented. Henry S. Parmelee was the first to patent an automatic sprinkler in 1874.

Over the years, fire protection engineers began to develop different suppression systems and in response to emerging industrial hazards and warehouse storage techniques. These included systems for the protection of transformers, library book stacks, roll paper storage, rubber tire storage and aerosol storage.

The National Fire Protection Agency (NFPA) is a self-funded, non-profit organization created in 1896 with three objectives: to provide the science, to improve methods of fire protection, and to circulate information on this important subject. They lead globally in advocating for the eradication of injury, death, property loss and economic loss because of fire, electrical and related hazards.

In fact, NFPA 13 provides the current standard for fire sprinkler installation.

And, it is the NFPA which has created and disseminated the classifications of types of fires in the U.S. There are five different types of fire designated by the NFPA as Class A through Class D, and Class K.

In Europe, the European Standard Classification of Fires delineates their fire classifications. Various types of fire in both systems are determined primarily by the kind of fuel being burned, i.e. flammable liquids or combustible metals.

How Fires Grow and Spread

Three primary components are required for a fire, heat (a source of ignition), fuel and oxygen. These can be visualized as a triangle, often referred to as the “Fire Triangle.” Air is 21% oxygen. A fire generally only needs 16% oxygen to burn, so that element is almost always available. As noted above, once fuel has ignited, it combines with oxygen in a chemical reaction to produce light, flames, more heat and smoke.

Heat, or ignition sources, include cigarettes, cooking equipment, lightening, electrical discharge, excessive heat from the sun, and so forth.

Fuel is anything that burns.

The thing about fire is that once it ignites and starts to burn, the combustion reaction with oxygen itself creates heat which keeps the process going, spreading and speeding the fire along. And the heat dries out the fuel, making it burn faster.

The only way to stop a fire is to take away one of these three components. Many suppression systems are designed to “smother” the fire by removing oxygen from the equation. And with the right types of fires, water can remove heat from the equation by cooling the fire. Fuel is the hardest link to break, but good management of combustibles can help prevent fires.

Fires go through four stages:

  • Ignition: This is where heat, oxygen and fuel come together in an ongoing chemical reaction. A fire at this stage generally has no more than a small flame. It can be controlled by a fire extinguisher.
  • Growth: As the fire burns, it begins to consume more fuel, creating more heat. The fire grows in size and the heat it produces can bring almost all combustible materials in an enclosed space to their ignition points simultaneously. When this happens, a “flashover” can occur, which engulfs the space in flames.
  • Fully Developed: Most, if not all, fuel and oxygen is being or has been consumed in this stage, and temperatures have peaked. This is where most of the heat damage occurs.
  • Decay: Frequently the longest stage, this is where the fuel has been consumed, the heat decreases and the intensity of the fire is reduced. It is a dangerous stage because the fire can reignite if combustibles remain or if there is an influx of oxygen to the fire.

Once burning, a fire can spread through any or a combination of three processes: conduction, convection and radiation. All require a transfer of heat.

Conduction is where burning fuel contacts some material that transfers, or conducts, the heat to other fuel that then ignites and burns. Metal is a good heat conductor and can transfer the heat from flames to other combustible materials, which then ignite and burn. In a building with metal support beams, for example, fire can spread from room to room through conduction.

Convection occurs when the heated gases produced by the combustion reaction (including air) rise, and cooler air, which is denser sinks. What makes this so dangerous is that in a building, the heated gases have nowhere to go, so they run along the ceiling. This creates a thick layer of heated air which will then start to descend, igniting all the fuel in the room.

Radiation allows fire to spread through heat being transferred along electromagnetic waves. Some examples of electromagnetic waves are microwaves and radio waves. Heat energy radiates out in all directions, and can even be carried through glass, such as windows. Combustibles inside will then burn. Radiation helps to explain the type of fire spread seen between buildings. With radiation, no direct contact with flames or burning or smoldering fuel is necessary for ignition.

General Approaches to Fighting Fires

There are four ways to extinguish a fire. The first is to cool what’s burning—the fuel. Water is probably the most common method used. This is where automatic fire sprinklers play a huge role. And since this is your first line of defense against fires, having a regular fire sprinkler inspection is critical. You need to know that your system is working at peak efficiency at all times.

While water may not be suitable for all types of fires (e.g. grease, electrical), fire sprinkler systems can be tailored to fit the needs of a business, down to the specific room or even a cabinet housing sensitive equipment. They can discharge the appropriate fire suppression agent for each particular need.

Smothering the fire by removing the oxygen is the second method of putting out a fire. Different types of fires, as noted above, may require specific suppression materials and methods to accomplish this.

The third way to extinguish a fire is to starve it by removing the fuel.

And the fourth method is to interrupt the chemical reaction of combustion. This requires the use of specific “clean” suppression agents, called halocarbon agents. An added benefit of these clean agents is that no residue is left behind that would otherwise damage sensitive equipment or other valuable articles. 

Types of Fires and their Characteristics

How many types of fire are there? In the U.S., there are five types of fire classification. Class A pertains to general combustibles like trash and fabric. Class B fires involve flammable liquids or gases. Class C are fires from energized electrical equipment. Class D fires arise from combustible metals like potassium and magnesium. And Class K is for fires in cooking appliances from cooking oils, fats or grease. To further explain these classes, we will delve into their different characteristics.

Class A Fires

This class of fire involves ordinary combustibles, such as paper, wood, fabric, many plastics and rubber. Ash is left behind from these fires. Industries susceptible to such fires would include paper mills, textile mills, lumber yards, professional firms (legal, architectural, accounting) and retail businesses.

Class B Fires

These happen when flammable liquids or gases, such as gasoline, propane or alcohol ignite. Class B fires can spread rapidly. Water is not a good way to extinguish these fires. You need to remove the oxygen by smothering, such as with foam. Industries that may be susceptible to this type of fire would be garages, industrial warehouses, and shipping.

Further, Class B fire extinguishers are required on recreational vessels. Marine fire suppression requirements for those vessels less than 65 feet in length are set forth, in part, under the Code of Federal Regulations 33 §175.320. And fire extinguisher service is a must, since these extinguishers need to be tested, maintained and possibly recharged on a regular basis.

Class C Fires

Electrical equipment that is energized, i.e. is actively drawing power, is what causes Class C fires. They can be triggered by things like faulty wiring, blown or overloaded circuits or failure to maintain equipment. Water cannot be used to fight these fires as it conducts electricity, creating a life-threatening situation. Clean agents, as described above, are best to combat them.

No matter how it starts, a vehicle fire is a Class B fire due to the flammable liquids on board—gasoline, diesel fuel, steering fluid, transmission fluid, etc. But commercial buses, trucks and truck tractors are required to carry B:C vehicle fire extinguishers pursuant to the Federal Motor Carrier Safety Administration’s regulations (CFR 49 §393.95). This is especially noteworthy since the most common cause of vehicle fires is mechanical or electrical issues. The industries most susceptible to Class C fires tend to be construction, trucking and broadcasting.

Class D Fires

Class D fires involve combustible metals. These include lithium, potassium, magnesium, titanium and zirconium. They burn extremely hot, and this extreme temperature can separate water into its component parts of oxygen and hydrogen, creating additional O2 to feed the fire. It can even cause explosions. So, water should not be used on Class D fires. These fires are fought by breaking the chemical reaction with dry powder agents. Even clean, dry sand can be used. Some industries in which Class D fires may break out are laboratories, manufacturing and industrial storage facilities.

Class K Fires

This last Class describes fires that occur in cooking appliances, such as stoves, that involve cooking oil and/or grease. These fires can flare up quickly, particularly if oil or grease is spilled or splashed onto the flame or eye of the stove. They can also flare up inside appliances that have not been properly cleaned, or where the oil or grease is allowed to overheat.

Class K fires spread fast. Water only makes them spread faster, so only a Class K fire extinguisher should be used. These extinguishers use wet chemicals that neutralize the fire through a process call saponification. Basically, this means that the Class K fires are put out by converting the oils, fats or grease into soap which smothers the fire.

Class K fires can occur in any eating establishment, in offices, and in homes.

Safety Requires Awareness and Proper Tools

A better understanding of fire types and their origins helps to maintain personal and public safety. Extinguishing these various fires requires a range of fire extinguishers, with each utilizing the most appropriate fire suppression agents inside. Water-type extinguishers work best for Class A fires. The multi-purpose dry chemical extinguisher works well on both Class A and Class B fires. Wet chemical extinguishers are commonly used for Class A and K fires. For both Class B and C fires, CO2 extinguishers may also be used. However, extra care should be taken with these, especially in closed spaces as CO2 extinguishers can reduce the oxygen in the surrounding air. Finally, the dry powder type of fire extinguisher is for use on Class D fires. This fire suppressant type can also be delivered using a shovel or scoop.

Always know your escape routes in case of a fire emergency. Be sure to have the appropriate fire extinguisher on hand, no more than 40 feet away and near a point of escape. Be sure you know how to operate your extinguishers. Remember PASS:

  • Pull the pin (point away from you and unlock)
  • Aim low (at the base of the fire)
  • Squeeze the lever slowly and evenly
  • Sweep the nozzle from side to side

Aside from awareness, the key to your personal and structural safety is ensuring your safety equipment is regularly checked, always charged and ready for use.