Hazard Fire and thermal radiation
Type of fire
Having established the location of a fire, the incident commander and firefighters will need to give early consideration to the type of fire they are confronted with. Identifying the type of fire is critical as it will help form the basis of an initial incident plan or strategy and clarify decisions on the method for extinguishing the fire, the type of media needed and how best to apply it.
To accurately identify the type of fire, it is imperative that firefighters possess a sound fundamental understanding of the physics and chemistry of combustion, including areas such as:
- Physics of combustion
- States of matter and energy, density, heat transmission and temperature
- Chemistry of combustion
- Atoms and molecules, elements, compounds and mixtures, pyrolysis and flammable range
This knowledge, combined with experience, will enable firefighters to understand the relationship between the types of combustion, phases of fire and the products of combustion (smoke) and how these are likely to interact in a fire situation. They will then be able to make effective judgements and/or decisions to ensure that any fire is appropriately, safely and efficiently extinguished.
In simple terms, firefighters should have an understanding of what a fire is and the conditions required for it to start (ignite) and grow (develop) so that they can tackle and extinguish the fire and be safe and effective in their role.
Fire is the result of a chemical reaction called combustion. Combustion is a chemical reaction between oxygen and a fuel source that produces chemical energy in the form of heat and light.
To understand the process of combustion and the subsequent behaviour of a fire, firefighters need to have a basic understanding of the elements required for combustion to occur. In the first instance, a combustible fuel must be present, along with oxygen in sufficient quantities and a source of ignition.
This is commonly represented by the 'triangle of fire'. For many years, this simple triangle of fire (oxygen, fuel and heat) has been used to illustrate our basic understanding of fire. While it remains useful, from a technical perspective, there is a fourth component in the chemical chain reaction, which is extremely important for fire suppression and extinguishment.
The three elements of oxygen, fuel and heat need to be sufficiently balanced for combustion to occur. They must then continue to interact in such a way that they create a self-supporting chemical chain reaction to maintain the combustion process. This is described as the fire tetrahedron. Each component of the tetrahedron must be in place for self-sustaining combustion to continue.
Figure 1: The fire tetrahedron
A fire will typically develop in four clearly defined phases (or stages). By being able to easily recognise these different phases (or stages) through observation of signs and symptoms and/or use of their senses, a firefighter will better understand the method, tactics and techniques that may need to be employed to deal with the respective phases.
An accidental fire normally begins with an incipient phase during its early stages. This stage of fire growth may occur very slowly but in some instances could proceed quite quickly.
The growth phase incorporates fairly rapid fire development, depending on an adequate availability of fuel, heat and oxygen (fire triangle). This stage normally concludes in flashover unless restricted by a lack of any of fuel, heat or oxygen.
Fully developed phase
The fully developed stage represents a period of burning where the maximum heat is released (termed 'steady state' fire) and flames fill the compartment or area. This is usually a period where the majority of the energy in the fire load is released.
The decay stage follows the fully developed fire. During this period, much of the energy in the fire load continues to be released but to a lesser degree as the burning rate decreases.
Time/temperature curve - the phases of fire development
A time temperature curve demonstrates the phases of fire and shows the demarcation between each stage of fire development. During the incipient phase of a fire, which will be an upward curve, the time can vary depending on the type of fuel, the size of the compartment and the amount of oxygen supplying the fire. Flashover may occur at the end of the growth phase and the start of the free-burning (or fully developed) stage. Backdraught can occur in the smouldering (or decay) phase.
Figure 2: Fire Development curve
The type of fire encountered by firefighters varies enormously, both in terms of what is involved and its size. However, by having good knowledge and giving consideration to the classifications of fire (as detailed in BS EN 2: 1992 - Classification of fires) firefighters can anticipate or make very early judgements on the most appropriate extinguishing media.
BS EN 2: 1992 - Classification of fires broadly identifies classes or groups based on the physical properties of the material/s actually burning:
- Class A fires: fires involving ordinary combustible materials, such as wood, cloth or paper
- Class B fires: fires involving flammable liquids, greases and gases
- Class C fires: fires involving gases
- Class D fires: fires involving combustible metals, such as magnesium, titanium or sodium
- Class F fires: fires in combustible cooking fuels such as vegetable or animal fats
Electrical fires are not considered a class of fire on their own and may better be described as fires involving electricity. Once the source of electricity is isolated and any residual current dissipated or earthed, a fire can be classified and treated as one of the primary classes.
Although this standard that sets out the types of fire that may be encountered is based on the material or substance type, many incidents attended by firefighters are likely to involve a combination of material or some of these groups in isolation. For example, a well-developed fire in a take-away or restaurant may potentially involve a combination of all of the substances identified in each of the classes A-F.
One of the tasks for the incident commander and firefighters will be to identify the primary material group involved in the fire, including the amount of fire loading, how it is stored and the form in which it is found. For example, large structural timbers may behave differently when burning, compared to timber shavings exposed in a fire of the same intensity.
The process of identifying the fundamental material or substance group involved will be critical to the subsequent decision-making process. It will help to ensure that the overall method of intervention and extinguishment selected is appropriate and is the most efficient way of dealing with the fire.
The method of extinguishing a fire is based on interrupting one or more of the essential elements in the combustion process. With flaming combustion, the fire may be extinguished by reducing temperature (cooling), eliminating fuel or oxygen (starving or smothering), or by stopping the uninhibited chemical chain reaction. If a fire is in the smouldering mode of combustion, only three options exist: reducing the temperature, eliminating fuel or eliminating oxygen.
Once the type of fire is clearly identified and the incident commander and crews have selected an effective method and approach to putting the fire out, they will need to consider selecting the most appropriate extinguishing media to achieve the objective. This may include:
- Dry powder
- Carbon dioxide
- Beating out
- Controlled burn
- Allow to burn out
The method for extinguishing the fire and the type of firefighting media selected will influence the way the media is applied and the choice of firefighting techniques.
Water is usually the default media type because of its abundance and its rapid impact on the chemical process of combustion. Techniques and equipment can include simple hose lines and branches.
More complex media may require more complex techniques of application because of lack of availability, insufficient amounts being available or the manner in which they need to be produced, such as producing foam.
Identifying the type of fire, selecting an appropriate method to extinguish it along with appropriate media and the most appropriate techniques and equipment to apply it are frequently decisions taken simultaneously without any significant delay on the speed or timeliness of an initial intervention.
This may also depend on the type of incident as there may be an opportunity for initial intervention to contain the fire so that crews can perform the immediate life-saving rescue of casualties. It may then be possible to withdraw to consider the safest and most efficient, method, media, techniques and equipment, such as a major foam application or allowing the fire to burn out.
By determining what is involved (the type of fire) at the earliest opportunity, subsequent decisions for the fire and rescue service personnel on firefighting method, appropriate firefighting media and the techniques of application can become natural and intuitive processes.
Extent of fire
Locating a fire, its type and overall extent are critical steps in the initial stages of any incident. The information and intelligence established by the incident commander and crews at this early stage will set the foundation for implementing a basic plan to bring the situation under relative control.
Where the situation expands, for example in a large developing fire, the initial incident plan may need to be revised as more factual and predictive information is obtained or where more resources become involved and the incident command structure grows.
Protecting life and preventing deaths and injuries at a fire are clearly the primary objective for fire and rescue services in any fire situation, whether this is to protect members of the public, ensure the safety of firefighters, or minimise the social and environmental impacts of fire.
For further information, see National Operational Guidance: Incident command - Safety Management
Often the best way to protect lives and minimise damage is simply to extinguish the fire quickly. However, the ability to contain, gain overall control (surrounding the fire) and extinguish a fire will vary enormously and depend on many factors including the type of fire and its broad environment. For example:
- A fire in a high-rise building may be complex with regard to access, egress and the number of resources required to deal safely with the situation.
- A wildfire incident may cover a wide geographical area and require the deployment of significant resources from a fire and rescue service or numerous fire and rescue services over a period of weeks
Even in the simplest of circumstances, fire and rescue services will need to consider the best way to achieve timely intervention and the correct speed and weight of intervention for a given incident (context of operations) and ensure the appropriate deployment of resources to deal with the fire in a safe and effective manner.
The speed and weight of intervention in any organisation will largely depend on the capacity of a fire and rescue service to deliver crews, emergency fire vehicles, equipment and media to an incident ground to control, contain and extinguish a fire.
It is imperative that the incident commander and firefighters have a good knowledge of the number and range of resources that they have at their disposal and carefully consider what they may need to deal with fires that they may encounter in their service areas.
During the initial assessment and evaluation of a fire, the incident commander will have considered the level of risk, the resources they require and how quickly they will need them - the speed and weight of intervention to have an impact on the fire. The initial incident plan will also depend on the available or responding resources being assembled in a considered manner. For example, if a delay occurs for whatever reason, the fire could grow and spread rapidly, affecting firefighters, casualties or members of the public.
If the fire develops rapidly or is too large for the initial attendance, the incident commander will need to exercise their operational judgement and determine the actions that can be taken safely with the resources at hand, for example, whether the crews should continue with an offensive strategy, limit their actions to protecting surrounding exposures or wait for the fire to burn out.
One of the most crucial decisions that an incident commander will have to make during firefighting operations is whether it is necessary to attack a fire internally or externally. To some degree, this will be influenced by the environment in which the fire has occurred, largely dictating whether the incident commander adopts an offensive or defensive firefighting strategy.
In deciding the most appropriate fire attack, incident commanders will need to look at the various factors and consider the following:
- Risk versus potential benefits
- Whether it is safe to commit firefighters to an area or structure
- Whether there are any obvious casualties or lives at immediate risk
- Whether the size, type or intensity of the fire limits or prohibits entry
- Whether there are sufficient resources in attendance to undertake a safe and effective intervention (personnel and emergency fire vehicles)
- The nature of the building construction
- Whether there are adequate water supplies available to sustain an attack
- Additional hazards and their location in relation to the area on fire
- Access points and maintaining egress points
- Fire safety features in a building
- Information from a responsible person
- Specialist advice
An objective for all fire and rescue services, their incident commanders and firefighters, is to protect property. In some situations it will result in the incident commander having to exercise operational judgement by making difficult decisions such as sacrificing property to save life or, with a fire in a derelict structure, focusing resources on protecting surrounding buildings or risks to infrastructure.
A priority for an incident commander and firefighters will be to prevent the fire from spreading. This will depend on the level of risk and how far the fire has already spread. For example:
- In a domestic dwelling, if the fire is confined to a single room, the aim should be to prevent it spreading to neighbouring rooms or compartments
- In a high-rise structure, if multiple flats or apartments are already involved, the incident commander may decide to focus on containing the fire to a single floor.
- If a fire has spread substantially through a building or geographical area, it may be necessary to focus on protecting and limiting firespread to the neighbouring structures, high-risk areas, critical equipment or infrastructure
Incident commanders should use all the factual and predictive information at their disposal to anticipate, look ahead and identify a point at which crews can intervene safely with an appropriate firefighting method and suitable firefighting media. For example, they should:
- Look for, capitalise on or seek to create, natural gaps, breaks or separations between buildings to minimise the effect of radiated heat. See National Operational Guidance: Fires in buildings for further information.
- In a wildfire situation, use the information to predict the path and/or head of the fire and identify a safe point to intervene and create a firebreak by removing the fuel to starve the fire, either by cutting or burning away sources of fuel. See National Operational Guidance: Wildfires
- A fire involving flammable liquid will usually require the use of foam as the primary extinguishing agent. The crews will aim to lay a blanket of foam over the surface area or the pooling fuel to prevent ignition or to smother the fire by preventing the vapour of the substance mixing with the air and source of heat.
Preventing or limiting firespread in all instances is vital. It is worth considering the use of thermal imaging cameras, which can play a valuable role in helping to identify sources of heat and potentially highlight any areas of unseen firespread through structural voids such as roof space. They can be deployed externally during the scene survey or internally by breathing apparatus teams. In addition, they may act as a key control measure to help crews remain safe in this context.
Dealing with a fire will invariably result in firefighters engaging in a range of activities or tactics, employing a number of different methods, techniques, extinguishing media or items of equipment to complete the respective tasks that may be necessary to extinguish the fire.
The suppression of the fire will usually be accomplished through removing one of the elements of the fire tetrahedron to stop the process of combustion.
The fire will essentially be extinguished by cooling, starving or smothering with firefighting media, or through a combination of those methods, which inherently or by design will inhibit or interfere with the chemical chain reaction that is combustion.
Other methods such as using a fire blanket (smothering), creating firebreaks (starving) or beating out the fire can be very effective ways to suppress and extinguish the fire. It is also possible that the incident commander or firefighters may undertake controlled burns to remove fuel sources from an advancing fire, such as cutting away timbers of a roof structure to limit or prevent spread.
Alternatively, an incident commander may simply choose to focus on protecting surrounding exposures or risks and let the fire burn out, effectively starving the fire of the fuel element of the fire tetrahedron. The firefighting media selected will be a direct result of the initial assessment and evaluation carried out by the incident commander once they have identified the type of fire they are likely to be dealing with. There are four main categories of media:
- Carbon dioxide (CO2)
- Dry powder
Fire and rescue services will usually be able to choose from a range of firefighting methods and media to support extinguishing the many different types or classes of fire they may encounter. However, the emergency fire vehicles, equipment and techniques employed by most fire and rescue services will more often than not revolve around water as the most frequently used extinguishing agent.
Water is generally accepted to be the most effective firefighting media because it is the cheapest, most efficient and readily available media for extinguishing fires of a general nature. It is used by fire and rescue services for the majority of fires, although the techniques of application can differ.
Water is the most common fire-extinguishing agent because it has a range of physical and chemical properties and can be found in a range of sources - hydrants, lakes, ponds, streams, rivers, sea, even swimming pools. It is non-toxic and, in comparison with other types of media, due to its unique chemical composition, has a high specific heat capacity and high latent heat of vaporisation.
The level of fire loading, and its reaction to the building or structure in which it is located during any incident involving fire, will have an impact on the decision-making process and incident plan. Therefore, it is important to consider the impact that the nature, amount, and orientation of the fire loading will have on a fire situation.
The nature and diversity of the substances and materials that will be encountered in contemporary buildings (sandwich panels, timber framed, steel portal framed, etc.) will have an impact on the process of combustion and may produce energy, achieving peak temperatures and heat release rate at a faster rate than in conventional buildings.
See: National Operational Guidance: Fires in Buildings
This is important from a firefighting perspective, as this can mean that firefighters may encounter rapid fire development conditions at an incident whilst the structure is simultaneously deteriorating in stability and integrity.
An assessment of the fire load, its location with regard to ignition sources and the likelihood of any firespread should be considered at an early stage as part of a dynamic risk assessment.
Items involved in fire
Any fire can have the potential to involve sealed metal objects which, when heated, can pose an explosion and/or projectile hazard. These sealed metal objects can vary in size, use and construction and are all subject to the laws of pressure and heat.
The following lists a few examples of sealed metal objects:
- Domestic radiators (with valves closed)
- Central heating boilers http://www.hse.gov.uk/services/localgovernment/boilers.htm
- Vehicle airbag inflators
- Electrical transformers
Additionally, emergency service personnel should be made aware of the more obscure sealed metal objects which may have a potential to cause harm, when heated. These objects include welded box sections in the construction of vehicles and skips and sealed metal objects used in building construction.
Fires in vehicles pose a variety of hazards to firefighters due to the number of potential materials and inbuilt systems involved and the location of the vehicle in proximity to other hazards e.g. buildings, other vehicles, open dry land and members of the public. Fires in vehicles can develop rapidly, producing high heat release rates which can have an effect on surrounding structures. Examples of vehicle fire hazards include:
- Plastic or composite fuel tanks (running fuel fire risk)
- Compressed gas struts
- Compressed gas airbag inflators
- Pyrotechnic gas generators (Sodium Azide)
- Magnesium components
- Petroleum and alternative fuel (LPG, electricity, hybrid, biofuels)
- Hydrofluoric Acid
- Air conditioning systems
Contain, control or extinguish
Having identified the type of fire and its location, varying degrees of dynamic activity will take place as fire and rescue services and other responders begin to build a picture of what they are faced with in a particular scenario.
The process of information gathering will be intensive at this stage, but diligence will support the development of the approach, tactics and techniques required to extinguish the fire safely and efficiently.
During this period it is critical that the incident commander and firefighters ascertain the overall extent of the fire, including the phase of development, how it is behaving and the likelihood of firespread.
Many factors may affect the development and extent of a fire in any given situation. In the first instance, it is important to note the impact of the location of the fire and its interaction with the environment in which it has occurred, such as a wildfire in the open environment or a fire in the built environment, a vehicle or a vessel.
Although the fundamental principles related to the physics and chemistry of combustion are broadly the same with any fire, its behaviour and development may vary significantly depending on the environment in which it occurs. For example, a fire in the open environment will be strongly affected by the overall meteorological conditions and the geography and topography of a wide area. The ambient temperature and time of day combined with the angle or gradient of a slope will have a dramatic effect on the pre-heating of fuels and consequently on the likelihood of firespread.
One of the primary effects of a fire in the built environment will be the impact of compartment boundaries (ceilings, floors and walls) and openings (doors, windows and hatches) on a fire's development and behaviour. See National Operational Guidance: Fires in buildings and the Building Research Establishment knowledge sheets for further information.
To understand and establish the likely extent of a fire and how it can spread, firefighters should have a fundamental knowledge of the physics of combustion. In simple terms, this states that fires can spread or grow through the transmission of heat in four fundamental ways:
- Direct contact
An understanding of the theories of convection, conduction and radiation, combined with a good basic knowledge of what fire is (combustion) and the typical phases of fire, will enable an incident commander to predict the likely pattern of firespread in a given scenario.
Firefighters will also need to know how fire behaves. There are many factors that can affect a fire's behaviour in a given situation. These include the materials or substances involved, the fire loading, ventilation, wind and other meteorological conditions and the size of the building or area.
Having a good knowledge of the key fire phenomena such as flashover, backdraught and fire gas ignition, and how they can occur, is a requirement for all firefighters. Even more important is their ability to interpret the key behavioural indicators that are the step and transient events that lead to a fire.
By understanding how a fire behaves, firefighters will be safer and more effective at predicting the potential extent of any fire, limiting its spread and subsequently selecting the most appropriate time and method to be used to intervene and suppress and extinguish the fire.
The extent and development of a fire will be heavily influenced by the degree of ventilation to the compartment or area involved. In its most fundamental form, ventilation can be defined as the amount of air available to the fire; this is clearly linked to the chemical process that is combustion:
- A ventilation-controlled fire occurs when there is not enough air to support the complete combustion process of the fuel in a fire compartment
- A fuel-controlled fire is where there are adequate amounts of air but not enough fuel to support the combustion process of the fuel in the fire compartment
The impact of ventilation, whether pre-existing such as through open doors or windows, through the effect of building design and systems such as heating, ventilation and air conditioning (HVAC), or where it is used as an intervention technique by firefighters, will have an effect on the state of the fire and can also assist in making the conditions more acceptable for casualties, occupants and firefighters. Where ventilation is used as an offensive intervention tactic, incident commanders should ensure that there is a clear ventilation strategy to prevent unplanned or unwanted outcomes.
The extent of a fire and its likely development will also be acutely influenced by what is actually involved, such as materials, substances or processes. Ascertaining information about the type of fuel and the amount of material present - the fire loading - along with how long a fire has been burning, should therefore always be considered by the incident commander and crews before deciding on the appropriate intervention and whether to adopt an offensive or defensive approach.
The level of fire loading will, given adequate ventilation, help an incident commander to predict the likely extent and severity of firespread that may be experienced during firefighting operations.
Problems of high fire loading may be worsened by poor housekeeping, by lack of site management by owners and occupiers, or by emerging issues such as hoarding in domestic properties; these factors increase the potential for rapid firespread or unusual fire development.
Most of this information can be acquired from various sources, including the responsible person, but incident commander and firefighter knowledge, judgement and situational awareness, and their ability to read and interpret key fire behaviour indicators (including the signs and symptoms of phenomena such as flashover and backdraught), are vital.
Once the incident commander and firefighters have an understanding of where the fire is, what is involved and to what extent it is involved, they will then be in a position to make a more accurate assessment of the resources that will need to be deployed. As the picture becomes clearer, they will be able to predict any additional resources required, taking into account the necessary speed and weight of intervention. This will allow crews to begin the process of containing, controlling and extinguishing the fire in a safe, appropriate and effective manner.
- Control measureAppropriate speed and weight of intervention