Are you old enough to remember Mr Ed? If so then you will remember, “A horse is a horse of course of course...” The implication is, all horses are the same really. Similarly, many people think all fires are the same. Only when you see that there are different fire extinguishers, you start to realise all fires are not all the same.
In fact, in Australia, fires can be classified into five distinct categories, each determined by the agent that fuels them: Class A, Class B, Class C, Electrical, and Class F. This classification system helps to assess hazards and determine the most effective method to extinguish the fire. It also helps to identify what is the best or most suitable form of passive fire protection and what the FRL is.
Let’s take a look at the different types of cellulosic fires and what you need to know.
Class A Fires
Class A fires are the most common type of fire and the sort that the vast majority of us are familiar with. They involve (cellulosic) combustible materials such as wood, paper, fabric and common items found in rubbish.
Class A fires are typically caused accidentally by people when striking a match or starting a bonfire, knocking over a candle, or a stray spark from a fireplace, or a lightning strike on a tree.
Class B Fires
Class B fires include ignitable fluids or gases such as petroleum grease, oil, alcohol, paint, propane, or gasoline. However, Class B excludes fires that involve cooking oils or grease. These sorts of fires may happen anywhere flammable liquids or gases are used or stored.
Class C Fires
This class of fire includes combustible gases such as LPG (liquefied petroleum gas), butane, propane, or LNG (liquefied natural gas, acetylene), hydrogen, butane or methane.
Electrical fires involve electrical gear and might be started by old wiring in walls, frayed electrical cords, faulty switches, destroyed or worn-out breaker boxes, or faulty appliances and machines. These fires are exceptionally common in both homes and industrial settings.
Class F Fires
These are ordinarily chip-pan fires and involve combustible cooking media such as vegetable or olive oil, fat, lard, butter and grease normally found in kitchens – especially cafes, restaurants and commercial kitchens.
What is a cellulosic fire?
Cellulose is a natural polymer that exists abundantly in nature as a constituent of plants and microorganisms. Cellulose-based fibres such as paper, lumber, leaves, wood and such are often utilised in the building and textile industry, filtration, and often as part of fibre-reinforced composites.
So, it should be no surprise that the term ‘cellulosic fire’ is coined after cellulose, the fundamental trigger or primary fuel. In other words, this sort of fire is fuelled by materials that consist of high amounts of cellulose such as paper, lumber, leaves, wood etc. These items all catch fire easily and also spread flames easily.
Fundamentally, a fire needs three things to burn: oxygen, heat, fuel, (also referred to as the fire triangle) and sometimes a chain reaction.
Cellulosic fires are mostly common in residential and commercial buildings. Fires of this nature have a slow flame and in comparison to other fires – spread gradually. From the point of ignition to the point where the flame spreads easily (approximately 500°C), it can take just 5 minutes. The temperature can then continue to climb to around 1100°C with a radiation value that has been measured at 50 kW/m² (Kilowatt/square metre).
Cellulosic fire curve
Corresponding with a growing interest in how to prevent fires as well as the devastation caused by fires, there has been improvement in testing. Using the information acquired from these tests, a series of time/temperature curves for the various exposures has been created.
For each building space, fire curves can be designed that indicate the temperature dependency over time and such curves are valuable tools for designing fire safety in buildings. It is these curves that then help the planning for fire control measures. The curves are used to determine how hot a temperature may get, and the impact it will have on the building elements exposed to heat.
The stages of a building fire
By most norms including the International Fire Service Training Association (IFSTA), there are 4 phases of a fire. Here is a brief overview of each stage:
Incipient: This is the first stage and begins when heat, oxygen and a fuel source combine to either cause a chemical reaction or reach an ignition point to start a fire. Also known as ‘ignition’, it is typically represented by an exceptionally small fire, which frequently (and ideally) goes out on its own, before the accompanying stages are reached. Your best opportunity at suppression or escape is from recognising a fire in this stage.
Growth: At this stage the structure’s fire load and oxygen become active fuel for the fire. Numerous factors influence this phase including where the fire started, what combustibles are near it, ceiling height and the potential for ‘thermal layering’.
Despite being the shortest of the 4 stages, it is during this phase when a deadly ‘flashover’ can occur, potentially trapping, injuring or killing firefighters. A flashover is defined as the point where the ambient temperature is so high it can cause almost simultaneous combustion of all fuel sources in the vicinity, thus causing the fire to grow almost exponentially.
Fully Developed: A fire is considered fully developed when the growth stage has consumed all flammable materials in the vicinity. This is the hottest phase of a fire and the most dangerous for anybody trapped within.
Decay: Usually the longest stage of a fire, it is characterised by a significant decrease in oxygen and/or fuel. Both will eventually lead to the end of the fire. Two common perils during this stage are the presence of non-flaming combustibles, which can conceivably light another fire if not extinguished completely; and a backdraft when oxygen is reintroduced to a volatile, confined space.
Cellulosic Fire Protection
When we look at cellulosic fires, there are two key things to note in fire protection:
- Behaviour that can be noted in duration, growth rate and peak combustion
- The fuels, the structural elements exposed and the area being protected
Cellulosic fires and the protective materials utilised are classified into categories A, B and F for products that offer protection against cellulosic fire.
Passive fire protection (PFP) is a fundamental part of fire safety and structural fire protection for any building is part of the Building Code of Australia. In other words, the building design should incorporate passive fire protection such as fire boards, fire collars, or intumescent paint for cellulosic fire in components such as the timber structural frame of the building; cellulosic fire shutters; cellulosic compartment walls, partitions and floors; ceilings; fire-resisting glazing; and fire doors and hardware.
When used in combination with active fire protection, passive fire protection works to suppress and stop fires, prevent the spread of flame, permit people to evacuate in an orderly and safe way, and limit damage to the structure itself. While they work independently, one system is not better than another; both systems need to be functioning properly to assist in the suppression of a fire.
Compliance and Legal Obligations for Building Owners
The Building Code of Australia is a uniform set of technical provisions for the design and construction of buildings and other structures. Understanding them as well as the fire resistance levels (FRL) for various components in a building is critical to guaranteeing compliance. Given legal effect by State and Territory legislation, the Building Code of Australia (BCA) necessitates that documentation in regards to a building must be created and retained with respect to a wide scope of performance requirements, including fire resistance of building elements, fire hazard properties and resistance to the incipient spread of fire.
In compliance with legal obligations, building owners must ensure that all their building works conform to the BCA, which has explicit provisions pertinent to fire safety systems and components utilised in a building. Specifically, all fire rated materials utilised in a building or structure must comply with Australian Standards, with AS4072.1 and AS1530-4 explicitly pertinent to fire rated elements.
Consult with Permax if you need more insights on how to effectively comply with legal requirements on fire resistance and protection.