Fire can be devastating in building interiors without effective fire protection measures. This is why New York City made automatic sprinklers mandatory for high-rise business occupancies with Local Law 26 of 2004. However, it is important to note that not all fires are the same: their properties change depending on the source, and the most effective method to extinguish fire also changes accordingly.
In the US, fires are grouped into five classes based on their properties: A, B, C, D and K. Note that European and Australian standards also have fire classes, but their names and properties differ slightly. Thus, in any application where fire classes are indicated, it is important to check which standard is being used.
When the wrong extinguishing method is used against fire, the consequences can range from having no effect to causing an explosion. For example, water should not be used when fire is caused by an electrical fault.
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Class A Fire: Ordinary Combustible Materials
Class A fires are produced from combustible materials of everyday use, including paper, wood, fabrics and plastics. As a result, these fires are the most common; for example, burning garbage will most likely produce a class A fire.
As you might expect, these ordinary fires can be controlled with the most intuitive method: dousing them with water. Monoammonium phosphate is also an effective agent against Class A fires, and it is commonly used in fire extinguishers.
Class B Fire: Flammable Liquids and Gases
Class B fires are caused by flammable liquids like gasoline, kerosene and paint; or by flammable gases such as methane and propane. These substances are common in industrial environments, but they can also be found in residential and commercial settings.
Water is not effective against Class B fires because it can disperse the flammable fluid, spreading the fire instead of controlling it. Therefore, you must use a method that removes oxygen or a method that extinguishes fire with a chemical reaction.
Monoammonium phosphate can also be used against Class B fires. It displaces the air in contact with fire, cutting the supply of oxygen.
Sodium bicarbonate is an example of the second alternative, where fire is extinguished with a chemical reaction.
Class C Fire: Electrical Sources
Electrical faults can cause intense heating, potentially starting a fire. As you might guess, these fires should not be sprayed with conductive agents under any circumstances, and this includes water. To eliminate a fire of electrical origin, there are two important steps:
Disconnecting the electric supply that provides power to the fault.
Using a non-conductive agent to extinguish the flames.
Class C fires tend to occur in industrial locations with large electrical equipment such as motors and transformers. However, they can also occur in residential and commercial settings when an electrical installation has been installed incorrectly or lacks adequate protection.
Monoammonium phosphate and sodium bicarbonate, the two extinguishing agents described in the previous section, are also effective against Class C fires. Both substances are non-conductive and can be used safely when electrical equipment is involved.
Class D Fire: Combustible Metals
Fires caused by combustible metals represent a unique challenge, since many of these metals have chemical properties that pose additional hazards. Combustible metals are most commonly found in laboratories, and they also have applications in many industrial processes. Therefore, a Class D fire is more likely research and manufacturing applications, especially when the metal is present in the form of powder.
Lithium and potassium are combustible metals that explode when exposed to water, forcing the use of other extinguishing agents.
Other examples of combustible metals are titanium, magnesium, aluminum and zirconium.
Class D fires are extinguished with dry powder agents, which absorb heat while blocking off the oxygen supply that sustains the combustion reaction. Many of these dry powder agents are based on sodium chloride or graphite.
Class K Fire: Food Preparation
Class K fires involve liquid substances used in food preparation, including cooking oil, grease, lard, butter and olive oil. These fires are not included in Class B, since they have unique properties that require special attention. As you might expect, Class K fires are of special concern in the restaurant industry and commercial kitchens in general.
Class K fires are normally controlled with wet chemical extinguishing agents. Water mist can also be used successfully against them, but a stream of water should be avoided because it can accidentally spread the ignited substance.
Fire protection is a technically complex topic, subject to stringent standards in the US. You can only ensure code compliance and building safety when protection systems are designed by qualified fire protection engineers. For example, your building must have adequate extinguishers for all the fire classes that can be expected.
Having code-compliant fire protection systems not only improves building safety. You can also get better coverage conditions from insurance companies, and reliable fire protection makes commercial spaces and apartments more attractive for tenants.
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