- In electric power transmission, a flashover is an unintended high voltage electric discharge over or around an insulator, or arcing or sparking between two or more adjacent conductors. Electric power transmission, a process in the delivery of Electricity to consumers is the bulk transfer of electrical power An insulator, also called a Dielectric, is a material that resists the flow of Electric current. An electric arc is an Electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive
A flashover is the near simultaneous ignition of all combustible material in an enclosed area. For an Authority Having Jurisdiction, combustibility is defined by the local code
Flashover occurs when the majority of surfaces in a space are heated to the point (known as fire point) at which they give off flammable gases that are hot enough to sustain combustion. In Physics, heat, symbolized by Q, is Energy transferred from one body or system to another due to a difference in Temperature The fire point of a Fuel is the temperature at which it will continue to burn after ignition for at least 5 seconds Pyrolysis is the Chemical decomposition of organic materials by heating in the absence of Oxygen or any other reagents except possibly Steam Flashover normally occurs at 500 °C (930 °F) for ordinary household combustibles.
The classic example of flashover is where a piece of furniture is set alight in a domestic room. Furniture is the Mass noun for the movable objects which may support the human body (seating furniture and beds, provide storage or hold objects on horizontal The fire on the furniture produces a layer of hot smoke across the ceiling in the room. Smoke is the collection of airborne solid and liquid Particulates and Gases ref> ''Smoke Production and Properties'' - SFPE Handbook of Fire Protection Engineering The radiated heat from this layer causes pyrolysis (heating of the other surfaces in the room, causing them to give off flammable gases). Thermal radiation is Electromagnetic radiation emitted from the surface of an object which is due to the object's Temperature. Pyrolysis is the Chemical decomposition of organic materials by heating in the absence of Oxygen or any other reagents except possibly Steam When the surface temperatures become high enough, these gases ignite and, in the space of a few seconds, every surface in the room may be on fire.
When the phenomenon occurs in open air, it is called a firestorm. A firestorm is a Conflagration which attains such intensity that it creates and sustains its own wind system
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Different types of flashover
The original Swedish terminology related to the term 'flashover' has been altered in its translation to conform with current European and North American accepted scientific definitions as follows:
- A lean flashover (accepted scientific terminology is rollover) is the ignition of the gas layer under the ceiling, leading to total involvement of the compartment. Rollover (also known as flameover) is a stage of a Structure fire when fire gases in a room or other enclosed area ignite The fuel/air ratio is at the bottom region of the flammability range.
- A rich flashover occurs when the flammable gases are ignited while at the upper region of the flammability range. This can happen in rooms where the fire subsided because of lack of oxygen. The ignition source can be a smouldering object, or the stirring up of embers by the air track. Smouldering (or smoldering in American spelling is a Flameless form of Combustion, deriving its heat from oxidations occurring on the surface of a The internationally accepted scientific definition of such an event is known as 'backdraft'.
- A delayed flashover occurs when the colder gray smoke cloud ignites after congregating outside of its room of origin. The results can be very unpredictable, and if the ignition occurs at the ideal mixture, the result can be a violent smoke gas explosion. The internationally accepted scientific terms for this process are either smoke explosion or fire gas ignition depending on the severity of the combustion process.
- A hot rich flashover occurs when the hot smoke with flammable gas ratio above the upper limit of flammability range and temperature higher than the ignition temperature leaves the compartment. Upon dilution with air it spontaneously ignites and the resultant flame can propagate back into the compartment, resulting in an event similar to a rich flashover. The internationally accepted definition of this process is known as auto-ignition which is another form of fire gas ignition.
Dangers of flashovers
Flashover is one of the most-feared phenomena among firefighters. Firefighters are rescuers extensively trained primarily to put out hazardous Fires that threaten civilian populations and property to rescue people from car accidents collapsed Firefighters are taught to recognize flashovers and avoid backdrafts. A backdraft is a situation which can occur when a Fire is starved of Oxygen; consequently combustion ceases but the fuel gases and Smoke remain at high For example, they have certain routines for opening a closed door in a building on fire, such as sitting beside the door instead of in front of it, and to be ready to fight shooting flames.
Despite superior protective gear, a firefighter has less than 17 seconds to evacuate a flashover environment if wearing proper NFPA approved gear. Firefighters are rescuers extensively trained primarily to put out hazardous Fires that threaten civilian populations and property to rescue people from car accidents collapsed
Flashover indicators
Here are some of the signs that firefighters are looking for, when they attempt to determine whether a flashover is likely to occur.
- If the room is closed or mostly closed, so the oxygen of the room has been consumed, a flashover is plausible. Flashovers in open air are very rare.
- "Rollover" or tongues of fire appear (known as "Angel Fingers" to firefighters). Rollover (also known as flameover) is a stage of a Structure fire when fire gases in a room or other enclosed area ignite
- A rapid build up of heat is generally the best indication of a flashover.
Firefighters memorize a chant while in training "Thick dark smoke, high heat, rollover, free burning. " So they memorize the four signs of a flashover.
The color of the smoke is often considered too, but there is no connection between the color of the smoke and the risk of flashovers. Traditionally black, dense smoke was considered particularly dangerous, but history shows this to be an unreliable indicator. For example, there was a fire in a rubber mattress factory in London in 1975, which produced white smoke. A mattress is a mat or pad usually placed atop a Bed, upon which to sleep or lie A factory (previously manufactory) or manufacturing plant is an industrial Building where workers manufacture goods London ( ˈlʌndən is the capital and largest urban area in the United Kingdom. Year 1975 ( MCMLXXV) was a Common year starting on Wednesday (link will display full calendar of the Gregorian calendar. The white smoke wasn't considered dangerous, so firefighters decided to ventilate, which caused a smoke explosion and killed two firefighters. In firefighting, ventilation refers to the tactic of creating a draft with an opening above or opposite the entry point so that heat and smoke will be released permitting The white smoke from the pyrolysis of the rubber turned out to be extremely flammable.
See also
External links
- Living Room Flashover Video
- Rapid Fire Progress & Flashover related fire development
- Realistic hot fire training to deal safely with flashover and backdraft
- Flashover / Backdraft training
- Photo and video of a flashover in a living room (Forschungsstelle für Brandschutztechnik (University of Karlsruhe)
Dictionary
flashover
-noun
- The near simultaneous ignition of all combustible material in an enclosed area.
- An unintended electric discharge or arc over or around an insulator