Flashover
From Wikipedia, the free encyclopedia
- 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.
A flashover is the near simultaneous ignition of all combustible material in an enclosed area.
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. Flashover normally occurs at 500 °C (930 °F).
The classic example of flashover is where a piece of furniture is set alight in a domestic room. The fire on the furniture produces a layer of hot smoke across the ceiling in the room. The radiated heat from this layer causes pyrolysis (heating of the other surfaces in the room, causing them to give off flammable gases). 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.
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[edit] Different types of flashover
The original Swedish terminology specifies four types of flashover:
- A lean flashover is the ignition of the gas layer under the ceiling, leading to total involvement of the compartment. 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.
- 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.
- 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.
[edit] Dangers of flashovers
Flashover is one of the most-feared phenomena among firefighters. Firefighters are taught to pay much attention to recognize flashovers and avoid backdrafts. For example, they have certain routines for opening a closed door in a building in 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 two seconds to evacuate a room that has a flashover.
[edit] Flashover indicators
These are some of the signs firefighters are looking for, when they attempt to determine whether a flashover is plausible.
- 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.
- 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. The white smoke wasn't considered dangerous, so firefighters decided to ventilate, which caused a delayed flashover and killed two firefighters. The white smoke from the pyrolysis of the rubber turned out to be quite flammable.
