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Posted By Steven
I am having trouble in understanding how the fire ratings are worked out for fire extinguishers. Class A extinguishers are rated against time taken to extinguisher a wooden crib of certain dimensions ie 21A rated extinguisher will put out a wooden object 2.1m square.
However I cannot get my head round how a class B extinguisher is rated – must be TF its Friday syndrome.
An explanation would be appreciated.
steve
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Rank: Guest
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Posted By Derek Housley
Hi, BS En 3-7:2004 on Portable Fire Extinguishers covers this area. The test is carried out in laboratory type conditions
In brief a circular tray of varying diameters is filled with a flammable liquid ( 1/3 water 2/3rds fuel) and fired.
The amount of liquid starts at 21 litres with a 920 mm tray. If the fire is extinguished 21B rating is acheived, the tray size is then increased to 1,170mm with 34L of liquid and fired etc.
The process is then repeated increasing the dimensions of the fire each time, until the extinguisher fails to put out the fire. The extinguisher would then be normally be rated at the rate that it knocked the fire out at. The tests are repeated to exclude "fluke" results
The highest rating is 233B, for a tray of (yes) 233L of mix and 3m dimension.
There is a toleration of plus or minus 10 mm on the tray size. Hope that helps in clearing the smoke
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