Water mist extinguishers provide a multiclass rating option to dry chemical or clean agent devices and can be rated for use on energized electrical equipment (Class C fires). While it seems incongruous that electrically conductive water could obtain a Class C rating, protecting the operator from electrical shock while applying water to an electrically energized appliance or piece of equipment is similar to a hand-held fog nozzle being used on energized electrical equipment. When the water is sprayed and the droplets of water are not connected in a solid stream, the electric current path is broken between the droplets, preventing current flowback to the operator of the extinguisher.
Water mist extinguishers were introduced in the United States in 1997. Initially, these extinguishers were simply modified stored-pressure water extinguishers that included a spray nozzle allowing the extinguisher to
meet all of the requirements of American National Standards Institute/Underwriters Laboratories (ANSI/UL) 8, Water Based Agent Fire Extinguishers, and ANSI/UL 711, Rating and Fire Testing of Fire Extinguishers, in a way that enabled the extinguisher to earn a 2-A, 1-B:C rating.
Later, requirements were added so that a water-based extinguishing agent in an extinguisher had to have an
electrical resistance of less than 1 million ohms (1 microsiemen). Plain water used in the first water mist extinguishers was changed to deionized water, but the same principles applied to the extinguisher.
A water mist extinguisher tested to ANSI/UL 8 and ANSI/UL 711 is considered safe for the operator to use on electrically energized equipment or appliances—that is, they will not deliver an electrical shock back through the discharge stream. Also, they offer enhanced Class A firefighting capability due to the exceptional cooling capability of the water spray. The deionized water extinguishing agent is nontoxic, environmentally friendly (zero ozone depletion potential and zero global warming potential), has no disposal problems, and is inexpensive.
The extinguishers are easy to use and have a long discharge duration. They are rechargeable and have a low maintenance cost.
This Coffee Break Training tip is courtesy of the Fire Equipment Manufacturers’ Association.Source: http://www.usfa.fema.gov/downloads/pdf/coffee-break/cb_fp_2012_46.pdf