Fire extinguishers and fire supression for computer equipment:
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We use the Fike ECARO-25 fire suppression system. It is "clean agent" gas system that does not harm the electronics and is safe for people in the computer room. Our suppression system is tied to the fire alarm system which has a battery backup. I recommend visiting http://www.fike.com for more information about computer room suppression system.
We had a fire a few years ago and the chemical extinguisher did more damage to the electronics than the fire. In all our areas with electronics we installed CO2 extinguishers (in addition to ABC extinguishers near by). I understand the CO2 extinguishers leave no trace behind.
The "standard" for protecting computer and networking equipment (and other high-value assets) during a fire has been total flooding with Halon. Unfortunately Halon is also somewhat harmful for the environment and its production has ceased (although it is still sold, mostly from existing supplies and reclaimed Halon from existing systems). Halon is incredibly effective at extinguishing nearly all types of fires - it works by displacing the oxygen required for the reaction.
A number of manufacturers have produced Halon replacements which have lower environmental impact and are purportedly just as effective at fire suppression. Most of these replacements are less toxic and have a lower impact on ozone depletion than standard fire suppressants. My research led me to recommend DuPont's line of fire suppression technologies which are available for total flooding systems as well as portable extinguishers from various manufacturers. The price of an extinguisher using FE-36, for example, is somewhat higher than comparable dry chemical systems but the extra expense is usually appropriate for protection of expensive electronics.
It's important to purchase an appropriately sized extinguisher for the location and type of fire. For example you would need a 9.5 lb FE-36 extinguisher for 1A-10BC classification for a 200 sf room (minimum room size). You may need larger or smaller extinguishers based on the types of combustibles present and size of the room.
The NFPA website <http://www.nfpa.org/> used to have extensive fire extinguisher information available but I couldn't find it just now; perhaps it's now part of their subscription service. The EPA and OSHA websites have information available about the safety and environmental impacts of the chemicals used in fire suppression equipment. I would echo the recommendation of talking to a local service provider - your school may already be working with one to service your existing equipment.