DerBlitzkrieger

yes, generally speaking, all sprinkler heads are under pressure, as long as the system is operating correctly. now, the majority of sprinkler systems are "wet" systems, in that there is water in the piping all the time. other types of systems are "dry," where the water pressure is replaced by air, preaction, and deluge. i wont go into specifics of the last two, but they are basically just specialized types of dry systems without air pressure. you dont see them too often.

anyway, sprinkler heads are rigorously tested by multiple sources. at the factory they are burst tested to 500 PSI. FM approves, and UL lists all sprinkler heads to at least a 175 PSI operating pressure, and some heads are rated to 300 PSI. and then again, the sprinkler contractor is required to pressure test the entire sprinkler system at 200 PSI for two hours before the system is accepted as being "in service"

its rare for a sprinkler head to just fail. im not saying it wont happen, its just not very likely. more often than not, sprinkler heads go off because something hits the glass bulb, be it a forklift truck, or a coat hanger.

the new style glass bulb sprinklers are quite delicate. the bulb itself is only 3mm thick in most instances, and its under pressure on only one axis. there was a story a couple of years ago about a kid who tossed a nickel at a sprinkler head in a department store. he managed to hit the glass bulb, and break it, much to everyones suprise, im sure.

now, getting back to sprinklers failing. tyco in particular has a 10 year warranty on all of thier products. the NFPA requires a representative sampling of sprinkler heads in a building to be removed every 50 years for testing. so like i said, its rare for a sprinkler head to fail and release water.

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300,000 Kilometers per second: It's not just a good idea. Its the law.