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In a movie they all go off at once
It's a curious thing, but in just about every movie in which fire sprinklers are seen, they all go off at once. As soon as someone sets off one of the fire sprinklers, ALL the sprinklers in the building are started automatically, soaking everything and everyone in the place. The movie mistake is so common that many people actually believe that's what fire sprinklers really do, and it may even put people off the idea of having a fire sprinkler system fitted.
In a way, it's an easy mistake for story writers to make. They've never seen a fire sprinkler system doing what it does if there's a fire, and they've probably seen so many other movies in which the fire sprinkler cliché is used, that they make the assumption that it's the truth. In some ways it helps the story-writing, because one person can do a single action which can cause widespread effects. However, moviemakers should beware, because the perpetuating of the myth is putting people's lives at risk.
let's get the fact straight:
Fire Sprinklers DO NOT all go off at once!
It may be a bit inconvenient when trying to write a plot, but really, you've managed OK without airships being a dreadful fire risk (they're full of non-inflammable helium), and if a car goes off the edge of a cliff, it's acceptable for it to burst into flames after it hits the ground rather than in anticipation just after leaving the cliff edge. So, for a quality story writer, it should be reasonably easy to factor in the truth about automatic fire extinguishing sprinklers!
How a fire sprinkler system really works:
Generally, a fire sprinkler system consists of water pipe plumbing going to each of the sprinkler heads, each of which has a fire detector consisting of a small bulb or link which fails if it's heated up. The system is a bit like the handbasins in a building; most of the time, all of the taps/faucets are turned off. If someone turns one of them on, water comes out, without affecting the others. Essentially each fire-detecting sprinkler works on its own. If the heat near the bulb or fusible link gets too much, it will break, opening a valve which is like a trap-door. Once open, water will spray out, under pressure. There's a plate built in which spreads the water out in a fine spray, which is quite good at putting out a fire. The advantage of this selective system is that only the areas that are on fire get sprayed. As only one or two outlets are spraying, it saves the water pressure, concentrating the effort on the affected area where the fire is.
You can see it from a design point of view; there's no signal wires having to go all around the building, and the only connection required is a central plumbing system to connect up all of the sprinklers. The wonderful simplicity means there's nothing much to go wrong.
If your computer has been soaked by a fire sprinkler, don't panic. At least it didn't burn. Data Recovery people can rescue your data from a hard disc drive that's been wet, and it's much easier than trying to recover it from ash.
It may seem a bit strange having a sprinkler system in a library, because, surely it's going to ruin the books? Well firstly it's only going to ruin the books that are in the section where the fire started (because the sprinklers don't all get set off just because one of them has been triggered). Plus, fire does more damage than water. If a fire starts, you have a minute or so to put it out with a fire extinguisher, and after that, if the sprinkler goes off, it's only happened if the temperature on the ceiling is hot enough to melt/burst the detector.
Fire sprinklers are not smoke alarms. With a smoke alarm, the smoke is detected which results in the alarm making a loud noise. It's a curious fact that most people who die in a fire are killed by smoke, not fire. They die in their sleep. However a smoke alarm saves people because they wake up, realise there's a fire, and then get a chance to make their escape.
Sprinklers are not set off by smoke, but by heat. It requires quite a high temperature to set off a sprinkler, but if it reaches that temperature then the reason is usually a fire which needs putting out. The amount of water required to put the fire out at that point is considerably less than would be applied by the fire brigade, especially if the fire got a chance to spread and got a hold on the place.