Friday the 13th came a week early.

[Jay 2K Winger]

Actually, since each sprinkler has a heat sensor (solder melts or volatile liquid boils and shatters the vial holding it, allowing the valve in the sprinkler head to open), it means the sprinkler system is also an array of heat sensors. If the water starts flowing, it means one of the heat sensors has detected a fire, therefore the alarm needs to go off. One flow sensor in the pipe feeding the sprinkler system can trip the alarm if any sprinkler detects a fire.

Of course, mechanical damage to the sprinkler system will also make the water flow, so the fire alarm is also a "clumsy forklift operator" alarm.

Actually, the fire alarms CAN go off if they detect a drop in the water pressure.

The high school I attended when I lived in Plano, TX was across the street from a golf course. It seemed like once a week during the spring, we'd have an impromptu fire drill because our water system was on the same as the golf course. The sprinklers there would go on to water the fairways and such, and the water pressure would drop. Fire alarms at the school detect this, doesn't know the difference, and BEEPBEEPBEEPBEEP.