Posted By Stuff4blokes Smash a head and lots of water comes out, fast. Break a pipe and it happens in a much more spectacular way. Break a 6" main and it'll blow most things away. Bells go off, alarms sound and evacuations start, possibly fire brigade turns up too. Laundry bill for the plonker who caused the mayhem. Very, very annoyed managers and workforce especially if on performance bonuses. If you're a contractor expect claim for damage which may be big. Sprinklers operate on heat. Hot work sets them off if too close. Most are made of steel or brass and hurt lots if you bash your head. Few people ever set a sprinkler system off by accident for a second time! Bottom line: don't mess with sprinklers. Very early in my career I managed to fill up an open-topped MG sports car with water when I opened the 2" drain and neglected to find out where the pipe went after it disappeared through the wall. That cost a bit, but I've never done it again.