Final answer:
In a preaction fire sprinkler system, the preaction valve is opened to allow water into the piping system in response to a fire alarm signal before the sprinkler heads activate.
Step-by-step explanation:
In a preaction fire sprinkler system, the preaction valve is opened before the sprinkler heads. Preaction systems are a type of fire protection system where the piping is filled with air or nitrogen under pressure, and water is held back by the preaction valve. This valve is electronically operated and typically opens in response to a fire alarm signal, allowing water to enter the piping system. The sprinkler heads themselves will only activate and release water when they are heated to their operating temperature by a fire. The design of preaction systems provides an added level of protection against accidental discharge and is especially useful in areas where water damage must be minimized.