Where must valves not be installed to avoid isolating fire sprinkler piping?

In the context of fire sprinkler systems, it is crucial to ensure that the system remains operational and effective in the event of a fire. Valves that can isolate parts of the sprinkler piping system could prevent the system from functioning properly, thereby compromising safety and fire suppression efforts.

The correct choice highlights that valves should not be placed in any location where they might inadvertently isolate the fire sprinkler piping. This is in accordance with the principle that fire sprinkler systems must remain fully operational at all times. If valves were allowed in certain areas and could potentially be closed, they might inadvertently cut off water supply to critical sections of the system.

While considering the other options, installing valves near the main shutoff is generally acceptable, as these serve a specific purpose in managing the entire system rather than isolating it. Installing valves in fire-rated ceilings can also be done, provided they are appropriately protected, to ensure they do not interfere with the integrity or function of the fire protection system. Lastly, while high water pressure areas might require special considerations, this does not inherently prevent valve installation. Hence, the outlined rationale solidifies why the answer indicates that valves must not be installed in any location that risks isolating the fire sprinkler piping system.