Final answer:
The minimum usable residual pressure for firefighting operations is about 20 psi, or 137,895 Pa, to effectively deliver water to fire-affected areas and operate firefighting equipment properly.
Step-by-step explanation:
The realistic minimum usable residual pressure from a water distribution system for firefighting operations is generally accepted to be around 20 pounds per square inch (psi), which equates to approximately 137,895 Pascals (Pa) or 1.38 × 105 N/m2. This pressure is needed to ensure that water can be effectively delivered to the higher floors of buildings and sufficiently cover the areas affected by fire. Firefighting hoses and nozzles are designed to operate within certain pressure ranges to be effective, and this minimum pressure is crucial for them to perform their function.