NFPA 25, the standard governing the inspection, testing, and maintenance of water-based fire protection systems, explicitly mandates this requirement. The 5-year flow test is crucial for evaluating the internal condition of private fire service mains. It helps detect potential issues such as tuberculation, deposits, or other obstructions that could impede water flow during a fire. The test compares current hydraulic performance against the system's original acceptance tests or previous flow tests to ensure the water supply remains adequate for the fire protection systems it serves.

B. Once every 6 years: This frequency is not specified in NFPA 25 for this type of test and is therefore incorrect.

C. Once every 10 years: This interval is too long and does not meet the minimum safety and maintenance requirements established by the standard.

D. Once every 20 years: This extended interval would allow for significant, undetected degradation of the piping, posing a severe risk to system reliability.

1. NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems, 2023 Edition. Chapter 7: Private Fire Service Mains, Section 7.3.1, "Flow Tests." The text states, "Underground and exposed piping shall be flow tested to determine the internal condition of the piping at a minimum of 5-year intervals."

2. Cote, A. E., & Harrington, G. P. (Eds.). (2021). Fire Protection Handbook (21st ed.). National Fire Protection Association. Volume 2, Chapter 10, Section 4, "Private Fire Service Mains," subsection "Testing," page 10-58. The handbook reiterates the NFPA 25 requirement, stating, "A flow test of the main is required every 5 years to determine whether the internal condition of the pipe has changed."

3. Oklahoma State University, College of Engineering, Architecture and Technology. FST 3333 - Fire Protection Systems. Course materials covering NFPA 25 requirements for private fire service mains align with the 5-year flow test interval specified in the standard.