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Real back flow preventers have dual valves and test ports and are required to be tested and certified annually. The little Mickey Mouse ones on the black tank rinse are simply a band aid. Usually if you have positive pressure when turning off the hose and everything works nothing happens. Many frost proof hydrants when used as the water source and turned off drain back creating a small vacuum which can exacerbate a real problem.
Isolating and separation of the two systems and hoses as many said they do just makes sense and protects you and the next person attaching a hose to the bib.
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