Pro Plumbers! Under what circumstances does a RPZ backflow preventer operate? RPZ backflow preventers are required for medical and restaraunt water systems in...

RPZ backflow preventers are required for medical and restaraunt water systems in my state. I think through that they are supposed to prevent biologically or chemically contaminated water from backing up previous the meter into the fresh water system.

What I want to know is, under what circumstances would that ever ensue? It seems to me that the water pressure surrounded by the lines would not allow contamination to run backwards through the system. Would the plumbing of the building have to be completely under hose down for this to ever happen?

I realize this isn't the 'plumbing' section, but if in that is a plumbing section, I couldn't find it!
Question almost electric showers...?
Answers:    the meter could fail and allow it to backflow..they r required here where on earth i live on all houses..

lic. gen. contractor