No, potable water systems in many places are totally open in the sense that water from the house can flow back out into the mains. Only when you throw in a check vavle does it become closed. A PRV has one, the utility may install one either in their water meter or as part of the installation either now or in the future. They do tend to replace the meters, and newer ones often come with a check valve, thus, some municipalities require an expansion tank to be installed so there isn't a rash of service calls when their T&P valves start leaking.
T&P Valve looking at two sides.