What is the purpose of a pressure vacuum breaker?

The purpose of a pressure vacuum breaker is primarily to protect against both back pressure and back siphonage, which is vital in maintaining the safety and integrity of the potable water supply. By creating a physical barrier, a pressure vacuum breaker prevents contaminants from the system from flowing back into the clean water supply due to changes in pressure, whether that be a drop in pressure or a maintained vacuum. This is particularly important in applications where hazardous substances might be introduced into the plumbing system.

The correct answer encompasses the broader safety function of the device, as it not only addresses the need for relief during sudden pressure drops but also handles conditions that could lead to back siphonage effectively, ensuring comprehensive protection. The other choices focus on aspects that do not capture its full protective capacity against both pressure changes and siphoning effects, thereby underselling the critical role of the pressure vacuum breaker in plumbing design.