Which of the following is a key characteristic of a properly functioning hose-bib vacuum breaker?

A properly functioning hose-bib vacuum breaker is designed primarily to prevent backflow of contaminated water into the clean water supply. This is accomplished by having a mechanism that allows air to enter the system if a drop in water pressure occurs, thereby breaking any potential siphon formed by the vacuum effect.

When the hose-bib is not in use, the vacuum breaker effectively seals off the water supply, preventing any contaminants from being drawn back into the drinking water. This characteristic is crucial for maintaining water safety and is essential in various settings such as residential homes and commercial food establishments.

Other characteristics of a hose-bib vacuum breaker, such as allowing continuous water flow or automatic self-cleaning, do not directly contribute to its primary function of preventing backflow. In fact, if continuous water flow were allowed, it would negate the very purpose of preventing contamination. Similarly, requiring manual operation would also compromise its effectiveness and convenience, as it must be able to perform automatically to ensure safety at all times.