Vacuum breaker
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An attachment commonly placed on a bibcock valve, that prevents water from being sucked backward from a garden hose into the public water system. This prevents contamination should the public water system's pressure drop.
A vacuum breaker typically contains a plastic disc that is pressed forward by water supply pressure, and covers small vent holes. Should the supply pressure drop, the disc springs back opening the vent holes (which let in outside air), and preventing backflow of water.
A more complex valve that accomplishes much the same purpose is the backflow preventer.
