Editing Backflow prevention device (section)

{{Short description|Device that prevents undesired water flow}}
{{refimprove|date=March 2013}}
[[File:Back siphon example hosepipe up hill.svg|thumb|In this situation, dirty water from the hillside pool will ''back-[[siphon]]'' or ''back-flow'' down the [[hosepipe]] and into the clean water tank. To prevent this from happening, the house's external [[Tap (valve)|tap]] should be fitted with a backflow prevention device.]]

A '''backflow prevention device''' is used to protect [[potable water]] supplies from contamination or pollution due to [[backflow (plumbing)|backflow]].

In [[water distribution system]]s, water is normally maintained at a significant pressure to enable water to flow from the tap, shower, or other fixture. Water pressure may fail or be reduced when a water main bursts, pipes freeze, or there is unexpectedly high demand on the water system (for example, when several [[fire hydrant]]s are opened). Reduced pressure in the pipe may allow contaminated [[ground water|water from the soil]], from storage, or from other sources to be drawn up into the system.