A backflow prevention device is used to protect potable water supplies from contamination or pollution due to backflow.

In water supply systems, 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 hydrants are opened).

Reduced pressure in the pipe may allow contaminated water from the soil, from storage, or from other sources to be drawn up into the system.