A backflow preventer is used to protect water supplies from contamination or pollution. In water supply systems, water is normally maintained at a significant pressure to enable water to flow from the tap, shower, hose, etc. When pressure fails or is reduced as may happen if a water main bursts, pipes freeze or there is unexpectedly high demand on the water system, the pressure in the pipe may be reduced and may allow contaminated water from the ground or other sources to be drawn into the system. To prevent this, there should be an air gap or mechanical backflow prevention assembly installed between the delivery point and storage or the water. Where submerged mains inflow is permitted, a backflow prevention assembly is required. In this way the backflow prevention assembly protects the potable water system from contamination hazards which can be severe or even fatal. There are over 10,000 reported cases of backflow contamination each year.