Backflow prevention is the term used to prevent the reverse flow of water from a potentially polluted or contaminated source into drinking and bathing water supply systems.

Backflow is caused by cross-connection. This is any point on a water system where a polluting substance may come in contact with potable (drinking) water. This may include any system piping that allows access to the potable water supply, any connected auxiliary water supply, submerged inlets, by-pass arrangements, removable sections and temporary devices.

This is prevented by installing a backflow prevention device such as a one-way valve which prevents the contaminants from entering the potable water supply. These are supplied in testable and non-testable types depending on the risk associated with the possible contamination. Testable devices require annual testing to ensure satisfactory operation.