Containment vs. isolation
Two complementary protection strategies: containment protects the public main at the service connection, while isolation protects users inside the building at each hazard.
Containment and isolation describe where backflow protection is placed. Containment means installing a backflow preventer at or near the service connection, typically just after the meter, so that whatever happens inside the property cannot reach the public distribution system. Isolation, sometimes called fixture or internal protection, means installing protection at each individual cross-connection inside the building, such as at a boiler feed, an irrigation tie-in, or a chemical dispenser.
The two strategies protect different populations. Containment guards the community by sealing off the entire property, but it does nothing for the people inside the building, who still share internal piping with every internal hazard. Isolation protects building occupants at each specific risk point, but a missed or failed internal device leaves the main exposed unless containment also exists. Many programs therefore require containment at the service while plumbing codes require isolation at fixtures, and the two work in layers.
Testers and property owners should identify which role each assembly on a property performs, because the roles carry different expectations. Containment assemblies are usually the ones tracked and tested under the purveyor's program, typically annual where required, set by the water authority. Isolation devices may fall under local plumbing enforcement instead. Knowing which authority governs which device prevents gaps where everyone assumes someone else is responsible for a given preventer.
Related terms
| Cross-connection control program | The organized system of surveys, device requirements, testing, and recordkeeping through which a water purveyor or authority manages backflow risks. |
| Degree of hazard | The classification of a cross-connection by the severity of harm its contaminant could cause, typically as a health hazard or a non-health hazard. |
| Water purveyor | The utility or organization that owns, operates, and supplies a public drinking water system and is responsible for the quality of water it delivers. |
| Annual testing | The periodic field testing of backflow prevention assemblies, typically annual where required, set by the water authority, that verifies each assembly still works. |
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