Backflow Prevention and Cross-Connection Control
The Core Principle
Cross-connection control exists to protect potable water from contamination or pollution. A cross-connection is any actual or potential connection between potable water and a source of contamination. Backflow occurs by backsiphonage (negative pressure in the supply) or backpressure (downstream pressure exceeds supply pressure). Candidates must distinguish these two mechanisms because device selection depends on which can occur.
Devices and Applications
Know the hierarchy: air gap (the only true physical separation, and the preferred method), reduced-pressure principle assembly (RP/ RPZ) for high hazard under both backflow types, double check valve assembly for low hazard, pressure vacuum breaker and atmospheric vacuum breaker for backsiphonage only, and hose-bibb vacuum breakers for sillcocks. Understand test cock locations, relief valve discharge, and that AVBs cannot be under continuous pressure.
Traps in Questions
- Confusing pollution (aesthetic/health degradation) with contamination (health hazard).
- Selecting a double check for a high-hazard application.
- Forgetting that a PVB cannot be installed where backpressure is possible.
- Assuming test frequency is universal; it is set by the authority having jurisdiction.
- Overlooking thermal expansion or a boiler connection as a cross-connection.
Worked Example / Rule of Thumb
Match hazard to device: high hazard plus possible backpressure → RP assembly; low hazard plus backsiphonage only → PVB; irrigation with no chemical injection and low hazard → double check (if local code allows). Rule of thumb: an air gap is always acceptable; a device is only as good as its installation orientation and drainage.
Remember: a backflow assembly must be tested by a certified tester at installation, after repair, and at intervals set locally, with results reported to the water purveyor.