Cross-connection is any actual or potential connection between a public water supply and any other source or system through which it is possible to introduce, by backflow, any used water or substance other than the intended potable water.
Backflow, or the reversal of the normal flow of water, can be caused by "backpressure" or "backsiphonage."
"Backpressure" occurs when the downstream water pressure exceeds the supply pressure. This can occur through a rise in the downstream pressure, a drop in the supply pressure, or a combination of both. Increases in downstream water pressure above supply water pressure can be created by booster pumps or temperature increases (e.g. in a boiler).
"Backsiphonage" occurs when the supply line pressure falls below atmospheric pressure (14.7 psi at sea level). In this situation, atmospheric pressure is creating a greater pressure on the downstream side in the reverse direction of flow, and the supply water system falls below atmospheric pressure. Decreases in the pressure of the supply water system may be caused by fire fighting, breaks in the water main, or water mains being shut down for maintenance.