Water distribution systems are designed with the intention of the water flowing in a certain direction-from the distribution system to the consumer. However, hydraulic condition within the system may deviate from the "normal" conditions, causing water to flow in the opposite direction. Therefore, it is possible (and common) for water to flow in the opposite direction in unprotected systems. This is called backflow.
A cross-connestion is an unprotected actual or potential connection between a potable water system used to supply water for drinking purposes and any source of system containing unapproved water or a substance that is not or cannot be approved as save, wholesome, and potable. By-pass arrangements, jumper connections, removable sections, swivel or changeover devices, or other devices through which backflow could occur, shall be considered to be cross-connections.
Back siphonage is caused by mainline piping failures or drafting due to high demands. For example, it is a condition that can occur when the drinking water system pressure drops below that of the fire protection system, drawing the fire protection system water back into the drinking water system.