A cross connection is an actual or potential physical connection between our drinking water and anything that could pollute or contaminate the water supply. For example, food and beverage processing equipment, without backflow preventers, has the potential for backflow to occur and contaminate the water supply.
Backflow is the undesirable reversal of flow of non-potable water or other contaminants through a cross connection and into the water distribution system. When the flow is reversed or "backflow" occurs, contaminated water can flow backward, drawing a contaminant into the water supply. There are two types of backflow - backpressure and back siphonage.
Backflow is typically caused when an unforeseen change in water pressure allows the water to flow in reverse of the normal direction in the water distribution system, for a contaminated source to a drinking water supply. A drop in pressure could be caused by a variety of circumstances, including a water main break or the need to fight a fire.