: Backflow occurs when the water pressure in the distribution system drops below the water pressure at the end of the service. This can occur when the distribution system is shut down for any reason (for instance, when there is a water main break), and it creates a situation where materials or contaminants can enter the distribution system.A backflow assembly (also known as a Reduced Pressure Principle Assembly, or RPPA), is a device that reacts to the pressure drop and prevents material or contaminates from entering the distribution system if a pressure drop occurs.