What are the different types of backflow preventers, and where are they typically installed in a plumbing system?

Backflow preventers are devices installed in plumbing systems to prevent the reverse flow of water, which could contaminate the potable water supply. There are several types of backflow preventers, each designed for different applications and levels of hazard. An atmospheric vacuum breaker (AVB) is the simplest type, consisting of a float check valve that opens to atmosphere when the water pressure drops, preventing back-siphonage. AVBs are typically installed on lawn sprinklers and other non-continuous pressure applications. A pressure vacuum breaker (PVB) is similar to an AVB but includes a spring-loaded check valve that closes when the pressure drops, providing greater protection against back-siphonage. PVBs are also used on lawn sprinklers and other non-continuous pressure applications. A double check valve assembly (DCVA) consists of two independently operating check valves, providing protection against both back-siphonage and backpressure. DCVAs are used in situations where there is a low to moderate hazard, such as connections to fire sprinkler systems or commercial dishwashers. A reduced pressure principle assembly (RPZ) is the most reliable type of backflow preventer, providing the highest level of protection against both back-siphonage and backpressure. RPZs include two independently operating check valves and a pressure-relief valve that opens to atmosphere if either check valve fails. RPZs are used in situations where there is a high hazard, such as connections to hospitals, chemical plants, or sewage treatment plants. Backflow preventers are typically installed at the water meter, at the point of connection to the public water supply, and at any location where there is a potential for contamination of the potable water system.