Which type of backflow preventer is NOT approved for protecting the potable water supply for a lawn irrigation system?

The atmospheric-type vacuum breaker is not approved for protecting the potable water supply of a lawn irrigation system due to its limitations in safeguarding against backpressure. This type of device is designed to prevent back-siphonage, but it does not provide adequate protection in situations where backpressure could occur, which is a risk in irrigation systems.

The other types of backflow preventers mentioned, such as pressure vacuum breakers, reduced pressure zone assemblies, and double check valve assemblies, are specifically designed to handle various conditions, including both back-siphonage and backpressure. This makes them suitable for installation in situations where the risk of contamination to the potable water supply is significant, such as in lawn irrigation systems. Therefore, while the atmospheric-type vacuum breaker has its uses, it falls short of the necessary standards for ensuring complete protection in this context.