In a plumbing system, when is indirect drainage required?

Indirect drainage is an important aspect of plumbing design that helps prevent contamination of the potable water supply. This method is particularly required for fixtures that release waste water which may pose a risk of backflow or cross-connection, such as sinks, dishwashers, and similar appliances. The indirect drainage system allows waste water to be discharged into a receptor, such as a floor sink or a hub drain, rather than directly into the drainage system. This creates a separation between the waste source and the drainage, thereby reducing the chances of any potential backflow contaminating clean water supplies.

The requirement for indirect drainage is particularly significant in settings like food service where food-related appliances cannot directly drain into the sewage system due to health standards. Understanding this helps to ensure that plumbing systems comply with safety regulations, thereby maintaining public health standards and preventing contamination. In this context, it is clear why the correct answer involves fixtures that include sinks and dishwashers, which need specific considerations for waste discharge and protection of the overall plumbing system.