What should be installed between a fire hydrant and the main?

The correct choice highlights the importance of having an auxiliary valve installed between a fire hydrant and the main water supply. An auxiliary valve serves critical functions in water distribution systems by allowing for isolation of the fire hydrant from the main line. This isolation is essential for maintenance or repair work, as it permits the hydrant to be serviced without disrupting water supply to the rest of the system.

Moreover, the auxiliary valve helps prevent potential backflow issues by allowing controlled flow and access to the hydrant. In emergencies, firefighters can also quickly open the auxiliary valve to access water without needing to manipulate the main supply directly, thus ensuring rapid response to fire incidents.

The presence of an auxiliary valve is a standard practice in the design of hydrant systems to ensure safety and operational efficiency. Other components, while essential in certain contexts, do not fulfill the same duties as the auxiliary valve in terms of providing control and operational flexibility at the hydrant.