Which type of appliance enables the use of only one supply line at a time when necessary?

The Siamese appliance is designed to allow firefighters to connect multiple supply lines to a single water source, but it is particularly useful for situations where the fire service needs to tap into more than one hydrant or water source at once. Although it can take in multiple lines, it feeds them into a single outlet to supply water to the fire engine.

In scenarios where only one main supply line is needed, the Siamese can be configured to choose which line to draw from, effectively allowing the use of just one supply line at a time. This feature is critical in managing water flow and pressure, ensuring that resources are used efficiently and effectively during firefighting operations.

The other choices mentioned, such as Wye, pressure regulating valve, and hydrant adapter, serve different purposes and do not inherently limit the use to just one supply line at a time. For instance, a Wye can split one supply line into two, while a pressure regulating valve primarily adjusts the pressure of the water coming through, and a hydrant adapter is used for connecting hoses to fire hydrants without the specific constraint of limiting supply lines.