Which of the following can cause a loss of fire-main pressure?

A ruptured pipe can cause a significant loss of fire-main pressure because it creates a direct path for water to escape the system. When a rupture occurs, water flows out of the pipe rather than being delivered effectively throughout the fire-main system. This not only decreases the pressure but can also lead to reduced water availability for firefighting efforts, which is critical in emergency situations.

In contrast, while a blocked pipe can restrict flow and affect pressure, it doesn’t necessarily create a leak, so pressure may be maintained until the system is compromised further. An insufficient water supply might lead to low pressure, but it is not a structural failure of the system like a rupture. Similarly, a closed valve can obstruct flow but does not inherently reduce the system's pressure until the point of flow restriction. Thus, a ruptured pipe is a more definitive cause of pressure loss in the fire-main system.