Which pressure measurement indicates the condition of the system when no water is flowing?

Static pressure is the correct choice because it refers to the pressure in a system when water is not flowing. This measurement represents the potential pressure available in the system and is crucial for understanding how the system will perform under different conditions. It is taken at rest conditions and does not include any pressures lost due to friction or flow, thus giving a clear indication of the pipe's ability to deliver water when flow is introduced.

In contrast, residual pressure is measured during flow conditions and indicates the pressure remaining in the system while water is moving. Flow pressure, meanwhile, is specifically the pressure in the system when water is actively flowing and is influenced by factors such as friction losses through the piping and fittings. Supply pressure indicates the pressure at the source, usually at the pump discharge or water main, which may differ from static or residual pressure based on the conditions of the system. Understanding these distinctions is important for effectively evaluating and managing fire suppression systems.