How does a sprinkler system work as an open loop system?

What you may be referring to, since the question was posted in electrical engineering section, is in reference to the electrical switches and devices installed on sprinkler systems. The systems have water flow devices, low air switches, valve tamper devices and the like. each device is designed to monitor changes in state in the system. (sprinkler waterflow, low air pressure in a system, an improperly open or closed valve, to name a few.) these devices are designed to send an electrical current or signal of high or low voltage to a device(bell or siren) or to an alarm or control panel. The terminals on these devices are similar. They have a "common" terminal. Next, they have an "normally open" terminal; which means the circuit is open normally, but closes during a change in state. The circuit is supervised with a end of line resistor in the circuit, hence an "open loop"