During the event the city water main has a break in the pipe, the pressure in the pipe can change and cause a backflow and cause things to siphon into the water mains. When a house is on fire, the fire department connects to the fire hydrant and starts pulling water from the system, this can cause a difference in pressure and cause a backflow in nearby buildings connected to the same water system. Fire sprinkler systems are fed and supplied from the same water for drinking. Water that supplies a fire sprinkler system is stored in the pipe waiting to put out a fire. Water that sits in a sealed system not being used becomes stagnate and grows bacteria. This water is supplied from the same pipe that supplies fresh drinking water and a backflow device is needed to keep the filled stagnate water from ever going backwards and contaminate the system in the event of a backflow . Every year you notice the city going around and flushing out the fire hydrants on the side of the road. Twice a year each fire hydrant is flushed to ensure it works, but also to flush out that stagnate water.

Water mains are fed from water sources and then feed buildings; this is a confusing network of pipes. The water supplier supplies water in a city water main to towns, buildings and houses. The water leaves the water tower then travels in water mains throughout the neighborhoods to each building in town. From the street back to the city is known as water main, from the street to each building is known as a water service. Then from the meter to the rest of the house is known as a water distribution system. Backflow devices are used once you have a water supply in a building and then connected to the system before feeding different applications.

Most people have seen a backflow device and never even knew it. A vacuum breaker on the end of a hose spigot is a form of backflow device. After you shut off the hose, the remaining water discharges on the lawn threw this vacuum breaker, like wise in the event of a backflow the vacuum breaker will pop open and break the siphon and not allow any water to go backwards into the building and into the water main system. Not all hose spigots have this built in vacuum breaker, and if your hose spigot is older than twenty years then there is a good chance you do not have this built in safety.