The figure below depicts a possible backsiphonage scenario. In this drawing, a motor vehicle collides with a fire hydrant installed on the public water main (B). The crash causes the hydrant to break releasing water from the public main. The ensuing drop in water pressure in the public water main creates suction that draws dirty water from a second floor bathroom in a nearby home into the public main (A).
The second story bathtub presents a cross connection hazard because...
It has elevation (second story). It is a tall column of water that must be constantly supported by the water pressure in the public main. Even a momentary loss of support will cause this column to fall.
The faucets might be installed below the bath tub rim level or overflow drain. In these situations, the faucet (a connection to potable drinking water supply) can become submerged creating a nexus for the introduction of unsanitary water into the drinking water supply.
A temporary hose might be affixed to the faucet and allowed to submerge in the bathwater. This situation would create the same conditions as a submerged faucet described above.