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The background and the basics since the introduction of water treatment and piped reticulation systems utilities have strived to supply pure drinking water or potable water. With treatment plants it has been with the refining of filtration processes and chemical dosing. In addition to piping systems being developed in plastics, all these invitations, coupled with national standards and legislation, brought us the water quality we have today. There is one final area that is often over looked, but it is again a vital part of insuring people can consume pure water, without risk of contamination caused by cross connection, and for the last 50 years there has been a great deal of interest and effort put into addressing this issue.
 Double check valves were introduced around the turn of the century, believe it or not but to isolate fire mains and industrial water supply from the portable water supply. Unfortunately little interest was shown then until the 1930's when a device consisting of two check valves with a relief valve between them was conceived and successfully proved and by the late 30's the move was on to develop vacuum breakers and backflow prevention devices. This was all happening in the USA, which then continued where ordinances were being enforced and the Safe Drinking Water Act was bought into legislation. Here in Australia backflow really emerged in the early 1980's and currently today we have the Australian standard AS3500 which covers application installation and testing procedures for backflow devices to which most water utilities have adopted, and TAFE colleges run accreditation courses where Licensed plumbers are instructed in all aspects of backflow prevention. AS3500.0 defines potable water as:- "Water that is suitable for human consumption" which tends to be a broad statement, however the Health Department do have a more specific definition and requirements. A cross connection in simple terms is the actual or potential connection between the water supply installation and a source of actual or potential contamination. Backflow is caused by the unwanted reverse flow of a liquid in a piping system and can be caused by back siphonage or back pressure.
 Back siphonage is caused by a drop in supply pressure creating a negative pressure in the water system, this can occur through a vacuum in the system due to high water usage (draw off) of a broken main pipe causing a lowing of mains pressure, also flushing of the water main can also cause the same negative pressure resulting in back siphonage high velocity water perhaps due to under size piping can cause a venturi or aspirator effect from pipe intersections thus causing vacuum and subsequently reverse flow. Gravity can also cause back siphonage, when supply stops or is lost and an elevated fixture is opened allowing air into the system water will reverse flow backflow can also be caused by back pressure where an increase in downstream pressure creates a flow reversal this could be caused by a pump pressure higher than mains pressure, thermal expansion from boiler or indeed elevated water storage tanks.
The fundamentals of backflow/cross connection control are:-
LEVELS OF PROTECTION
The majority of water utilities in Australia although adopting the Australian Standard AS3500, have so with regard to containment protection only which ensure it safe guards their water supply, however there has been a noticeable increase in all protection level in the public Hospital sector. Respectfully,  John Gill "Plumbers Protect The Health Of The World." 
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