Figure 6-9. Siphon action in plumbing system.
DIRECT CROSS CONNECTIONS
Direct cross connections, when found, are most Iikely in industrial or commercial
buiIdings where both potable and nonpotable water systems exist. An example would
be a small community whose potable water supply is not sufficient for fighting large
fires, but which has a readiIy available source of nonpotable water with which to
augment the potable system in an emergency. Such systems could be so designed that
the fire hydrants may be supplied with either potable water, nonpotable water, or both.
Such an arrangement would always present the hazard of a direct cross connection.
Another example of a solid pipe connection is a cooling system whereby potable water
is supplied to a cooling jacket and subsequently discharged into a sanitary sewer
system. Department of the Army construction criteria prohibit any physical connection
between the potable water system and the sewer system, or cross connections with an
unsafe supply. Nevertheless, conditions that allow backflow frequently occur because
of improper construction, subsequent changes in piping, or carelessness of users.
INDIRECT CROSS CONNECTIONS
Indirect cross connections, because of their very nature, are more difficult, to
detect than direct cross connections; however, they are more Iikely to occur because
they are usually the result of an oversight in the installation or use of water supply and
plumbing fixtures. The following are a few examples: