A clear-looking hose connection has a contamination route

A fictional commercial installation uses a flexible hose between a potable outlet and equipment containing a nonpotable treatment solution. The equipment can develop pressure, and the hose end can also become submerged. A worker says a clear appearance and one ordinary check valve prove that every contamination risk has been removed. The current hazard classification, protective device suitability and commissioning evidence are not supplied. This classroom exercise introduces no chemical and changes no connection. Identify the difference between backsiphonage and backpressure, why an actual protective design is needed and what evidence should be recorded without inventing a universal device type for an unspecified contamination category.