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The height of an air gap between the lowest part of the outlet of a tap, combination fitting, shower head or other fitting discharging over any appliance or other receptacle, and the spillover level of that appliance, where a fluid category 4 or 5 risk is present downstream.
Maximum permissible fluid category against which a Type AUK3 tap gap is accepted as adequate backflow protection:
Backflow by back pressure: not acceptable for protection against back pressure from any fluid category
Backflow by back siphonage: fluid category 5
Interpretation given in Regulators’ Backflow Specification - backflow prevention arrangements and devices: backflow prevention arrangements