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A non-mechanical backflow prevention arrangement of water fittings with a vented, but submerged, inlet; the air gap being measured vertically downwards from the lowest point of the air inlet to the critical level.
Maximum permissible fluid category against which a Type AC air gap is accepted as adequate backflow protection:
Backflow by back pressure: fluid category 3
Backflow by back siphonage: fluid category 3
Note: overflow and warning pipes shall discharge through or terminate with an air break, the dimensions of which should satisfy a Type AA air gap.
Interpretation given in Regulators’ Backflow Specification - backflow prevention arrangements and devices: backflow prevention arrangements