Type AC – Air gap with vented submerged inlet and circular overflow

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