Type AB – Air gap with weir overflow

A non-mechanical backflow prevention arrangement of water fittings complying with Type AA, except that the air gap is the vertical distance from the lowest point of the discharge orifice which discharges into the receptacle, to the critical water level of the rectangular weir overflow. 

Maximum permissible fluid category against which a Type AB air gap is accepted as adequate backflow protection:

Backflow by back pressure: fluid category 5

Backflow by back siphonage: fluid category 5

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