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A mechanical backflow prevention device for fitting to the outlet of a hose union tap and consisting of a single check valve with air inlets that open if the flow of water ceases.
Maximum permissible fluid category against which a Type HA device is accepted as adequate backflow protection:
Backflow by back pressure: fluid category 2
Backflow by back siphonage: fluid category 3
Interpretation given in Regulators’ Backflow Specification - backflow prevention arrangements and devices: backflow prevention arrangements