(1) Where the critical level is not marked on an atmospheric vacuum breaker, pressure vacuum breaker or spill-resistant pressure vacuum breaker, the critical level shall be taken as the lowest point on the device.
(2) Where an atmospheric vacuum breaker is installed, it shall be located on the downstream side of the fixture control valve or faucet so that it will be subject to water supply pressure,
(a) only when the fixture control valve or faucet is open, and
(b) for periods of use not to exceed 12 h continuous.
(3) An atmospheric vacuum breaker shall be installed so that the critical level is at least the distance specified by the manufacturer at which the device will operate safely but not less than 25 mm above,
(a) the flood level rim of a fixture or tank, or
(b) the highest point open to atmosphere in an irrigation system.
(4) A pressure vacuum breaker or spill-resistant pressure vacuum breaker shall be installed with its critical level at least 300 mm above,
(a) the flood level rim of a fixture or tank, or
(b) the highest point open to atmosphere in an irrigation system.