The basic principle of operation involves raising and maintaining the internal pressure of the enclosure to a level slightly above the atmospheric pressure outside the enclosure. This ensures that any flammable gases or vapours cannot enter. The minimum over-pressure specified in standards is 50 Pa (0.5mBar) for types px and py, and 0.25mBar for type pz.
The safe gas used to provide the over-pressure will normally be air, but an inert gas such as nitrogen may also be used where necessary
Note: The manufacturer shall specify the minimum and maximum normal overpressure in service, the maximum overpressure during purging and the maximum leakage rate at the maximum normal overpressure.
