Pertains to compartments pressurized at a maximum pressure of 3 bar (gauge) and with a maximum pressure x volume of 2000 bar/litres with inert gases, for example sulphur hexafluoride or nitrogen or a mixture of such gases, used in indoor or outdoor installations of a.c. switchgear and controlgear with rated voltages above 1 kV up to and including 52 kV where the gas is used principally for its dielectric and/or arc-quenching properties.