Applicable to welded composite enclosures of cast and wrought aluminium and aluminium alloy enclosures pressurized with dry air, inert gases, e.g. sulphur hexafluorlde or nitrogen or a mixture of such gases, used in indoor or outdoor installations of high-voltage switch gear and control gear with rated voltages of 72.5 kV and above, where the gas is used principally for its dielectric and/or arc-quenching properties.