There are two basic tasks of the enclosure: to keep the gas pressure inside and to be gas tight so that SF6 will not be released to the atmosphere. Both tasks can be well covered by using sheet steel materials. Cast steel has porosities that cannot guarantee such high gas tightness values as required for GIS.
Steel is relative easy to weld and to form. Therefore, the steel enclosures have been welded by steel sheets that have been mechanically formed to cylinders and then were welded.
Used for the conductors as an extruded pipe or in cast technology. For enclosures aluminum is used in cast technology or with welded sheet materials using longitudinal welds or spiral welding processes.
Extruded pipes can be used as conductors in a straight bus bar section. When conductors are used inside disconnecting or ground switches, the required shape and design requires a casting technology.
The disadvantage of cast aluminum in the early years was that the insulation gas was released to the atmosphere due to its porosity.
Today, most enclosures are manufactured in cast aluminum as the top performer for GIS In pic shown Straight conductor graphic that manufacture by aluminium.