The purpose of a surge arrester is to limit damage resulting from transient overvoltage. Usually they are applied to power transformers and (some) switchgear assemblies. For GIS (gas insulated switchgear), a special high-gradient metal-oxide block is used (often multiples, connected in series, with an SF6 or similar gas). Likely causes of high transients include: lightning strikes, switching (particularly high-speed and/or high-frequency operations), and capacitor failures.
Converting energy to motion for more than half a century
If the GIS station is connected to an overhead line then it is obvious that due to change in surge impedance,
a SA has to be installed at the junction.