Gas insulated switchgear encloses busbars, circuit breakers, disconnectors and instrument transformers inside an earthed metal housing filled with pressurised SF6. Thanks to the dielectric strength of the gas, clearances between live parts shrink dramatically and a complete substation fits into a tenth of the space it would occupy outdoors.
It is used where land is expensive or scarce — urban substations, industrial plants, underground installations — and where the environment is aggressive, because with everything enclosed, salt or industrial pollution does not reach the insulation.
Its maintenance has particularities: the interior cannot be reached without recovering the gas, so diagnostics rely on measurements taken from outside — partial discharge by TEV or ultrasound, SF6 quality analysis, thermography of busbar joints — and gas density is monitored permanently.