Switchgear

Switchgear is the collective term for the switching and protection equipment — circuit breakers, disconnectors, earth switches, busbars, and their supporting structures — that controls the flow of power through a substation. It is the primary plant that sits between the incoming lines and the transformers, and its job is to isolate faults, enable maintenance, and route power through different circuit configurations.

AIS vs GIS

The two main physical forms are determined by the insulating medium:

• Air-Insulated Switchgear (AIS) uses ambient air as the insulator between live conductors. The equipment sits outdoors in an open switchyard with safety clearances measured in metres. AIS is cheaper to build, easier to inspect visually, and simpler to extend — but it requires a large footprint and is exposed to weather, pollution, and wildlife.

• Gas-Insulated Switchgear (GIS) encloses all live components in sealed metal chambers filled with sulphur hexafluoride (SF₆) gas, which insulates at roughly three times the voltage per metre that air does. A GIS installation can fit into a fraction of the space an equivalent AIS yard would need. GIS is the default choice for urban substations, underground installations, and sites where land is scarce or environmental exposure is a concern. The trade-off is cost, complexity of maintenance (the enclosures must be opened in controlled conditions), and the environmental impact of SF₆ — a potent greenhouse gas — if it leaks.

Design life and replacement

Switchgear is primary plant with a design life of 30–40 years. In UK transmission, switchgear replacement is driven by condition assessment through Ofgem’s RIIO price controls, typically on the same cycle as transformer refurbishment. AIS switchgear from the 1960s–70s is still in service at many sites; GIS from the 1980s onward is reaching its first major overhaul window.

Relevance to digital substations

From an IEC 61850 perspective, the switchgear type affects the physical layout of the process bus. AIS yards have long cable runs between outdoor equipment and the control building, which is where stand-alone merging units and fibre-optic process-bus cabling deliver the most value — replacing hundreds of metres of copper trunks. GIS installations are more compact, with shorter distances between the switchgear and the protection panels, but the sealed enclosures mean that retrofitting sensors or merging units requires careful integration with the GIS manufacturer.