strung on support structures. The air space required increases with system voltage and with the lightning surge voltage rating. For medium-voltage distribution substations, metal-enclosed switchgear may be used and no live conductors exposed to the environment at all. For higher voltages, gas-insulated switchgear, in a gas insulated substation (GIS) reduces the space required around live buses. Instead of bare conductors, buses and apparatus such as switchgear are built into pressurized tubular containers filled with sulfur hexafluoride (SF6), or an alternative gas. This gas has a higher insulating value than air, allowing the dimensions of the apparatus to be reduced. In addition to air or SF6 gas, apparatus will use other insulation materials such as transformer oil, paper, porcelain, and polymer insulators. Structure Outdoor, above-ground substation structures include wood pole, lattice metal tower, and tubular metal structures, although other variants are available. Where space is plentiful and appearance of the station is not a factor, steel lattice towers provide low-cost supports for transmission lines and apparatus. Low-profile substations may be specified in suburban areas where appearance is more critical. Indoor substations may be gas insulated substations (GIS) (at high voltages, with gas insulated switchgear), or use metal-enclosed or metal-clad switchgear at lower voltages. Urban and suburban indoor substations may be finished on the outside so as to blend in with other buildings in the area. A compact substation is generally an outdoor substation built in a metal enclosure, in which each item of the electrical equipment is located very near to each other to create a relatively smaller footprint size of the substation. See also Anchor portal Electric power transmission Power-line communication HVDC converter station Traction substation High-voltage transformer fire barriers References Further reading