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Dielectric Gases 369

FIGURE 12 Circuit breakers using SF 6 gas. (Courtesy of Westinghouse Electric Corporation.)

A search for other gases/mixtures for use in circuit breakers with metal casings, generally called metal clad
breakers showed that while no other gaseous medium out- or dead-tank circuit breakers because the casing or tank is
performs SF 6 in circuit breakers, some gases/mixtures can at ground potential. This naturally led to the development
be considered. The performance ratings (based on the crit- of other substation components such as disconnect
icalinterruptioncurrent I c foragivensurgeimpedance Z 0 ) switches, arresters, interconnecting links (buses), and ca-
relative to SF 6 of some such media are listed in Table III. bles for getaways using the metalclad technology.
The totally enclosed metalclad substations using SF 6
gas (Fig. 13) are significantly more compact than the open-
C. Compressed-Gas Insulated Substations
air substations and switchyards. Often, the land required
The excellent dielectric strength and arc extinguishing is reduced by as much as 40 times, and the resultant sav-
properties of SF 6 facilitated the development of circuit ings in the cost of land are thus significant. Furthermore,