Page 92 - Practical Power System and Protective Relays Commissioning

90 Practical Power System and Protective Relays Commissioning

FIGURE 7.7 Schematic of a vacuum circuit breaker.

As the arc in an air CB is an electric conductor made up of ionized air
particles, the arc is extremely difficult to form in a vacuum. This type of
breaker has limited interrupting ratings compared to oil CB.

7.3.4 SF 6 Circuit Breaker
SF 6 (sulfur hexafluoride) is an excellent gas for insulation in high-voltage
equipment due to its excellent dielectric and arc-quenching properties in a
gas insulated system (GIS), where it is used in CB, disconnect switches, bus
ducts, current and voltage transformers, etc.
SF 6 is an industrial product developed through the chemical combination
of fluorine and sulfur. The resulting product is purified, liquefied by com-
pression, and stored in steel cylinders.

7.3.4.1 Properties of SF 6 Gas
SF 6 gas has the following characteristic:
odorless,
colorless,
nontoxic,
nonflammable,
heavier than air, and
good characteristic for electrical arc quenching.
Normally it is stored in pressurized cylinders.

7.3.4.2 Contaminants
Contaminates, such as water, air, and oil, come as a result of poor vacuum
processing of the gas and reduces its electrical dielectric.

7.3.4.3 Related Standards
IEC Standard 376 recommends the following limits:
CF4 maximum of 500 ppm,
oxygen and nitrogen (air) of 500 ppm,