Basic Information about SF6 Circuit Breakers
Some of the outstanding properties of SF6 circuit breaker gas are
Electrical and Thermal Properties
- High dielectric strength
- Outstanding arc-quenching ability
- Excellent thermal stability and
- Good thermal conductivity
Physical and chemical attributes
- Chemically inert
- non condensable at low temperature
Advantages of SF6 circuit breaker gas when used in power switch gear
- Increase safety
- Reduction in size
- Simplified design
- Reduction in weight
- Reliability of operation
- Noiseless operation
- Ease of installation
- Ease of handling and
- Ease of maintenance
Dielectric Properties OF SF6
At atmospheric pr.
- D.S. of SF6 is about 2.5 times that of air
- 30 % less than that of oil
D.S.of SF6 is equal to oil at 650 gm/cm2 and 15% higher at 1.25 kg/cm2
SF6 Decomposition Products
Although SF6 gas is an inert and stable gas, in the presence of electrical discharge such as an arc, spark or corona, a portion of the gas decomposes in to lower fluorides of sulphur which can react to form no. of chemically active by products.
As can be seen above, the byproducts consists of both the
- gaseous byproducts ( SF2, S2F2, SF4, SOF2, SOF4, SO2F2 , HF, S2F10
- solid byproducts ( AlF2, FeF3K CuS , CuF2
Amount of each decomposition product depend on many factors:
- Magnitude and duration of discharge
- The materials of construction of the equipment
- Contamination levels of moisture and air inside the equipment.
Removal of SF6 Products
The byproducts are corrosive and are likely to affect the organic materials more.
Absorbent materials used in Circuit Breakers:
- Activated Alumina ( effective for removal of SOF2, H2S, SF2
- Molecular services (sodalime- CaO.NaOH) –[ removal of stable gases such as SF4, SiF4 & S2F2 ]
The serve the dual purpose of absorbing the moisture as well as the arc products.
So the suggested mix is 50 /50 of sodalime and activated Alumina. The suggested weight of the absorbent is 10% of the weight of the gas.