Page 93 - Practical Power System and Protective Relays Commissioning

Circuit Breakers Theory Testing and Commissioning Chapter | 7 91

water of maximum 15 ppm,
acidity of 0.3 ppm maximum, and
hydrolysable fluorides maximum of 1.0 ppm.

7.3.4.4 Precautions
1. Ventilation
As SF 6 is heavier than air, it replaces air in the room and can cause
asphyxiation.
2. Toxic arc by-products
For any fault inside SF 6 CB, care should be taken for the products of
SF 6 in arc chamber of the CB, and people should wear protective clothes
and protective equipment.
3. SF 6 gas contamination by compressor oil.
Compressors are the source of oil contamination; oil test unit is
required to test the oil contamination.

7.3.4.5 Testing for Contamination
1. Moisture content
Moisture is measured using dew point measuring instrument where
readings are directly in ppm moisture.
2. Oxygen content
Analytical portable trace oxygen analyzer will be used for measure-
ment, and the measurement will be in 1 1000 ppm by weight.
3. Gaseous by-products
During internal faults, tests have to be done for the gas products.
4. Gauge calibration
4.1. Density monitor checks
The density monitor should be recalibrated using variable com-
pressed air.
4.2. Pressure and gauge checks
Gauge recalibration should be performed. Refer to Fig. 7.8.
a. This figure shows monograph for dew points and ppm as a
function of pressure, for example, at a pressure of 3 bars.
b. 2 20 C will give a moisture content of 350 ppmv on line of

monograph.

7.4 CIRCUIT BREAKER OPERATING MECHANISM

1. A stored energy mechanism uses large springs.
2. A penumatically operated mechanism employs compressed air in the
operation of the CB.
3. A hydraulic operated mechanism employs compressed oil.