MA-037 - CB Watch 3 Operating Manual
v7.0 - May 2020
Page 44 of 118
SF6 GAS LEVEL MONITORING
General Description
If the circuit breaker being monitored uses SF
6
gas (or a gas mixture) to extinguish the arc,
then the CBW3 can monitor the gas tanks for leaks.
During commissioning, a circuit breaker is filled with gas to its nominal filling pressure. The
performance and even operation of a circuit breaker can be severely affected if the pressure
of the gas contained in the circuit breaker falls too low.
During the CB’s
life, it is therefore
necessary to monitor the pressure of the gas not only to alert maintenance that a re-fill
operation is needed before the CB locks itself (preventing operation) but also to detect any
gas leak early so as to avoid SF
6
gas emissions to the atmosphere and thus reduce cost,
avoid penalties and save the environment.
Measurement principle
Because gas pressure varies with temperature (see figure below), pressure values cannot be
compared over time unless they are temperature compensated. The variation of pressure
with temperature is linear in the range of service (-25
o
C to +50
o
C). Temperature
independent comparisons are therefore made using either
“
pressure normalised at 20
o
C
”
or
“
density
”
(expressed in kg/m3 or gr/l).
The gas pressure and gas temperature are measured by the sensor and then the gas density
or normalised pressure are calculated using the Beattie-Bridgeman equation to take into
account the thermodynamic laws of the gas (or gas mixture) used.
Figure 59
–
Thermodynamic law applied to SF6 gas
These diagrams show lines of equivalent density where, along each line, the gas has the
same density for various temperature/pressure combinations.
