SF6 density: pressure vs. gas temperature
At constant density the absolute pressure follows a straight line with the gas temperature (isochore). Lines through 0.70 (fill), 0.62 (alarm, assumed) and 0.60 MPa (lockout = minimum functional pressure, Siemens 3AP1 FG 145 kV) at 20 °C. Dots: measured pressure at the measured gas temperature. A point that crosses to a lower line means gas has left the breaker.
Contact wear and operating times
Top: accumulated interrupted current Σ I^1.8 per pole against an allowance of 20 interruptions at 40 kA (assumed; the manufacturer's endurance curve sets it). Bottom: opening time of every operation against the rated 23 ms corrected for the control voltage.
132 kV line bay
Busbar, disconnector, current transformers, the three-pole SF6 breaker with its spring mechanism and density monitor, and the line, with live sensor values. Red = beyond the twin's alarm limit.
Condition sensors
Bars against the alarm limits in the twin file (red line).