Figure 1 shows major and minor failures and their causes [1].
A major failure is a complete failure of a circuit-breaker which
causes loss of one or more of its fundamental functions. A major
failure will result in an immediate change in the system operating
conditions, or in mandatory removal of the circuit-breaker
from service for non-scheduled maintenance. Any other failure
is considered a minor failure. Figure 1 shows that only 7% of
major failures are due to the leakage of SF6 gas. On the other
hand, 40% of minor failures originate in the SF6 gas insulation
systems.
Gas-insulated systems (GIS) have been in service for about 40
years, and have proved highly reliable. However, some in-service
failures of circuit breakers are related to GIS defects. Figure 2
shows the distribution of the main failure causes in GIS. Assuming
the use of a sufficiently sensitive partial discharge (PD) detection
system, failures due to foreign particles and moisture (about
27% of the total) could have been predicted.
Figure 3 shows that circuit breakers using hydraulic actuators
suffer significantly from motor current, storage position, oil
level, and oil leakage problems. A very high proportion of such