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This paper shows the result of a failure survey including the entire population of gas-insulated substations in the Norwegian power transmission system. The survey comprises the complete service experience up to now-nearly 12000 circuit-breaker years. Information about 180 failures has been recorded during 36 years: 77 internal flashovers/arcing faults have occurred, four of which burned through the encapsulation and another six caused the pressure-relief devices to operate. No severe injuries have been recorded. The overall failure rates and their causes are comparable to the findings of other reliability surveys, although the occurrence of severe arcing faults appears to be clearly higher than noticed elsewhere. The 300-kV and 420-kV gas-insulated substations put into service around 1980 have a significant and negative influence on the overall failure rate, whereas a large number of 145-kV substations installed since the early 1990s have proven very reliable. Even though a large portion of the population is above or approaching the designed service life of 30 years, no significant aging problems could be revealed with a basis in the failure statistics.