Svenska kraftnät has installed about 300 disconnecting circuit breakers (DCB) in the Swedish 245 and 420 kV transmission network. They are installed without any disconnectors in the substation. The first DCBs were installed in the early 2000s and are approaching the age of 12 years, which is the time for the first scheduled maintenance, a function test that requires de-energising of the breaker. For a shunt reactor breaker in a double-breaker double-busbar configuration this means disconnection of one of the busbars at the substation, whereas for a line breaker it means disconnection of the incoming line, which is often difficult.

Thus for disconnecting circuit breakers especially line breakers, there is need for an in-service condition test, which have been developed and tested at Svenska kraftnät. It requires some modification to the DCBs from the manufacturer and has been a Svenska kraftnät standard for many years.

This paper describes the difference between the conventional off-line function test and the new in-service condition test, mainly regarding the possible failures that can be detected. The failures can be classified in three major types: mechanical, electrical and related to the insulation gas. The present condition test measures the movements during the opening and closing operations and it can assess the condition of the mechanical parts, whereas the condition of breaking chamber and the electrical resistance of the contacts are not possible to measure without disconnecting the circuit breaker. The gas pressure and other parameters can be added separately to the condition test. Also this paper provides normal values of the parameters measured in an in-service condition test, such as coil current, motor current and timing parameters.