Vacuum circuit breakers are compact and require little maintenance. However, after several years of service, an inspection of vacuum interrupters is recommended to verify that the vacuum level in the vacuum chambers is still adequate. Rise of internal pressure exceeding a certain threshold value causes loss of current switching capability and electric insulating properties. It is common practice to perform pressure measurements in the factory on new vacuum chambers with a measuring system usually based on the well known magnetron principle. However, pressure measurements on new vacuum chambers after manufacturing and verification of vacuum in circuit breakers after several years of service require different test procedures and diagnostic techniques. Users of vacuum circuit breakers are interested in a simple method suitable for application on the site which aim at checking whether a given pressure threshold value is not yet exceeded. In this paper an overview of ideas, patents and procedures for internal pressure diagnostics of vacuum circuit breakers is presented and the feasibility for application on the site for the circuit breaker vacuum monitoring system is discussed
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