Eaton DST-2-VR+ Series Instruction Book

Contents

12 Instruction Book IB182942EN July 2018 www.eaton.comDST-2-VR+VR-Series+ Replacement Circuit BreakerSECTION 4: DESCRIPTION ANDOPERATIONVR-Series+ vacuum replacement circuit breakers are designed to beused with existing installations of equivalent air-magnetic metal-cladswitchgear circuit breakers. The front mounted spring type storedenergy mechanism facilitates inspection and provides improvedaccess to components for servicing. The long life characteristics ofthe vacuum interrupters and proven high reliability of spring-typestored energy mechanisms assure long, trouble-free service withminimum maintenance.4.1 VACUUM INTERRUPTERVacuum interrupters offer the advantages of enclosed arcinterruption, small size and weight, longer life, reduced maintenance,minimal mechanical shock, and elimination of contact degradationcaused by environmental contamination.In the closed position, current flows through the interrupter movingand fixed stems and the faces of the main contacts. As the contactspart, an arc is drawn between the contact surfaces. The arc is rapidlymoved away from the main contacts to the slotted contact surfacesby self-induced magnetic effects. This minimizes contact erosion andhot spots on the contact surfaces. The arc flows in an ionized metalvapor and if the vapor leaves the contact area, it would condenseinto the metal shield which surrounds the contacts.At current zero, the arc extinguishes and vapor production ceases.Very rapid dispersion, cooling, recombination, and deionization of themetal vapor plasma and fast condensation of metal vapor causes thevacuum to be quickly restored and prevents the transient recoveryvoltage from causing a restrike across the gap of the open contacts.4.1.1 VACUUM INTERRUPTER ASSEMBLYEach vacuum interrupter assembly (also referred to as pole unit) isassembled at the factory as a unit to assure correct dimensionalrelationships between working components. Three interrupterassemblies are used per circuit breaker. Each vacuum interrupterassembly consists of a molded insulator frame and includes thevacuum interrupter, its lead assembly, bell crank, operating rod,stand-off insulator, and contact load spring. The vacuum interrupteris mounted vertically with the stationary vacuum interrupter stemupward and the moving interrupter stem downward. The pole unitsare fastened to the circuit breaker’s stored energy mechanismframe. Silver-plated copper laminated shunts transfer current fromthe moving interrupter stem to the upper primary bushings via aHolm-free, non-sliding conical current transfer. A silver-plated coppercasting is attached to the stationary stem, completing the primarycircuit to the lower disconnect assemblies. The operating rod,loading spring, and bell crank transfer the mechanical motion fromthe circuit breaker’s stored energy mechanism to the moving stemof the vacuum interrupter.4.1.2 CONTACT EROSION INDICATORThe purpose of the contact erosion indicator is to monitor theerosion of the vacuum interrupter contacts, which is very minimalover time with Eaton vacuum interrupters utilizing copper-chromecontact material. The VR-Series+ vacuum interrupter assemblyincorporates both the original vacuum interrupter erosion indicatorand the contact-spring wipe into one all-encompassing indicator. Theadequacy of the remaining contact erosion and wipe can easily bedetermined by observing the moving end of the vacuum interrupterassembly on a closed circuit breaker. The procedure to determinethe adequacy of the “T” cutout on the vacuum interrupter assemblyis depicted in Figures 6.1 and 6.2. If the wipe is inadequate (no partof the “T” cutout is visible) then the vacuum interrupter assemblymust be replaced. Field adjustment is not possible.Figurt 4.1. Vacuum Inttrrupttr AsstmblyFigurt 4.2. Vacuum Inttrrupttr Asstmbly (All Thrtt Polt Units)