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This paper describes the application of three-dimensional magnetic field analysis, including eddy current effects on the axial magnetic field electrodes. The accuracy of the analysis is verified with both basic and full models of the electrodes. The analytical results were compared with the measured values obtained by the Hall sensor system. The effect of the electrode slit length and the electrical conductivity of the contact on the magnetic flux density and phase difference was analyzed with the full model. The magnetic field in the gap was simulated when the arc ignites between the electrodes. These results demonstrated that the magnetic design of the electrodes can be carried out on an engineering work-station.

Using a scanning electron microscope (SEM) and a laser microscope, we observed micro-flakes and particles on machined Cu-Cr contacts which were considered to be built-up edges due to machining. A large number of microparticles of 5.7 - 8.7 X [10.sup.3] from contact surface of 1 [cm.sup.2], were generated from the machined Cu-Cr contacts during opening and closing operations of normal current. The size of typical microparticles was some tens [[micro]meters] and those larger than 100 [[micro]meters] were occasionally observed. This implies that the surface of machined Cu-Cr contacts must be treated by any means when they are used for vacuum interrupters. Current conditioning is one of the effective surface treatment. Although the surface roughness of the contacts increases through the current conditioning, the improvement of insulating ability is brought by the elimination of micro-flakes and particles and by the formation of fine structure layer preventing partial detachment of the contact material.

The performance of vacuum circuit breakers in switching shunt capacitors depends much more on in-rush current than on interrupting current. This is because when contacts that were fused together by pre-arcs during contact closing are forcibly separated, large protrusions are formed on the contact surface, making it easier for microparticles to be detached from the protrusions. Microparticles are also produced on the entire surface of contacts by mechanical impact or cold weld. Current interruption of a certain magnitude, however, has a conditioning effect because moderate arcs can eliminate such microparticles and lower the protrusions. To clarify the relationship between the behavior of microparticles and the dielectric breakdown, a laser scattering technique was employed. We found a microparticle-induced breakdown phenomena that could explain the mechanism of long delayed restrikes that is occasionally observed in vacuum circuit breakers.

We observed and analyzed the dynamic characteristics of long-stroke stainless steel bellows for a vacuum interrupter when it is extended and contracted at high speeds. Dimensions of bellows tested are as follows: outer diameter 85 mm, inner diameter 62 mm, number of folds 44 and 55. Test conditions : contracted speed : 4 m/s, 2 m/s, extended speed: 2 m/s, 1 m/s. We found that increasing the number of folds on the bellows does not necessarily prolong the life of bellows and it is preferable that the bellows be designed so that the ratio of operation stroke and free length of bellows falls in the range between 0.3 to 0.4.