Significance of microstructure analysis for performance evaluation of Vacuum interrupter contacts

The Vacuum interrupter is the arc extinction chamber of the vacuum circuit breaker which is widely used in the medium voltage networks for interruption of short -circuit current. In the field of vacuum switching devices, the contact material is the most important parameter determining the switching behavior. During arcing, the contacts are eroded and hence there is a change in the surface microstructure. The comparison of the microstructure before and after short circuit operations reveals information about the effect of the arc on the contacts. The microstructure properties like grain structure of the parent metals, the distribution of the grains and grain boundaries in the contact are important in the current interruption process. These properties are studied through the scanning electron microscope (SEM). Even the effect of the contact manufacturing process on the above mentioned properties can be studied through the SEM analysis. This paper compares the post short circuit current microstructures of the similar contacts subjected to different values of short circuit current. The paper also compares the microstructures of contacts manufactured with different processes and subjected to the same short circuit current regime. The paper concludes by establishing the correlation between the micro structural properties and the arcing performance of the contacts of the vacuum interrupters.