Changes

The testing of electrical contacts for power engineering applications serves on the one hand the continuous quality assurance, on the other one the new and improvement development efforts for contact materials. To optimize the contact and switching performance , contact materials and device designs have to complement each other. The success of such optimizing is proven through switching tests.

The assessment of contact materials is performed using metallurgical test methods as well as switching tests in model test set-ups and in commercial switching devices. While physical properties such as melting and boiling point, electrical conductivity, etc. are fundamental for the selection of the base metals and the additional components of the materials, they cannot provide a clear indication of the contact and switching behavior. Metallurgical evaluations and tests are used primarily for determining material and working defects. The actual contact and switching behavior can however only be determined through switching tests in a model switch or preferably in the final electromechanical device.

The main characteristic for the appraisal of contact materials for power engineering is the optical evaluation of their microstructure in a metallographic mount. This provides a picture of the internal structure of the materials. It allows detecting structural in-homogeneity, grain boundary enrichments, cracks, material separations, or defects in the brazing interface. The metallographic view is however limited to the one two-dimensional plain in which the mounting cut was made.

<xr id="fig:Microstructure of a powder metallurgical Ag CdO material"/><!--(Fig. 13.11)--> shows the microstructure of a Ag/CdO contact material after being affected by electrical arcing. In the lower part the starting material structure is visible. In the upper part the de-mixing of the composite material through the effects of the switching arc is clearly demonstrated. This “switching structure” shows in certain areas depletion of metal oxide which increases the probability of contact welding during subsequenbt subsequent make operations. Additional analysis by X-ray probing in a scanning electron microscope (SEM) allows the micro analysis of the elements present in the contact surface region.