Understanding Field Experience of Contact Performance from Lab Fretting Tests of Model Contacts

Silver and copper are key materials for sliding electrical contacts in tap-changer applications, where low contact resistance is crucial. Such contacts must function for at least 500 000 mating operations, why relatively small contact forces are used. Besides the sliding motion when mating the contacts, small-amplitude vibrations may introduce relative motion at the contact interface when mated. In this study the performance of solid silver against three different counter materials is evaluated in a lab set-up, simulating electrical contacts subjected to fretting. The three counter materials are solid copper, silver plated copper, and silver plated copper with a top layer of silver iodide. The study has three objectives: 1. To reveal the lab test performance of the different material combinations, in terms of friction and contact resistance. 2. To increase the understanding of the fretting process and its effect in real applications. 3. To correlate field experience of contact performance with the experimental results. From the study it is found that there are significant differences in performance and behavior between the different contact materials, and that lab test conditions can be attributed to real life contact performance.