Analysis of Various Electrical Tests Performed on Contact Tips for Molded Case Circuit Breaker (MCCB)

A commercially manufactured circuit breaker was tested for electrical endurance, mechanical endurance, and short circuits. Silver-based contact tips and copper-based contacts of the molded case circuit breaker (MCCB) showed metallurgical changes after several electrical tests. Thermal conductivity and electrical conductivity through braze joints connecting Ag-based materials to ETP (electrolytic tough pitched) copper were also impacted. This paper presents a comprehensive microstructural examination following the performance of various electrical tests on the contact tips of the MCCB, namely the movable contact tip and the stationary contact tip. It consists of a movable contact tip of Ag–WC (silver–tungsten carbide) resistance brazed onto a copper base and a stationary contact tip of Ag–C (silver–graphite) furnace brazed onto the copper base. The filler materials used in both joints differ in terms of their composition. A maximum current of more than 1000 amps can be carried by these contacts. Electrical testing revealed some typical metallurgical changes, including surface degradation, wear, cracks near braze interfaces, oxide layer formation, and cracks propagating into the copper. In some instances, the temperature rises to the melting point of silver. Therefore, the failure of the contact tips was investigated based on microstructural and mechanical changes caused by the various electrical tests.