Material Characteristics of Molten-metal Bridges at Electrical Contacts

Abstract

Steel and copper are widely used as current collecting material for power supply in electric railways. In this field experimental studies on material wear and friction phenomena are important for improving stability and reliability of current collection in service operation. However few are reported to the effect of contact bridge formation, which is caused by separation of electrodes, in the material wear and friction phenomena, especially for different materials, such as, steel and copper.
In this paper, anode and cathode materials, which are made of same metal (either steel or copper), are separated with very low constant speed. to observe the bridge formation and contact surface after the bridge separation in detail.
As a result, melting bridge formation of 0.5mm height was observed while steel electrodes separating. In the similar test of the identical condition of the copper electrode, the observation of the bridge formation was difficult. However, the copper bridge that grew at 0.2mm height was observed, where test was conducted with separating speed slowed down to 1/10. The molten bridge was formed at the boundary on the anode side with high brightness, and this effect was seen for both steel and copper. Although this phenomena appears for both cases of steel and copper, the anode made of steel material is damaged more seriously.
Also in this paper, special strips of steel and copper, simulating the contact bridge, are studied for contact drop, contact resistance and temperature increase at contact point to melt down by increasing current slowly from zero ampere.
As the result, the resistance of the copper which is a diamagnetic material rose in a smooth quadratic curve for the increase in current (temperature), and the steel which is ferromagnetic showed the rapid increase above magnetic transformation and allotropic transformation temperature. The increase of heat quantity by decreasing of specific heat due to magnetic transformation and the consequent resistance increase are the cause of the formation of large melting bridge in the steel And Thomson effect is considered as the cause of damage of anode.