Effect of the electric arc and the ambient air on the contact resistance of silver, tungsten, and silver-tungsten contacts

Abstract

Contact pairs of Ag, W, and Ag-W were operated 6000 times in a 20-A 110-V ac circuit. The effect of contact-resistance change was observed by measuring the temperature rise (TR) at the fixed contact while passing the steady-state 20 A. The TR was measured before the switching had begun and after each 1000 operations. A large number of contact pairs were used in order to ensure a good statistical interpretation of the data. The chemical composition of the contact surfaces was measured using a Debye-Scherer x-ray analysis. The distribution of contact resistance for Ag contacts remained the same throughout the experiment. For W contacts, the distribution was initially higher than that for Ag: the result of conductivity and hardness differences. During the switching experiments, the formation of tungsten oxides on the contact surface resulted in high contact-resistance values. For Ag-W, the distribution was initially closer to that for Ag contacts: the result of a thin surface layer of Ag on the Ag-W. During the switching, the Ag-W contact-resistance distribution gradually increased in dispersion and had higher values than those measured for W contacts. The x-ray data showed that not only were tungsten oxides formed, but also there was a high probability that some of the free Ag on the contact surface formed an insulating silver tungstate.