Experimental investigation of the instantaneous contact welding failure phenomena and mechanisms for general-purpose relays

The instantaneous contact welding failure refers to the phenomena that the opening time of relay exceeds the threshold value after the de-energization of coil. The direct consequence is the significant increase of switching time of electric circuit. Understanding the features and mechanisms of instantaneous contact welding is beneficial for the failure analysis and further optimization of relays. In this paper, the electrical endurance experiment of ac general-purpose relay is carried out until the cumulative number of instantaneous welding failure reaches five times. The contact re-close phenomena in the strong welding condition are observed by using the contact voltage and dynamic force waveforms, and the effect of peak voltage, point-on-waveform (POW) and breaking delay time of contact re-close on the welding force are investigated explicitly. Finally, the possible breaking process of two types of contact re-close (melting re-close and boiling re-close) are presented based on the degradation of the contact surface morphology. It is determined that the evaporation of contact a-spots or the converging of molten metal bridges would be the root reasons for the welding in the breaking process, and the material transfer behavior results in frequent instantaneous welding failures at the end of experiment.