Effect of short circuit current duration on the welding of closed contacts in vacuum

High ac currents were passed through closed Cu-Cr (25 wt. %) contacts located in a vacuum ambient (pressure 10^-4-10^-3 Pa). The duration of the short circuit currents ranged from 1 second to 4 seconds. The contact welding was measured for a range of closed contact forces and current durations. The weld fracture strength was measured using a slowly rising opening force of 50 N/s. It was found that the contacts required a larger contact force to prevent welding if the current duration of 4 seconds was applied, than if a current of 1 second, 2 seconds and 3 seconds was applied. The data showed that there was a considerable scatter in the data for all conditions examined. It also showed that a weak weld could well follow a strong weld. A consideration of the transition to the steady state model of closed contact welding can explain the observed data.