From the molten bridge rupture to the local thermal equilibrium arc: optical emission spectroscopy of electric arcs during contacts opening

We investigate by time-resolved optical emission spectroscopy the properties of electric arcs forming at the opening of Cu contacts with a current of a few kA. The systematic observation of well-resolved transitions demonstrates that the dense Cu vapor that results from the rupture of the molten metal bridge between the contacts expands rapidly. Solving the radiative transfer equation through the line-of-sight to fit the lineshape of the detected spectra, we obtain that both the pressure and the temperature of the Cu-dominated plasma increase during the first 100 µs of arcing. At the same time, the peak wavelength and broadening of Cu I transitions exhibit a non-monotonic time-dependence. In particular, both the blueshift and narrowing of these transitions by Stark effect during the first 40 µs of arcing demonstrate a decrease in electron temperature. This result leads us to the conclusion that it takes at least 40 µs for the arc to be at local thermal equilibrium after the rupture of the molten metal bridge.