Abstract—Voltage-current characteristics of free burning arcs in SF6 and air have been known for decades. As the demand for an SF6-free solution is increasing, there is an accompanying need to determine arc parameters in the alternative gases. An unblown arc experiment has been established to determine the voltagecurrent characteristics of SF6 alternative gases, which have not yet been thoroughly studied. In this experiment free burning arc measurements were performed in a number of gases under consideration of SF6 alternatives, including CO2 and mixtures of CO2 / O2 with and without C4F7N or C5F10O additives at concentrations of up to 10 %. Measurements were also performed in air and SF6 for comparison. Arc voltage was measured in each gas at pressures ranging from 1 bar to 5 bar absolute, and electrode separations ranging from 20 mm to 95 mm. Voltagecurrent characteristic measurements for air and SF6 show good agreement with previously published results. A linear relationship of the arc voltage to the arc length is shown, as well as fourth root dependence of the arc voltage on the gas pressure. It was shown that neither the O2 nor the fluorinated additives to CO2 have any significant influence on the voltage-current characteristic. The minimum arc voltage in all measured gases was slightly higher than in SF6, but the arc in SF6 was the least stable and had the highest elongations resulting in high voltage peaks. The arc voltage in air had a similar minimum value to the CO2 based gases, but the arc was much more stable, resulting in lower effective voltage, especially at low currents.

A free-burning arc experiment was performed in a gas mixture composed of CO2 /O2 /C5F10O (85% / 10% / 5%). An optical emission spectroscopy diagnostic was developed to obtain spatially resolved arc spectra at a high frame-rate. Spectral measurements are compared with simulated spectra to estimate arc parameters including temperature and composition. These techniques are used to characterize the arc and its associated electrode jets over a range of conditions, in order to study the influence of ablated Cu/W on arc characteristics. The observed spectra indicate that the ablation of the contact tip primarily occurs through vaporization rather than the expulsion of droplets. The metal vapor content of the arc is investigated as the arc driving current is varied, and a transition threshold for increased ablation at electrode current densities above 50A/mm2 is determined. The expansion angle of the electrode jet is estimated from the concentration of metal vapor in the arcing region at different axial positions, using a basic conical expansion model. The dependence of the measured voltage on the arc composition is examined, and a positive correlation between Cu content and arc voltage is identified. This trend is attributed to the higher emissivity of the metal vapor, which suppresses the central arc temperature and increases its resistivity.