*Article* **Ablation-Dominated Arcs in CO2 Atmosphere—Part I: Temperature Determination near Current Zero**

**Ralf Methling 1,\*,†, Alireza Khakpour 1,†, Nicolas Götte 2,† and Dirk Uhrlandt 1,†**


Received: 11 August 2020; Accepted: 9 September 2020; Published: 10 September 2020

**Abstract:** Wall-stabilized arcs dominated by nozzle–ablation are key elements of self-blast circuit breakers. In the present study, high-current arcs were investigated using a model circuit breaker (MCB) in CO2 as a gas alternative to SF6 (gas sulfur hexafluoride) and in addition a long polytetrafluoroethylene nozzle under ambient conditions for stronger ablation. The assets of different methods for optical investigation were demonstrated, e.g., high-speed imaging with channel filters and optical emission spectroscopy. Particularly the phase near current zero (CZ) crossing was studied in two steps. In the first step using high-speed cameras, radial temperature profiles have been determined until 0.4 ms before CZ in the nozzle. Broad temperature profiles with a maximum of 9400 K have been obtained from analysis of fluorine lines. In the second step, the spectroscopic sensitivity was increased using an intensified CCD camera, allowing single-shot measurements until few microseconds before CZ in the MCB. Ionic carbon and atomic oxygen emission were analyzed using absolute intensities and normal maximum. The arc was constricted and the maximum temperature decreased from >18,000 K at 0.3 ms to about 11,000 K at 0.010 ms before CZ. The arc plasma needs about 0.5–1.0 ms after both the ignition phase and the current zero crossing to be completely dominated by the ablated wall material.

**Keywords:** circuit breaker; switching arc; optical emission spectroscopy; ablation; current zero; SF6 alternative gases; CO2; PTFE
