Arc Cooling Mechanisms in a Model Circuit Breaker

Axially blown electric arcs in a model circuit breaker are investigated by electrical measurements and fast imaging with a high-speed camera for visualizing the arc structure. Parameters for a dynamic conductance model (known as the “black-box model” in the arc literature) are extracted from the electrical measurements for currents between a few tens of amperes and a few kiloamperes. The model is used for the prediction of the conductance-dependent cooling power and arc relaxation time, as well as the voltage-current characteristics of the arcs in our system. In order to work out the relevance of the different cooling mechanisms, an alternative and more physically based dynamic model for axially blown arcs is incorporated. It turns out that convective cooling predominates turbulent heat conduction and radiation cooling in the region of validity of the model. The result is consistent with the observed strong dependence of the cooling power on the flow conditions that are controlled by the blow-gas pressure and the geometry. Furthermore, the comparison between air and SF6 arcs indicates a considerable dependence of the conductance-cooling relation on the gas type.