Fundamental investigation on the thermal transfer coefficient due to arc faults
In order to determine the pressure rise due to arc faults in electrical installations, the portion of energy heating the surrounding gas of fault arcs has to be known. The ratio of the portion of energy to the electric energy, the thermal transfer coefficient, well known in literature as kp factor, is adopted here. This paper presents a theoretical approach for the determination of the pressure rise in electrical installations. It is based on the fundamental hydro- and thermodynamic conservation equations and the equation of gas state taking into account melting and evaporation of metals as well as chemical reactions with the surrounding gas. With respect to the dependence of the arc energy on gas density, the radiative effect of fault arcs on the energy balance is introduced. In consideration of the radiation, the more reasonable arc energy is applied for the estimation of the gas temperature and of the thermal transfer coefficient in the energy balance. In order to identify conveniently which factors essentially influence the development of pressure, the thermal transfer coefficient is studied and evaluated as an alternative variable of the gas pressure. The results for a test container show that factors such as the kind of insulating gases and of electrode materials, the size of test vessels, and the gas density considerably influence the thermal transfer coefficient and thus the pressure rise
The transport and thermodynamic properties of a copper-nitrogen mixture
The transport and thermodynamic properties of a copper-nitrogen (Cu-N) mixture are calculated for temperatures between 2000 and 28000K and pressures between 1 and 10 atmospheres. The results show that, compared with the electrical conductivity for pure nitrogen, a small concentration of copper vapour can greatly enhance the electrical conductivity at temperatures below about 8000K. The internal energy and enthalpy per unit mass are reduced by 1/2 and 2/3 when X is increased from 0 to 1, where X is the ratio of the total number density for all copper species to that for all nitrogen species. Changes in the electron thermal conductivity increase the total thermal conductivity by about (1+X) when X<0.1, for temperatures between 9000 and 12000K.
Radiative energy losses from a high-current air-blast arc
The importance of total radiation losses from high‐current arcs burning in highly accelerated air flows representative of conditions existing in commercial gas‐blast switchgear has been investigated. Such losses have been measured both in the high‐pressure region upstream of a shaped orifice, where gas velocities are low, and in the region downstream where velocities become supersonic and pressure conditions approach ambient. The dominance of upstream electrode vapor as the source of plasma radiation losses is demonstrated and the importance of radiated losses within the arc energy balance is examined using measured values of axial electric field. For upstream electrodes of elkonite (sintered copper/tungsten) as used in high‐power gas‐blast circuit breakers, it is shown that some 30–40% of the electrical energy input upstream of the orifice is lost as radiation, while downstream this figure becomes 10–20%. The effect of reservoir pressure on arc electric fields is examined and the contribution to this eff...