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
Multidimensional Risk Assessment of Manhole Events as a Decision Tool for Ranking the Vaults of an Underground Electricity Distribution System
Even though an underground electricity distribution system is safer than an overhead system, several accidents have occurred in them. Assessing the risk of hundreds or even thousands of underground vaults is a hard task. Furthermore, given the large variability in external and internal environments and, hence, there being a wide range of possible consequences when an accident occurs, an approach to risk assessment under a multidimensional view is required. Moreover, in terms of decision making, the aggregation of the decision maker's preferences in modeling, by multiple-criteria decision-making methods, is more complete, comprehensive, and, in particular, includes considering the decision maker's desires. Therefore, this study puts forward a multidimensional assessment of the risks from underground vaults by generating a decision tool, which ranks the vaults in a risk hierarchy. Multiattribute utility theory was used to achieve this ranking. An application was generated to demonstrate the applicability of the model, under the following aspects of consequences: those that are human, financial, and operational; and disruptions to local vehicular traffic. The use of information arising from analysis of the differences between risks enabled the decision maker to make an in-depth analysis of the range of possibilities over which alternatives may be chosen in order to implement preventive actions.
The physics of fusion welding. Part 1: The electric arc in welding
The electrical characteristics of the arc as used in welding technology are considered, giving some attention to problems of arc reignition and stability, and to the means of assessing the suitability of power sources for welding operations. The behaviour of thermionic and nonthermionic cathodes is reviewed and recent developments in the investigation of nonthermionic cathodes are described. Work on the arc column is divided into sections on properties and fluid dynamics, following which the behaviour of the column during welding is reviewed in relation to specific problems such as arc force, arc stiffness, magnetic effects and arc blow. A final section covers the anode, particularly in relation to the tungsten/argon arc and its use in the gas tungsten arc welding process.