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
Arc Flash Pressure Measurement by the Physical Method, Effect of Metal Vapor on Arc Blast
The effects of arc flash or arc blasts have received much attention in the electrical safety industry. Although many papers to date have focused on noise (auditory damage) and prediction of pressure, no consensus standard or unified method exists to predict the pressure or thermoacoustic blast created by an arc. There is strong belief in the industry that metal vaporization is a major contributor to the damaging effects and hazards of an arc blast. Our work was to determine if the effects of metal vaporization are the factor in the pressure and thermal hazards resulting from an arc blast. First, the relevant literature reviewed was to compare existing methods for practical measurements of arc blast pressure. Published methods were evaluated to determine if these may be suitable for prediction of pressure in enclosed equipment. To date, few papers provide practical equations that have the necessary parameters to accurately predict pressure that can be used to evaluate the switchgear failure or the effect on workers. The Crawford–Clark–Doughty paper appears to be the most promising. Second, we performed two types of controlled laboratory experiments to evaluate the effects of metal vapor expansion during an arc blast. This is commonly thought to be a significant factor in the pressure generated from an arc blast. The first experiment measured the displacement of an ejected door from which the acceleration and maximum velocity were calculated. The second experiment measured the pressure generated from an arc blast inside a closed box. The second experiment also served as a comparison to existing models. These experiments were done using two electrode materials.
Key Considerations for an Economic and Legal Framework Facilitating Medical Travel
Medical travel has the capacity to counter increasing costs of health care by creating new markets and increased revenue for health services, potentially benefiting local populations, economies, and health-care systems. This paper is part of a broad, comprehensive project aimed at developing a global health access policy (GHAP). It presents key issues to consider in terms of ensuring economic viability, sustainability, and limiting risk to the many stakeholders involved in the rapidly expanding industry of medical travel. The noted economic and legal barriers to medical travel are based on a synthesis of themes found in an extensive review of the available literature. Economic considerations, when setting up a GHAP, include a dynamic approach to pricing that is fair to the local population. Legal considerations include the implementation of international quality standards and the protection of the rights of those traveling as well as those of local populations in recipient countries. By taking into account these opportunities, the GHAP will more adequately address existing gaps in the economic and legal regulation of medical travel.