The corporate cost of capital is used by valuators to discount future flows of income from an entity in order to derive a present-day, forward-looking value of that entity. The cost of capital is therefore determined as the weighted cost of the various sources of funding, being typically equity, debt and preference instruments. The tricky and important part is estimating the cost of equity, which usually needs the application of finance models. The study on the texts on mineral valuation or mineral project evaluation demonstrates that the capital asset pricing model (CAPM) is a general model for estimating the cost of equity. However, according to shortfalls and problems relating to it a relatively similar and simpler model i.e. the single-index market model is proposed. The single index market model is an important tool in contemporary research in finance. Much of the importance of the model follows from its 'beta' parameter which, ideally, measures the sensitivity of returns on a security to changes in a market model. To estimate the cost of equity of the mining and cement companies listed in Tehran Stock Exchange (TSE) The single-index market model is selected because of the shortfalls and problems of the CAPM as well as the lack of commercial services for determining the market premium. The regression analysis as well as the statistical analysis is carried out using Excel spreadsheet. The statistic significance of the model is tested using t and F test statistics. The results showed that the independent variable (the rate of return on the market index) has a genuine effect on the dependent variable (the rate of return for the stock) and there is a statistically significant linear relationship between the two variables at significance level of 5%. Finally, the cost of equity formining and cement companies is estimated 25.0% and 31.0% respectively. Knowing the cost of equity, calculating the discount rate will not be very difficult.

This paper presents the results of testing samples of shield-centering elements from medium-voltage surge arresters. The elements were made of TSE glass textolite. The elements have been dismantled from different operated surge arresters, which were subjected to discharge currents (short-circuit currents) of different intensity and duration. The discharge currents led to degradation of the tested elements with various degrees of advancement. The degradation was investigated using microscopic methods and energy-dispersive X-ray spectroscopy (EDS). Changes in the content of elements of the surface of textolite materials – as the degradation progresses – were documented.
It was found that high discharge current flows resulted in melting of the organic binder, epoxy resin, especially its surface layer. Partial charring and even burning of the resin was noticeable. Furthermore, it was found that with increasing degradation on the surface of the TSE laminate, the carbon and oxygen content, which are part of the organic resin, decreases. Simultaneously the amount of silicon, calcium and aluminium, which are present in the glass fibres, increases. The charring effect of the resin and the formation of conductive paths result in a decrease in the performance of surge arresters and their subsequent failure.