The choice of a section of power cable is generally the result of examination of two criteria: the criterion of maximum heating at full speed and the criterion of voltage drop compatible with the proper functioning of the controlled devices. But when the rate of use of the cable becomes appreciable, more than a thousand effective hours per year, a new criterion must be considered, it is the criterion of profitability. Under these conditions, the optimum section is obtained by: Section(optimum)=I(3ρr_i p/B)^(1/2) x11mm^2.

This article is devoted to the analysis of the behavior of the suspension springs of the locomotives U22C of the National Railway Company of Congo (SNCC in abbreviation) which springs undergo repeated breaks, reducing the availability of the machines. Operation and causing a lot of purchase and maintenance cost as a replacement. Indeed, the locomotives U22C which is discussed in this article have total weight of; that is 18.3 tons per axle and 4575kg per wheel. Since their start-up, these locomotives know a lot of breaks of suspension springs and our first reflex was to check the behavior of the springs on the strength of materials to ensure their effectiveness before considering other tracks of research of the causes of the breaks. Both the theoretical calculations and the press tests have shown that the static load of these springs cannot cause these breaks, the stresses and the weakening of these springs under this load being lower than the admissible limit. This being the case, other avenues of research that are not part of this article will have to be considered in order to find the real cause and remedy of them quickly.