Time to failure of electric cables in seawater

Abstract

In this paper, the standardized insulation resistance test in water for predicting the service life of low-voltage cables is investigated. The basic explanations for determining thermal ageing and estimating the service life according to DIN EN IEC 60216 are based on the assumption of a first-order kinetics of the thermal decomposition reaction of the insulation material. Thermal oxidation, pyrolysis and hydrolysis of the base polymer are among the most important degradation reactions of an insulating material. Hydrolysis reactions are higher order reactions, since the reaction rate is determined by the concentration of oxygen or water. A methodical approach is presented using the example of an iteration loop to optimize the prediction of the time to failure. The non-destructive electrical method, the insulation resistance test is used as the tool for determining a reference value. Therefore, uncertainty in the measuring method for insulation resistance measurement is evaluated. The precise determination of an insulation resistance as a limit value is described and subsequently the change in the insulation resistance of thermally aged and actually aging cables is approximated to a material constant and a temperature function with the aid of an adjustment function. On the basis of this, the material could be classified and evaluated, since the indication of the numerical value of the insulation resistance is only of limited value, since it is very strongly dependent on the measuring conditions.