Derating of cables at high temperatures

A method to characterize behaviour of cables used in nuclear power plants at temperatures exceeding the long-term rating was developed. The problem is to predict how long and at what temperatures cables function in emergency situations. The structure and materials of the relevant cables were reviewed and the properties of these materials at high temperatures were described from literature sources. The physical and chemical processes to be expected were outlined for designing tests. Initially we carried out a real-scale equivalent test with a closed circuit, where the pair cable was connected to a pressure transmitter. The instrument PVC cable under investigation worked normally up to 196 °C, where a short circuit occurred, and leakage current suddenly rose to a high value. However, we could not get any evidence of continuous insulation derating from room temperature to the short circuit temperature. This may be due to the experimental arrangements not being sensitive enough to slight phenomenan or to leakages at connections in the electrical circuit. In order to obtain data about the conductivity of PVC cable insulation at elevated temperatures, a part of the open circuit was heated in a test furnace step by step from room temperature to the short circuit temperature region of 200 °C. We discovered leakage current and insulation conductivity improvement in the PVC cable by using a sensitive electrometer and an insulation resistance meter applied in the open circuit experiment for pure cable. Electrical conductivity of PVC cable insulation materials from the literature and electrical conductivity of the PVC cable insulation layer as a function of inverse absolute temperature match quite well. The first series of tests was carried out for a PVC cable to check the validity of the theory and the test method. The testing programme continued and included seven different types of cables