Forensic Review and Characterization of Half-Century Old Network Cables

An electric utility company discovered delaminating underground low voltage secondary network cable insulation in service, in addition to damaged cables with exposed conductors. A condition assessment study was undertaken to characterize the insulation materials to provide insight into the observed cable condition and considerations for possible replacement. Degraded network cables are often implicated in manhole fire and explosion events. Fundamental information about cable materials and their thermal and chemical degradation behavior is often lacking. The cable materials were subjected to chemical analyses using Fourier Transform Infrared Spectroscopy (FTIR) and Gas Chromatography using a Mass Spectrometer detector (GC/MS) for investigating the composition and formulation of the insulation compounds. Further, the chemical analysis methods were used to determine the approximate maximum temperature to which the cable was exposed in service. Other mechanical and microscopical tools were used to support the analysis. Chemical analysis revealed that the insulation material had not been exposed to a temperature above approximately 40°C based on the presence of retained low molecular weight additives, including organic processing aids. There were no indications of oxidative degradation of the chlorinated polyethylene (CPE) insulation. Delamination occurred between the CPE insulation and pigmented CPE outer layer. Pigmented rubber compounds were extruded over the cable core for phase identification. Since these were not co-extruded, no chemical bond formed between the layers. Conductor exposure was determined to have been caused by pull-in damage during installation. For some of the chemical analyses, the cable insulation and pigmented insulation layers were subjected to heating in a closed container with direct injection into the GC/MS system to characterize the gaseous decomposition products at several temperatures approaching pyrolysis. The decomposition products included traces of light combustible gases, such as hydrogen and acetylene. The most abundant concentration of decomposition products was, however, a homologous series of hydrocarbons, with a molecular weight profile similar to that of fuel oils. Combustible gas detectors are available for use in underground structures, however these are not sensitive to heavier hydrocarbons. This research should increase fundamental understanding of network cable aging under normal and adverse thermal conditions, including identification of gaseous compounds that support fires and explosions.