Michael Tay Ming Kiong*, Lim T.B, Chow Y.S
International Association of Forensic Sciences, 16th Triennial Meeting
August 2002, Montpellier, France. (oral presentation)
Objectives: This paper presents a study of damages to a live buried 230-kV electric cable by an excavator bucket at a work-site where cable-laying work was in progress. The cable was buried under about 30 cm of earth. We found that the initial mechanical damages to the cable insulation by the excavator led to further damages by electrical over-heating, swelling and bursting of the current-carrying cable, and burning of combustible materials.
Materials and methods: A 13-cm thick, 1.5-m length of 230 kV electric cable was examined. This transmission cable comprised a 6-segmental copper conductor, surrounded by oil-impregnated paper insulation, and protected by a continuous corrugated aluminium sheath and an outer black medium-density polyethylene (MDPE) jacket. Stereomicroscopy, SEM/EDX and FT-IR were used for microscopic examinations and materials analyses. A suspected excavator operating in the vicinity during the time of incident was studied with regard to the swing movement of its bucket arm. Bucket teeth caps were removed for lab examination and tests.
Results: The middle portion of the questioned cable was found to have burst, with the Al and MDPE layers blown outwards. One side of the burst part had a gradually deepening continuous score; the other side had a more superficial score. Both scores ran longitudinally. The burst aluminum layer near the side of the superficial score had folded by compression. Our examinations reveal that the cable was first subjected to an external mechanical scraping force by a hard object, which moved longitudinally along the axis of the buried cable. This object initially penetrated into the MDPE layer in a gradual manner, then reached further into the aluminium layer, creating folds in the corrugated metal. This damage to the insulation resulted in leakage of insulation oil and a rapid built-up of heat in the current-carrying copper conductor, which was sufficient to melt the copper. An expanding force developed from within the cable resulting in an explosion and ignition of the combustible materials in the cable. Photomicrographs will be presented to illustrate the nature of damages by mechanical force, heat and explosion.
Conclusion: The creation of a long continuous score with gradual penetration into the cable indicates that the hard object had a long steady swing with large radius of curvature and possessed the significant energy and force required to penetrate through the thick PE and Al layers, even folding the thick corrugated Al layer. These initial damages could not have been caused by a manual implement such as a spade, hoe or rake; they are consistent with having been caused by the bucket tooth cap of an excavator operating in close proximity to the buried cable.