Cyclic Movement of Cable-Its Causes and Effects on Cable-Sheath Life

Results of a field survey and laboratory tests are presented in this paper showing the relationships between loading, conductor-temperature changes, movement in manholes and ducts, and sheath life for paper and lead cable. The record on underground 15-kv cable is included to emphasize the noninherent failures attibutable to physical proportions of underground structures. Improvements in modern cable have permitted operation at increasing temperatures with few inherent failures. The tests and operating experience indicate that unsatisfactory sheath life caused by fatigue of lead can result from cyclic tempratures allowed in standard cable specifications. A method is outlined for computing sheath strains for given cable off-sets and movements at the duct mouth. Fairly reasonable estimates of sheath life can be prepared. The tests indicate also that snaking of cable within the ducts may impose severe cyclic strains on the lead and become the determining factor for optimum sheath life.