Insulation failure in telecommunication cables resulting from water condensation in aerial terminal closures and its prevention using a humidity-controlling polymer

Abstract

Insulation failure in telecommunications cables after extended use in aerial terminal closures has been examined. Extensive cracking and sea salt deposition were observed on the polyethylene (PE) insulation of wires in a terminal closure recovered from the field after 27 years of service. The insulation failure mechanism appears to be the formation, by bulk water condensation with deposited sea salt dissolution on degraded PE insulation, of ionically conductive pathways among the PE insulation cracks. An experiment using wires with PE insulation that was deliberately cut to simulate cracking showed that controlling the humidity within the terminal closure is an efficient way to prevent insulation failure. A field experiment showed that placing a polymer that absorbs moisture when the humidity is high and releases it when the humidity is low in the closure effectively buffered changes in humidity, thus preventing water condensation. Another experiment using wires with deliberately cut insulation in a terminal closure with 100% RH and with the water-absorbent polymer placed inside showed that the resistance of the wires stayed at around 10/sup 8/ /spl Omega/ without fluctuation. Since the absorbent polymer continues to function for many years, this proposed prevention method is well suited for practical application.