Pneumatic Testing of the Insulation of Low-Voltage Underground Cables

The insulation of low-voltage (LV) underground cables may be damaged during handling or cable pulling. In a conducting medium (water or pollution), perforated insulation may lead to the development of an electrical arcing fault. Under certain conditions, gases generated by materials near the fault (e.g., through pyrolysis of insulating materials and/or electrolysis) can result in violent explosions. An analysis on the Hydro-Quebec system has shown that the primary cause of faults is damaged insulation, likely dating back to when the cable was installed. Electrical protection systems like fuses and circuit breakers do not effectively eliminate such (often intermittent) faults, which develop randomly and may occur over a long period (sometimes hours). There is no simple, economical, readily applicable method of verifying the condition of the insulation of cables once installed. Dielectric tests are not effective for detecting broken insulation in a dry environment. A technique is thus clearly needed to effectively verify the mechanical integrity of cable insulation after the cable has been run through ducts. This paper presents a new method of verifying the condition of the insulation of LV cables in order to ensure a quality installation and to prevent possible arcing faults. The approach involves injecting compressed air into the cable core and then checking how well pressure is maintained. Defective insulation is detected by a drop in pressure. An extensive laboratory test program demonstrated that this technique performs well. Prototype insulation testers implementing this method have been successfully tested on the Hydro-Quebec underground distribution system. Test results were conclusive and Hydro-Quebec has decided to pursue industrial production of the testers and to deploy them on its underground distribution system as of 2012.