Manhole Events Caused by Secondary Cable Insulation Breakdown

Utilities continue to experience electrically induced manhole events. Eighty-five percent of manhole events are caused by insulation breakdown, and 97% of the breakdowns occur on secondary cable insulation. The vast majority of manhole events involve only smoke, but a very small minority involve more serious phenomena. The restrictive conditions under which secondary cable insulation breakdown leads to dangerous manhole events needs to be understood. To approach this issue, thermal decomposition of secondary cable insulation was investigated in a simulated duct environment. Sustained decomposition is achieved under both aerobic and anaerobic conditions. The consequences of insulation degradation as a function of temperature with varying oxygen concentration has been monitored using a residual gas analyzer, and the thermal decomposition rate of the cable dielectric has been measured using TGA. Based on this investigation, we are approaching an understanding of the restrictive conditions which lead to serious manhole events.