Effect of limiting airflow in mitigating combustion-driven manhole events

"Manhole event" refers to a range of phenomena, such as smokers, fires, and explosions, which occur in underground electrical infrastructure, primarily in large cities. Whereas some manhole events are caused by failure of primary distribution cable, most are caused by failure of unshielded secondary cable in network systems. Based on analysis and knowledge of field conditions, manhole events can be divided into at least two classes, those in which exothermic chemical reactions dominate and those in which electrical energy dominates [4]. In an electrically driven event, the arcing energy is so great that it causes rapid vaporization of the conductor and nearby polymers, which sweeps the oxygen from the cable duct so the event evolves through electrical fault energy induced pyrolysis. In the more common combustion-driven events, the electric fault energy ignites the cable insulation near the fault, resulting in self-sustained combustion as a result of air flowing down the duct. This establishes a combustion zone, which consumes oxygen and liberates heat, causing anaerobic decomposition of polymeric materials downstream of the combustion zone, which results in generation of combustible gases. In combustion-driven manhole events, airflow through the duct plays an important role in the combustion propagation in the duct and the evolution of the manhole event.