Initiation of a typical network secondary manhole event
Manhole events in secondary network systems can evolve in many ways; however, the most common starting point is SBR (styrene butadiene rubber) or butyl rubber cable, typically with a neoprene jacket, which has been aged thermally (as a result of excessive loading) to the point that cracks have formed all the way to the conductor. One of the authors has seen multiple examples of such cable removed from several secondary networks. When such cable is combined with "dirt" and relatively conductive water (often salty water), tracking is likely to form across the insulation and jacket between cracks which expose the conductor in adjacent phases. "Dirt" in the duct helps bridge any gaps between adjacent phases and also holds the conductive water, which extends the time during which dry band tracking occurs between the exposed conductors. At 208 V phase-tophase, a carbonized track will form very readily with one or a few exposures to small amounts of conductive water, and electrical activity will continue until voltage is removed between the phases. Past work and field experience suggest that under such conditions, the jacket and insulation will burn (if sufficient oxygen is present) or pyrolyze (if not), with decomposition of the polymeric insulation and jacket moving down the cable in the direction of air flow in the duct at a speed in the range of 1 mm/ second [1].