Manhole explosions due to arcing faults on underground secondary distribution cables in ducts

Arcing faults on low-voltage insulated conductors are widely considered to be self-extinguishing and consequently not very hazardous to people or property. Nevertheless, a number of arcing faults recently occurred on Hydro-Quebec´s underground secondary 120/240 V and 347/600 V systems and caused manhole explosions in downtown Montreal. To trace the probable cause, a series of staged tests was performed in IREQ´s high-power laboratory. The results obtained proved that in wet, dirty ducts an arcing fault can take the form of an interrupted arc which is difficult if not impossible for common overcurrent protection systems to detect. Any such fault that has a long duration produces a significant amount of gaseous by-products which escape to the ends of the duct where they can ignite or form explosive concentrations