Static Electricity Produced in Ventilating Ducts and Fan Casings in Hazardous Areas
Ductwork and fan casings in certain installations may be adequately and effectively grounded inherently for static protection purposes depending upon the condition of installation and maintenance. Whether additional grounding is necessary, shall be tested.
All sections of ductwork and fan cases, not complying with above, shall be adequately and effectively grounded in all classified hazardous areas by means of flexible jumper or bonding connections between the various duct sections. They shall be connected to the ground system.
All ductwork extending through roofs shall be grounded. Where the vents or stacks emit flammable vapors and dusts, they shall be protected with air terminals for lightning protection.
Vapor-air mixtures being handled at high velocity in ducts made of nonconductive materials may build up hazardous static charges if dust particles or droplets are present. If such vapors are within explosive limits, trouble may be experienced if the charge gets high enough (at least several kV) to result in discharge. A more likely hazard will occur by a much lower charge (a few thousand volts is enough) causing a spark between metallic parts of the system. When the plastic duct section is connected to a metallic fan, and the fan wheel is not grounded, a charge may be picked up that would be sufficient to cause a spark between the fan wheel and the housing igniting an explosive vapor-air mixture at that point. Methods which can be employed to avoid this hazard include (1) ground the fan shaft using a brush, (2) select a suitable conductive plastic composition for the duct construction, or (3) make all the surfaces of the system that may be in contact with an explosive vapor-air mixture of nonconductive and non-sparking materials.
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