Static Electricity 2000 Edition

77–
30
STATIC ELECTRICITY
2000 Edition
9.1.3
If a granular material contains only particles larger
than 420 
µm, then ignitible dust clouds cannot be formed.
However, if a granular material consists of fine particles or
contains an appreciable fraction of fine particles, then
ignitible dust suspensions can be formed and ignition
sources cannot be tolerated.
9.1.4
Experience has shown that, where powders have resistiv-
ities of less than 10
8
ohm-meters, static electric charges usually
relax rapidly enough to prevent their accumulation in the
bulk granular material.
9.1.5 Conductive Intermediate Bulk Containers.
9.1.5.1
Conductive IBCs (e.g., those constructed of metal)
should be grounded during all operations in areas where an
ignitible atmosphere exists. Engineering and administrative
controls should be considered to ensure that grounding of
conductive containers and associated conductive equipment
is accomplished during all operations. For example, metal
funnels should be grounded and flexible fill pipes should have
any conductive components, including any internal stiffening
wires, connected to ground.
9.1.5.2
When a nonconductive material is transferred into a
conductive IBC that is grounded, any charge that has accumu-
lated on the material will remain on the material. The process
of relaxation is the slow migration of the charges through the
material to the opposite charges on the wall. While this relax-
ation process is occurring, which can last a few seconds or
many minutes depending on the conductivity of the material,
an electric field still exists at the surface of the material. In the
case of IBCs with open tops, this electric field can induce
charges on other conductors that might be present, including
ungrounded personnel. Thus, induction of surface charges
onto other ungrounded conductors can occur even when con-
ductive IBCs are properly grounded. Appropriate precautions
should be taken.
9.1.5.3
IBCs made of conductive materials and nonconduc-
tive liners should be used only if the liners are essential (e.g.,
to maintain compatibility between the IBC and the material
being handled.) The risk of ignition and the possibility of elec-
tric shock from propagating brush discharge depend very
much on the thickness and resistivity of the liner, the handling
procedure, the electrical properties of the material being han-
dled, and the incendive nature of any combustible material
that might be present. In general, propagating brush dis-
charge will not occur, provided the nonconductive liner has a
breakdown voltage lower than 4 kV. Each situation should be
considered individually.