Static Electricity (continued)
Most organic and polymeric materials have volume resistivities
greater than 108 ohms per meter and retain charge for peri-
ods of many hours. The magnitude of triboelectric charging
may be strongly influenced by surface contamination of the
contacting materials, relative humidity of the atmosphere, and
the energy of rubbing. For example, during the grinding of
material during powder manufacture, charge levels of about a
microcoulomb per kilogram are typical. This is a relatively low
level of charge. The more energetic process of pneumatically
conveying a powder along a pipe may give rise to charge lev-
els a thousand times greater and an electrostatic hazard may
arise. Hazards are due to sparks to ground after a relatively
large amount of static charge has accumulated on a body that
is improperly grounded.
In modern industry, highly insulating synthetic materials, such
as plastic powders and insulating liquids, are used in large
quantities in many applications. Such materials charge up
readily, and large quantities of electrical energy may develop
with an attendant risk of incendiary discharges. When, for
example, powder is pneumatically transported along pipes,
charge levels of up to about 100 microcoulombs per kilogram
can develop and potentials of thousands of volts are gener-
ated within powder layers and the powder cloud. Energetic
sparking from charged powder may initiate an explosion of
the powder cloud. Similar problems occur when insulating
liquids, such as certain fuels, are pumped along pipes, and it is
essential that strict grounding procedures are followed during
the refueling of aircraft, ships, and other large vehicles.
The capacity of a person for retaining charge depends upon
stature, but is typically about 150 picofarads. Even the simple
operations of removing items of clothing or sliding off a
chair can lead to body discharges to ground of about 0.1 μC,
which are energetic enough to ignite a mixture of natural gas
and air. Human body capacitance is sufficiently high that, if
poorly conducting shoes are worn, body potential may rise to
15,000V or so above ground during industrial operations such
as emptying bags of powder. Sparking may then occur with
energy exceeding the minimum ignition energy of powder or
fumes, so initiating a fire or explosion. Conducting footware
should be used to prevent charge accumulation on personnel
in industrial situations where triboelectrification may occur.
In the microelectronics industry, extremely low-energy
discharges, arising from body potentials of only a few tens of
volts, can damage microelectronics systems or corrupt com-
puter data. During the handling of some sensitive semicon-
ductor devices, it is imperative that operators work on metallic
grounded surfaces and are themselves permanently attached
to ground by conducting wrist straps.
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