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The human body has capacitance as it can store charge to a degree
dependent upon how well it is insulated from ground. The body has a resistance
dependent upon muscle tone, moisture content, contact resistance, and other
factors. It is generally agreed that it consists of a capacitor in series with a
resistor. A small amount of inductance, probably a fraction of a micro henry is
believed to have negligible effect on damage results in most cases.
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5.
Requirements of static-free work station:
Wrist-strap
connection to ground:
Many problems have occurred through improper connection of wrist straps.
Wrist straps should never be connected to the work surface as a path to ground
unless the surface is metal. Most work surfaces add far too much resistance for
effective personnel grounding. Using alligator clips for connection to a work
surface increases the problem through excessive contact resistance. The strap
connection point should never be left to chance. In general, a separate hard
wired path to a connecting socket or bus for the strap connection should be
provided.
Personnel grounding can also be accomplished through conductive flooring
used in conjunction with conductive footwear. Conductive flooring can be
achieved by using special conductive tiles, mats, or other conductive floor
coverings or finishes.
Applications for conductive
flooring/ footwear:
Conductive flooring options are commonly used in applications such as:
1. Processes where operator
mobility precludes effectivity of wrist straps.
3. High reliability or other critical static controlled areas
Grounded
work surfaces:
The first requirement is that the surface in use have sufficient
conductivity to be groundable. The primary purpose for a grounded work surface
is to effectively ground conductive items contacting the surface. It also
provides a redundant path to ground for the operator at all times when the
surface is touched. An additional benefit is assurance that the surface will not
acquire a static charge.
Antistatic materials are not generally recommended for work surfaces
because of the excessively high resistance, dependence upon humidity, and
limited life.
Static dissipative materials
in the range of 10 7 to 10 9 are recommended for most
applications. This range does not have the extreme sparking problems of
conductive materials yet is of sufficient conductivity to effectively bleed off
potentials.
Conductive work surfaces such as stainless steel are often favoured
because of durability, rapid charge bleed off, and other factors. An undesirable
feature is the propensity for sparking when a sufficiently charged conductive
item is brought near or in contact with the surface. Thus if conductive surfaces
are used, extreme care must be taken to prevent charge accumulation on
susceptible items to avoid charged device failure.
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