TB-2007 January 2009 Page 1 of 4
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SAFE GROUNDING
OF STATIC-
CONTROLLED
WORKSTATIONS
Although ESD-protective grounding is often crucial, it is far from the only
consideration in workstation grounding; the avoidance of shock hazard to
personnel is even more important.
By Edward H. Russell
Member, ESD Assn. Grounding Standards Subcommittee,
and Quality Assurance Specialist, Santa Barbara, CA
T
misunderstood; it’s used freely and
often without the necessary
modifying specifics. That’s
unfortunate because unless
grounding is understood and its
principles properly applied, an
improperly grounded workstation not
only can allow damage to sensitive
parts, but can present a shock
hazard to humans as well.
grounding. Some authorities relate it
to a connection with the earth, while
Figure 1. Earth grounding in a typical 120-V, 60-Hz electrical system
There are various concepts of
he term “grounding” is often
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others speak in terms of a common
point to which all electrical returns
are connected.
may exist. Electromagnetic,
electrostatic, signal, chassis and
power grounds may be addressed
separately in order to control noise,
interference, and undesirable
current flow.
electrical storms are intense, special
rods are installed on buildings to
divert lightning to earth through
heavy-gauge wire. A similar
In electronics, different grounds
In geographic areas where
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configuration is used in electrical
power systems. The National
Electrical Code recommends that
the neutral circuit of all AC systems
be terminated in some conducting
body in the earth. Here’s why: such
a grounding system...
1. Maintains an arbitrary zero
voltage reference for the AC system;
2. Protects a building from lightning;
3. Drains equipment’s static charge.
Fig. 1 shows how earth grounding is
configured in a typical 120-V, 60-Hz
electrical system. This type of
system is the one most commonly
used at workstations in industry
today and is, therefore, a primary
subject of this article.
An ESD Ground?
system grounds should not be used
for ESD-protective grounding, but
this is not necessarily the case. A
frequent alternative to using the
system ground for ESD is a
separate earth ground rod installed
near the workstation for ESD use,
and this is indeed effective in
draining away static charges.
However, such a ground can be
hazardous to personnel working
near electrical equipment; such a
situation could allow a potential
difference between equipment and
the separate ESD ground.
an electrical overstress (EOS)
hazard might exist between, say, a
soldering iron (grounded through the
power system) and a worksurface
using the separate ESD ground
described above. Fig. 2 illustrates
the potential problems.
both for people and parts. However,
a properly grounded piece of
electrical equipment placed on a
grounded worksurface reduces this
hazard. However, there is still the
possibility of ground current flowing
through the worksurface itself, and
static charges might not bleed off
One might think that electrical
In addition to personnel hazards,
Obviously, this is undesirable
