Safe grounding of static controlled workstations Datasheet by Desco

: DESCO '_- ‘-— [.1th Fund Hm Nouns] Ground
TB-2007 January 2009 Page 1 of 4
©COPYRIGHT 1989
MILLER FREEMAN PUBLICATIONS, INC.
ALL RIGHTS RESERVED
THIS ARTICLE APPEARED IN THE FEB./MARCH ISSUE OF
EOS/ESD TECHNOLOGY. IT HAS BEEN REPRINTED WITH
PERMISSION OF MILLER FREEMAN PUBLICATIONS,
1050 COMMONWEALTH AVE., BOSTON, MA 03215.
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
Figure 1. Earth grounding in a typical 120-V, 60-Hz electrical system
others speak in terms of a common
point to which all electrical returns
are connected.
In electronics, different grounds
may exist. Electromagnetic,
electrostatic, signal, chassis and
power grounds may be addressed
separately in order to control noise,
interference, and undesirable
current flow.
In geographic areas where
electrical storms are intense, special
rods are installed on buildings to
divert lightning to earth through
heavy-gauge wire. A similar
The term “grounding” is often
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.
There are various concepts of
grounding. Some authorities relate it
to a connection with the earth, while
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?
One might think that electrical
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.
In addition to personnel hazards,
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.
Obviously, this is undesirable
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
DESCO WEST - 3651 Walnut Avenue, Chino, CA 91710 • (909) 627-8178 • Fax (909) 627-7449
DESCO EAST - One Colgate Way, Canton, MA 02021-1407 • (781) 821-8370 • Fax (781) 575-0172 • Web Site: http://www.desco.com
TECHNICAL BULLETIN TB-2007
Circuil halos M: Equipmlni “blind-n ;— m” " ’W'HF" «SEW 1 1% E """u' ‘~ "-5.3."un'"- ”E «(be '\Eanhl \mimnc: muz/' _ “v, \— Equipmzm m =-------— - fiV-YV".___-__-.- [IIIII ankle-mum Emhz .,_,=e© mm=-cne
TB-2007 Page 2 of 4
We make our wrist straps with the
industrial environment in mind.
They are designed and
manufactured to give you years of
service in the tough electronic
assembly environment.
May we send you a sample?
Quality
Ground Cords An equipment ground conductor
is often used in case a fault
develops in the primary power-
system ground. Fig. 4 shows the
relation between equipment ground
(also referred to as safety ground)
and earth ground. This
methodology is designed to handle
situations in which 120-V power
accidentally contacts the frame of
electrical equipment. So-called fault
current will flow through the
equipment ground, triggering the
local circuit breaker. Installation of
Ground Fault Circuit Interrupters, or
GFCIs, increases safety further.
completely due to related effects.
Fig. 3 shows how current flow
through the work surface is
generated.
This problem can be solved by
bonding the two earth grounds
together, eliminating the resistance
between them. If electrical power is
nearby or incorporated in
workstations, work surface and
wrist strap ground circuits should
be connected only to the electrical-
power-system ground to minimize
potential differences.
Figure 4. The relationship between equipment (or safety) ground and earth
ground.
Figure 3. How current flow through a work surface is generated.
Figure 2. Separate ground systems present potential problems.
For the highest quality ground
cords for mats, work surfaces, and
equipment call Desco. Ask for a
sample.
Industrial Grade
Wrist Straps
DESCO WEST - 3651 Walnut Avenue, Chino, CA 91710 • (909) 627-8178 • Fax (909) 627-7449
DESCO EAST - One Colgate Way, Canton, MA 02021-1407 • (781) 821-8370 • Fax (781) 575-0172 • Web Site: http://www.desco.com
Gammon bui md Tu urlh wound Wuhllrlloe 2‘s
TB-2007 Page 3 of 4
Testing Is a Must
Electrical systems should
always be tested before depending
on a grounding system. Accurate,
reasonably priced and easily
operated systems analyzers are
available. The analyzer used
should both detect flaws in the
system and also verify the wiring
configuration per the National
Electrical Code. Electricians aren’t
needed for these tests unless
discrepancies are noted.
To maximize safety, the
analyzer should be able to detect
flaws such as an equipment
ground shorted to neutral in the AC
outlet box. It should also be
capable of testing equipment-
ground impedance by injecting AC
current into the equipment-ground
conductor and measuring between
it and the neutral conductor. ECOS
Corp. (Oak Park, IL, Phone 708-
383-2525, Fax 208-383-2137), a
consultant on AC distribution and
grounding, recommends 1 S
between ground and neutral.
Doing It Yourself
Though it should rarely be
necessary, there are adequate
means of measuring earth-ground
resistance in AC systems. Install a
5/8-in. by 6-ft. copper rod in the
earth near an electrical circuit. Using
a specialized ground-resistance
tester, measure the earth’s
resistance between your auxiliary
earth ground and the AC system’s
ground conductor. Ordinary meters
are not recommended for this
measurement due to the possibility
of invalid measurements; ground-
resistance testers can guard against
earth currents (and consequent
measurement inaccuracies); the
typical VOM cannot.
The resistance will vary
depending on the nature and
moisture of the soil and the physical
implementation of the system
ground. For ESD purposes, the
maximum resistance allowable for
rapid static decay is about 1 MS
however, if the AC system ground
meets Code requirements, the
resistance should measure less
than 100 S.
Figure 5. Grounding strategy for workstations without AC power.
Figure 6. Block diagram of a ground-integrity monitor.
Wrist Strap Ground
Grounds Mats,
Grounds Wrist
Straps, and
Tests at a Touch
Item No. 19350 is a wrist strap
tester that also grounds your
mat. Call us if you would like to
see one.
Item Nos. 09740 and 09835
provide a secure ground at
each work station, and comes
complete with grounding wire
and instructions.
09835
09740
DESCO WEST - 3651 Walnut Avenue, Chino, CA 91710 • (909) 627-8178 • Fax (909) 627-7449
DESCO EAST - One Colgate Way, Canton, MA 02021-1407 • (781) 821-8370 • Fax (781) 575-0172 • Web Site: http://www.desco.com
TB-2007 Page 4 of 4
DESCO INDUSTRIES INC.
Employee Owned
analyzer(s) previously described. If
a workstation frame’s resistance is
too high, an approach similar to
that shown in Fig. 5 should be
used. In any event, for added
safety, such metal frames should
always be bonded securely to the
equipment ground.
Monitoring
To ensure the continuing safety
of people and ESD-sensitive
electronics, all static-controlled
workstations should be periodically
tested for ground integrity.
However, even periodic testing
can’t detect an immediate break or
degradation in a grounding circuit.
When such a break is eventually
uncovered, it is often too late for
some parts, thus various constant
monitoring systems are available.
Figure 6 shows a block diagram of
such a monitor. These monitors
sound an alarm if an open circuit
of ground-circuit degradation
occurs. Unfortunately, some of
these monitors are undependable,
so it’s good policy to periodically
test not only ground integrity but
the associated monitors as well.
1. C.C. Kleronomos and E.C.
Cantwell, “A Practical Approach to
Establish Effective Grounding for
Personnel Protection,” IEEE IAS
Annual Conference Record, 1979.
2. R.H. Lee, “Electrical Safety in
Industrial Plants,” IEEE Spectrum,
June 1971.
3. C.F. Daziel, “Electrical Shock
Hazard,” IEEE Spectrum, February
1972.
4. National Electrical Code.
5. E.H. Russell, “Static-Safe
Worksurfaces and Personnel
Safety - Update,” Evaluation
Engineering, September 1986.
6. E.H. Russell, “Safely Grounding
Static-Control Worksurfaces,”
EOS/ESD Technology, June 1987.
Desco Makes Grounding
of Work Surfaces and Floor
Mats Fast and Easy
Powerless...
Workstations without AC power
can be grounded to a common bus
rod mounted near the workstations.
Their work surfaces should be
connected to the ground bus with
wire of 20-gauge minimum
diameter and preferably larger.
Heavy stranded wire may be
preferable to a less-durable solid-
conductor. The end of this
grounding bus should be bonded
to a convenient AC-equipment-
ground circuit. This type is shown
in Fig. 5.
Equipment-ground conductors
in workstations wired with 120-
VAC can be used for grounding,
provided the circuit has been
tested and has passed. In
addition, the equipment-ground
circuit should be bonded
electrically to a workstation’s metal
frame if that frame is to be used as
common ground point.
Continuity of less than 1 Scan
be measured with the wiring
Our ground cords are designed to hook directly
onto any nearby “green wire” utility ground.
Ask for Technical Bulletin TB-2000 for details on
proper grounding and installation of ESD protective
surfaces or TB-2005 for proper maintenance and
grounding of wrist straps.
Installing push and clinch snap for
use with multi-layer or soft
conductive mats Item No. 09863
Our complete line of field
installable snaps makes grounding
a “snap”. Now you can place your
single ground point any place on
your work surface or floor mat.
Our common point ground cords and testers
allow for “common point grounding”.
Universal
Screw-on Kit
Item No. 09864
19350
09825
19210
Snap assembly
installable with a rivet
hand tool.
09740 09835
DESCO WEST - 3651 Walnut Avenue, Chino, CA 91710 • (909) 627-8178 • Fax (909) 627-7449
DESCO EAST - One Colgate Way, Canton, MA 02021-1407 • (781) 821-8370 • Fax (781) 575-0172 • Web Site: http://www.desco.com