First Published in EOS/ESD
Technology Feb/March 1989
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
Member, EOS/ESD Assn. Grounding Standards Subcommittee,
and Quality Assurance Specialist, Santa Barbara, CA
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 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 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 an arbitrary zero voltage reference for the
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 completely due
to related effects. Fig 3 shows how current flow through the worksurface
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, worksurfaces
and wrist-strap ground circuits should be connected only to the
electrical-power-system ground to minimize potential differences.
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.
Testing Is a
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
Electric 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), a consulant on AC distribution and grounding, recommends
1 Ohm 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 AX 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
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 MOhm; however, if the AC system ground meets code
requirements, the resistance should measure less than 100 Ohms.
AC power can be grounded to a common bus rod mounted near the workstations.
Their worksurfaces 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 a common-ground point.
Continuity of less than 1 Ohm can be measured with the wiring analyzer
(s) previously described. If a workstatin frame's resistance is
too high, an approach similar to that shown in Fig 5 should be used.
In any event frames should always be bonded securely to the equipment
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. Fig 6 shows a block diagram of
such a monitor. These monitors sound an alarm if an open circuit
or 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.
For more information
on grounding issues. The author suggests the following sources:
1. C.C. Kleronomos
and E.C. Cantwell, "A Practical Approach to Establish Effective
Grounding for Personnel Protection," IEEE IAS Annual Conference
2. R.H. Lee, "Electrical Safety in Industrial Plants,"
IEEE Spectrum, June 1971.
3. C.F. Daziel, "Electrical Shock Hazard," IEEE Spectrum,
4. National Electrical Code.
5. E.H. Russell, "Static-Safe Worksurfaces and Personnel Safety-
Update," Evaluation Engineering, September 1986.
E.H. Russell, "Safely Grounding Static-Control Worksurfaces,"
EOS/ESD Technology, June 1987.