First Published in EOS/ESD Technology Feb/March
1989
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.
Edward H. Russell
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 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 completely due to related effects. Fig
3 shows how current flow through the worksurface 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, 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 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 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 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 MOhm;
however, if the AC system ground meets code requirements, the resistance
should measure less than 100 Ohms.
Powerless...
Workstations without 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 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. 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 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.
E.H. Russell, "Safely Grounding Static-Control Worksurfaces,"
EOS/ESD Technology, June 1987.
