Grounding Considerations for an Electrostatic-Protected Area

David E. Swenson

A revised grounding standard from the ESD Association should remove the guesswork from setting up an ESD-controlled environment.

Electrically interconnecting all conductive and dissipative items within an environment—that is, bonding them—and attaching them to ground creates an area free of differences in electrical potential. Also, attaching electrically conductive and dissipative items to ground equalizes any existing potentials with the 0-V reference of ground. These fundamental principles form the basis for establishing an area where items susceptible to electrostatic discharge (ESD) can be handled or processed with reduced risk of damage arising from local differences in electrical potential. This is known as an electrostatic-protected area (EPA).

In addition, grounded conductive and dissipative items cannot hold an electrostatic charge. So, any conductive or dissipative item that may have a charge will lose that charge when it comes into contact with any part of the grounding system.

The ESD Association standard for grounding, ANSI/ESD S6.1, was updated in a major way in 2005.¹ The new S6.1 gives ESD control practitioners and others tasked with implementing ESD control programs substantive guidance on the subject of electrical bonding and grounding within an EPA. Preparers of the revised document researched the U.S. National Electrical Code NFPA 70, of the National Fire Protection Association, and other national electrical codes from around the world in conducting their work.²

This article provides information to assist interested readers in using the new standard S6.1. It is not intended to replace the standard.

New Terminology

Several new terms are introduced in the updated ANSI/ESD S6.1 standard. A few of the terms used in this article are defined here since they may be unfamiliar.

A bonding conductor is the wire, strap, flange, or other electrically conductive mechanical device used to interconnect two otherwise isolated conductive or dissipative items.

A common connection point is a device or location, less than 1 W within itself, where the conductors of two or more ESD technical elements (see below) are connected in order to bring the ungrounded ESD technical elements to the same electrical potential through equipotential bonding.

The ESD grounding/bonding reference point is the ESD grounding system selected for use in a facility or situation which best suits the application. As discussed below, this could be the ac equipment ground, an auxiliary ground, or equipotential bonding.

ESD technical elements are all of the items, materials, devices, tools, and equipment used within an EPA to control static electricity. Examples of these are wrist straps, shoe grounders, and static-protective table and floor mats.

A Level 2 technical element is any ESD technical element that is connected in series to common-point ground or to a common connection point through another technical element. An example of this would be a person wearing shoe straps who is standing on an ESD-protective floor.

Choosing the Grounding System

The first decision that must be made in setting up an EPA involves choosing the grounding system that best suits the application. In the standard, several options are provided under the definition of an ESD grounding/bonding reference point. The preferred ground in most factory situations is the ac equipment ground. Sometimes it is necessary to use an auxiliary ground or equipotential bonding.

Ac Equipment Ground. This is most often defined as the third-wire terminal of a standard North American 120-V ac receptacle. The signifying color is generally green or green with a yellow stripe, although other colors may be used in certain countries. This wire is the equipment grounding conductor that connects the receptacle to the ground bus, which in turn is bonded to the neutral bus in the main service equipment (i.e., the power panel at the service entrance) (see Figure 1).

Figure 1. A typical North American electrical system with common-point ground attached.

In most factories, the electrical conduit is electrically equivalent to the equipment grounding conductor and may be used as ac equipment ground. Of course, a properly wired facility is needed. If there is any doubt about the wiring, it should be verified by a licensed electrician. Figure 1 provides guidance.

Auxiliary Ground. An auxiliary ground system is sometimes thought to be the best way to provide the necessary ground in an EPA because any ESD spikes or electromagnetic-interference signals will be kept away from electrical equipment that is not attached to that system. But after considerable research and discussion, the working group for the original grounding standard back in 1985 discovered that the difference in potential that could—and does—exist between different grounding systems could be the bigger issue. Members of that working group reported seeing differences in potential up to 500 V between the ac equipment ground and specialized auxiliary grounds. If this were the case at a workstation, it would create an immediate and ongoing problem with respect to the objective of maintaining a near-zero difference in potential for the optimal handling of ESD-susceptible parts.

This was the main consideration behind the S6.1 standard stating that auxiliary ground and ac equipment ground need to be bonded together whenever possible. While there may occasionally be situations where an auxiliary ground is needed for some sort of specialized activity, in those rare cases it is important to remember that a considerable difference in electrical potential between that ground and the electrical equipment being used in the area may exist.

Equipotential Bonding. This option is most appropriate for remote or field operations where equipment is being serviced and a properly set up EPA is not available. Electrically bonding all the ESD technical elements, the personnel within the work area, and the equipment under service creates a system with equalized electrical potential. The actual electrical potential relative to electrical ground may be elevated, but everything within the system will be at some shared potential.

This method equalizes any differences in electrical potential or electrostatic charge between components of the system so that ESD-susceptible items within the system can be safely handled. When there is no difference in potential within the handling system, no discharge is possible. Figure 2 schematizes the concept of equipotential bonding.

Figure 2. Equipotential bonding equalizes potential among items in a system, without reference to electrical ground.

Creating a Common Connection Point

Once the ESD grounding/bonding reference point has been established, the next step in setting up the EPA is to establish a common-point ground or common connection point. The common-point ground is a grounded device or location—electrically attached to the ESD grounding/bonding reference point—where the conductors of one or more ESD technical elements are bonded. The common-point ground device can be anything that works as an electrically equivalent location for mechanically attaching the technical-element conductors. Terminal strips, bus bars, copper cables, and the like can form a common-point ground or common connection point. All of the ESD technical elements—wrist straps, table mats, floor mats, and so on—are terminated, by means of their grounding wires, to the common-point ground.

The simplest such system is illustrated in Figure 3. A wrist strap and work surface are electrically connected through their grounding wires to a common-point ground that in turn is attached electrically to the ac equipment ground.

Figure 3. Schematic illustration of the basic EPA concept.

As an EPA becomes more complex, the concept of Level 2 technical elements comes into play. A Level 2 technical element derives its path to ground from a primary technical element on which it depends. A technician standing on an ESD floor mat or ESD-protective floor is considered at Level 2 because that person’s footwear or shoe-grounding device has to make contact with the person as well as the flooring system in order to provide an electrical path to ground from the technician’s body (see Figure 4). An ESD chair and a cart with conductive wheels also are Level 2 technical elements, as they follow their paths to ground through the flooring system.

Figure 4. A complex EPA with Level 2 technical elements.

Putting Technical Elements in Place

The third step in setting up the EPA is determining what ESD technical elements will be necessary to ensure that unprotected ESD-susceptible parts can be handled with minimal risk of exposure to any electrostatic influence. The applications at the workstation will dictate the type and range of technical elements needed to create an effective EPA.

The basic rules state that, for seated operations, a worker must use a wrist strap as the personal grounding device. Even if an ESD-protective floor is in place, a seated person must use a wrist-strap type of grounding system. This is because his or her feet may very often not be on the floor or have only minimal contact with the floor.

Standing or mobile operations may employ an ESD-protective floor and footwear system as a grounding system for personnel. If the floor and footwear system is the primary grounding method for personnel, the performance specification has to be the same as that set for a wrist strap grounding system. As recommended in the ESD control program standard S20.20, for an EPA rated at a protection level of 100 V human body model and greater, the resistance to ground for personnel should be no greater than 35 MΩ.³

Another technical element commonly employed in an EPA is an ESD-protective work surface. The application will dictate the specific type and form of work surface, but the idea is to create an area where unprotected ESD-susceptible items can be worked on without generation of a static charge or dissipation of any static charge that might be present. Another objective is to minimize electrical fields and differences in electrical potential within the workstation.

Other application-specific technical elements are garments, chairs, carts, shelving, and other items used to provide ESD protection that are bonded to the grounding system. Each of the additional technical elements must be attachable to the common-point ground either directly or as a Level 2 element. Understanding how an ESD technical element makes electrical contact with ground is critical to determining a methodology for verifying that the item is doing what it was intended to do.

Ground Measurements

The ESD Association standard for grounding specifies test methods for determining that ESD control performance in the EPA is adequate. A summary of measurement procedures follows.

Ac Equipment Ground. The impedance of the equipment grounding conductor from a receptacle being used to establish the ac equipment ground to the neutral bus at the main service entrance shall not be greater than 1 Ω. This measurement is made with an ac circuit tester that plugs into the receptacle. Although this is not an instrument an ESD coordinator usually has, it should be a common device for licensed electricians to possess. This type of instrument not only measures the impedance of the equipment grounding conductor (the third wire); most of them will also verify the wiring orientation. It is important to make sure of several things: that the hot and neutral are not reversed, that the neutral and equipment grounding conductor are present and not connected to each other at the receptacle, and that the hot and equipment grounding conductor are not reversed.

Although this type of testing is not required often, it is important for verifying the wiring during initial setup of the EPA and after any electrical maintenance in the area that might affect the wiring orientation.

Ac Equipment Ground to Common-Point Ground. The resistance of the ac equipment ground to the common-point ground shall not be greater than 1 Ω. This measurement is made using a standard ohmmeter capable of measuring between 0.1 Ω and 1 MΩ (see Figure 5). The probes of the ohmmeter should be placed on the ac equipment ground and on the common-point ground so that any additional resistance contributed by connecting devices is included in the measurement. This includes the wiring between the two defined points.

Figure 5. Measurement of the resistance of the ac equipment ground to the common-point ground.

Common-Point Ground to Technical-Element Groundable Points. The resistance from the common-point ground to each technical element’s groundable point shall be no greater than 1 Ω plus the value of any resistor that may be included in the grounding conductor from the technical element. The probes of the ohmmeter should be placed such that any added resistance is included in the measurement. The technical element’s grounding wire often contains a resistor of typically 1 MΩ. Added resistance is not a requirement of the standard, but manufacturers of ESD technical elements may place a resistor in wires for current-limiting purposes.

Auxiliary Ground to Ac Equipment Ground. For this measurement, the ohmmeter should be connected between the ac equipment ground and any auxiliary ground that is used within the facility. The measurement probes shall be placed to include the resistance of all the interconnecting and securing devices, and the measured resistance shall not be greater than 25 Ω. NFPA 70 refers to auxiliary ground as supplemental ground. The frame of a building, buried metal grid, or any other mechanism that contacts the earth may be a supplemental, or auxiliary, ground.

The 25-Ω resistance value may be too high for some applications. Therefore, care should be taken when creating a facility specification. From the perspective of grounding for electrostatic purposes, 25 Ω or less is an acceptable specification; however, for electromagnetic-compatibility applications, the value is too high.

Field Service. In field service applications that do not use the equipment grounding conductor, the ohmmeter is to be connected between the groundable point of the ESD technical element and the item being serviced. The measurement probes are placed so that the measurement includes the resistance of all the interconnecting and securing devices. The total resistance shall not be greater than 1 Ω. If a resistor is used in the bonding conductor from the ESD technical element, the resistance value shall include the value of the resistor.

A common connection point may be used if more than one ESD technical element is needed in the servicing operation. For example, a service technician may have a wrist strap and a portable dissipative mat upon which to repair a part or prepare a part for insertion into the equipment. The common connection point may be a separate device, the metal frame of the equipment, or the ground bus of the equipment. As long as all the items involved in servicing the equipment, including the person, are interconnected electrically, ESD-susceptible parts will not be exposed to any electrical potential differential.

Conclusion

The new ESD Association standard on grounding helps take some of the mystery out of establishing a grounding system in an EPA. This revision of the recommendations should simplify the jobs of those responsible for implementing ESD control programs. The time for them to become thoroughly familiar with the updated ANSI/ESD S6.1 is now.

References

1. ANSI/ESD S6.1:2005, “Grounding” (Rome, NY: ESD Association, 2005).
2. NFPA 70, “2005 National Electrical Code” (Quincy, MA: National Fire Protection Association, 2005).
3. ANSI/ESD S20.20:1999, “Protection of Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically Initiated Explosive Devices)” (Rome, NY: ESD Association, 1999).

David E. Swenson, president and cofounder of Affinity Static Control Consulting LLC (Round Rock, TX), is senior vice president and manager of professional education for the ESD Association. He also chairs the grounding-standard working group responsible for ANSI/ESD S6.1. He can be reached via e-mail at deswenson@affinity-esd.com or by telephone at 512-244-7514.