Fowler Associates Labs

A Thoughtful Approach To Field-Service Grounding

Exceptional care must often be taken to protect personnel and ESD-sensitive parts from the consequences of improper grounding. To help deal with grounding problems in the field, here is advice form a member of the ESO/ESD Grounding Standards Committee.

Charles R. Hynes
Member, ESO/ESD Assn. Grounding Standards Subcommittee
and ESD Specialist, Atrix Inc., Burnsville, MN

Grounding sounds simple, yet there are many considerations that must be dealt with when we attempt to solve electrical overstress and electrostatic discharge (EOS/ESD) problems by using proper grounding in and around static-sensitive devices.

Ironically, after building such EOS/ESD-sensitive products as laser printers, computers and copiers in plants with total ESD control, these products usually operate in ESD-hostile environments for the rest of their lives. When latent failures, power surges and ESD events occur in use and cause failure or malfunction, a field-service technician is called to the site to make repairs. He or she may be required to service a product that is not located on the forty-fifth floor of a high-rise office building. As a result, to control ESD and field effects the technician may encounter, he or she becomes dependent on equipotential or floating grounds of uncertain characteristics unless there is safe, positive access to an earth ground of known quality.

This article discusses the use of the equipment ground circuit of a building as a nonfloating ground in field-service situations, where zero or near zero static voltage levels are required. The scope is limited to situations and locations where, for practical purposed, no AC frequencies above 60Hz are within reach of a field-sevice technician.

Although the term impedance is occasionally used throughout the course of this article, it is used parenthetically in order to call the reader's attention to areas of importance to personal safety, in which higher AC frequencies are encountered as a practical matter, the formula that is derived form MIL-STD-454 (Ref 1) does not apply.

Resistance to Ground Limits

Webster's Dictionary (Ref 2) provides three definitions of an electrical ground: A. The position or portion of an electrical circuit at zero potential with respect to the earth; B. A conducting connection to such a position or to the earth: C. A large conducting body, such as the earth, used as a return for electric currents and as an arbitrary zero of potential.

In definition C, we are confronted with the fact that large conducting bodies may only provide an arbitrary zero potential. Too often we incorrectly assume that when we connect the ground cord from a static-controlled workstation to a cold-water pipe, we have provided sufficient grounding to prevent ESD and EOS damage.

According to the Navy Electrostatic Discharge Training Manual, "In order to limit residual voltages caused by static generation at a typical ESD-grounded workbench to approximately 10 V, the maximum resistance to hard ground should not exceed 10 M Ohms" (Ref 3). This upper limit was presumably set to establish a two-second decay time for an electrostatic discharge from a person and is based on the RC time constant used for the human-body model as shown in FIG 10-1 of that document. The minimum resistance to ground is governed by personnel safety considerations.

Soft vs. Hard Grounds

To reduce the hazard of severe electrical shock, any conductive worksurface must be properly soft-grounded. A conductive worksurface should not be directly connected to a hard ground, that is, a connection to ground through a path that has little or no resistance.

[EOS/ESD Technonlgy Editor's Note: The nature of the application must be taken into account when determining the need for series resistance to ground. Mr. Hynes is accurate with respect to field-service situations in which the presence or absence of 60-Hz power on objects in the work area is unknown. Obviously, in such a situation, personnel protection must take first priority because the uncertainties in the work situation place personnel at risk. However, in the case of a workstation in a controlled environment such as a factory, where proper precautions have been taken in the control of 60-Hz power, and where regular inspections take place and monitoring techniques are used, the likelihood of damage to components may be greater than the risk to personnel, and harder grounds are more permissible. In the latter case, the desired charge decay rate can become the controlling factor.]

All possible parallel paths to ground- even those beyond the immediate work area- from people, metal furniture, electrical equipment, floor mats, table mats, wrist straps, etc., must be considered. An unprotected paralleled path to ground could reduce the resistance needed for personal protection. As one example, an uncovered stud on the underside of a table mat or field-service mat can become an unimpeded parallel path to ground if the mat or field-service kit is placed on a grounded metal table or conductive computer cabinet. Also, try to keep one hand in a pocket whenever working around unknown voltages.

Alternatively, too much resistance to ground will affect the static-decay rate of the worksurfaces, and the worksurfaces will not drain static electrical charges within the interval required for safe handling of sensitive electronic devices.

As opposed to a hard ground or an overly resistive ground, a soft ground is mandatory. A soft ground is a connection to ground through a resistance high enough to limit DC current flow to less than 5 mA. The resistance needed to achieve a soft ground is dependent upon voltage levels and AC frequencies that could be contacted by personnel near the ground.

Under normal circumstances, where only 110-V, 60-Hz, AC power sources are within reach of a person, an absolute minimum of 250,000 Ohms resistance (impedance) should be used.

If higher voltages sources or higher frequency voltages are within reach, the resistance (impedance) needed to achieve a soft ground must be calculated. Where only 60-Hz AC frequencies are present, the following formula may be used:

Based on this formula, a 1-MOhm resistor provides ample safety margin, even if 220-V, 60Hz AC power sources are within reach.

Periodic checks should make sure that an electrically conductive leakage path, limited by the proper resistance, exists between the wrist strap and ground and between the floor or table mat and ground. All ground cords should be examined for wear and tear, and all resistors should be checked on a regular basis to make sure they are functioning properly.

Common-Point Grounding Systems

Figure 1. Common-point grounding per DOH-Handbook 263.

Two optional wiring diagrams are provided. Fig 1 is the diagram as recommended in DOD-Handbook-263 (Ref 4). Note that the wrist strap and the table mat are connected in series to ground, while the floor mat, through its own resistor, is connected in parallel to ground. In effect, this diagram provides 2 MOhms of resistance to ground for the person wearing the wrist strap and 1 MOhm of resistance to ground form both the table mat and floor mat.

In order to make the series connection between the wrist strap and the ground cord, newer wrist straps and ground cords with stackable snaps are available (see Fig 2). These snaps have female and male studs on opposite sides of the snap.

Figure 2. Newer wrist straps and ground cords offer stackable snaps.

In Fig 3, individual connections form a wrist strap, table mat, and floor mat, each through its own current-limiting resistor, are connected to a bus bar or common-ground junction box. This method ensures more rapid static decay from the person than does the series-parallel diagram in Fig 1. Either method is acceptable so long as total resistance connected in series does not exceed 10MOhms.

In Fig 4, we see a third method. This method of connection puts the current-limiting resistors of both the wrist strap and ground cord in series with the resistance of the table mat or covering. Since resistances in series are cumulative, it is possible that total resistance to ground could be so high that static-decay time from the person wearing the wrist strap might exceed acceptable time limits. (Note: Mats equipped with studs at both sides are intended for left- or right handed use for the worker's convenience, not for making series connections; see Fig 5.)

Figure 3. Individual grounding connections from a wrist strap, table mat, and floor mat are connected to a common ground through separate current-limiting resistors.	Figure 4. This method of connection puts the current-limiting resistors of both the wrist strap and ground cord in series with the resistance of the table mat or covering.

Figure 5. Don't make series connections with the alternate snaps provided by some worksurface makers at the right and left sides of their products.

Voltage Decay vs. Time

On one hand, the lower limit for resistance to ground is a matter of personal protection against possible shock or electrocution, should a defective power cord or piece of equipment come in contact with a worksurface or person. On the other hand, the higher limit is dictated by the damage susceptibility of the ESD-sensitive device.

From FED-STD-101, Method 4046 (Ref 5) and Mil-B-81705 (Ref 6), we find that a time limit of less than 2 sec has been established for discharging a +/- 5,000 V charge to 0 V when testing packaging material used for ESD-sensitive items. In theory, and probably backed by some obscure time-and-motion study, it takes a person at least 2 sec to pick up a package, open it, reach in and touch the device. This same time limit has been extended to discharging a person who has to work on a unit in the field.

ESD Waveform

However, as shown in MIL-STD-883, Method 3015 (Ref 7), we find that the decay rate is exponential (see Fig 6a). In fact, the complete discharge of a 5-kV charge may never reach zero if the earth itself is "an arbitrary zero of potential." Thus, depending on the quality of a ground at any given point, some residual voltage may remain on a grounded person or worksurface (Fig 6b).

Residual Voltage vs. Time

In order to limit residual voltage caused by tribolectrification and changes of capacitance at a typical ESD-grounded workbench to approximately 10 V, the maximum resistance to a a hard ground should not exceed 10 MOhms (Ref 2, p 120). Thus, all series resistance to hard ground must be considered, and the sum of the various resistances must be known (see Fig 7).

If we now add the resistance of a could-water pipe and its connections to the ground circuit and throw in the fact that the actual potential of the earth at the grounding point is only arbitrarily zero, is it any wonder when we find residual voltage on worksurfaces formerly considered safe? High-resistance or intermittent grounds can sometimes be as dangerous to sensitive devices as the total lack of ground and can lead to a false sense of security.

Controlling high resistance to ground in a production situation is simple compared to controlling it in a field-service environment. At least you can run overall resistance tests on your building ground circuit, and you can periodically recheck it. But a field-service person can't exercise this control from the forty-fifth floor of an office building.

Equipotential (Common) Grounding

To date, equipotential bonding has been the answer. Equipotential bonding eliminates the risk of ESD damage and may, in fact, bring an entire system to some common non-zero potential. However, a good ground may still be necessary in some situations to reduce residual voltage to an acceptable level in an acceptable time.

The most accessible earth ground in an office is the equipment ground of the electrical system in the building. But the first thing a field-service person usually does when servicing a unit is to unplug the unit from the building's electrical supply. This, in effect, disconnects the potentially best available earth ground in the area. The unit under service may now become a mass completely isolated from ground by rubber insulating pads on leveling legs, on a massive insulative plane of carpet.

The field-service person, under these conditions, may have established a floating ground at a different potential form any of the AC-powered equipment he may use while on site. This same condition has been found in production plants where separate static -control grounds and equipment power grounds are used.

A Potential Solution

Before I adopted a modified equipment-ground cord, residual voltage always disrupted the field-training demonstrations I conducted in high-rise hotels or office buildings. Materials that should have drained a charge to near zero in less than 1 sec might show a residual voltage of more than 100 V after 30 sec or more. Field meters that would normally return to zero would not do so even when equip-potentially bonded to the operator and the conductive worksurface being used in the demonstration area.

These problems were resolved by fabricating a substitute equipment ground cord. It consists of a standard table mat ground cord, with a 1- MOhm resistor, and a standard three-prong electrical wall plug. The spade of the plug, which would normally mate with the "hot" (phase) slot of a standard three-wire wall receptacle, was removed. The ground cord with resistor was connected to the equipment ground terminal on the plug. The normal spade used for the neutral return remained unconnected and was left in the plug to provide mechanical stability for connection to any standard three-wire wall receptacle.

Since one can never assume that a given wall receptacle has been properly wired, several commercial circuit testers such as an ECOS Model-7106 ACCUTEST or a Pilgrim GAM-2 should be used. A unit of this type will test for neutral-ground shorts (ground loops), neutral-ground reversals, ground-path impedance and common wiring errors.

Alternatively, with a little extra effort , a simple VOM can be used. However, circuit testers that simply check for proper wiring at a wall plug without testing resistance to ground are inadequate and should not be relied upon.

For added personal protection against electrical shock or possible electrocution, when using a building ground as an ESD ground, a short extension cord consisting of a heavy-duty plug, 12-AWG wire, and an insulated box with cover containing a ground-fault circuit interrupter can be fabricated from components available at any hardware store. Should a dangerous overcurrent situation develop, the GFCI will provide added measure of safety.

Figure 6. Charge decay is exponential (a), but if there is sufficient resistance between the charged object and ground, a residual charge may remain for a very long time (b). These are voltage waveforms; MIL-STD-883 now specified a current waveform (see sidebar).	Figure 7. All of the series resistances between worksurfaces (as well as other objects) and ground must be known and considered.

Hooking up the Mobile Ground System

Beginning at a wall receptacle of a building, here is the step-by-step procedure used to eliminate residual voltage in a high rise building:

1. Test a convenient electrical receptacle close to the worksite with one of the circuit testers mentioned earlier to make sure the receptacle is properly wired and that overall resistance to ground is appropriate.
2. Connect the short extension cord containing the GFCI to the outlet. Test the GFCI to make sure it is working.
3. Lay out the field-service or floor mat.
4. Test the resistors in all of the ground cords using a good ohmmeter.
5. Using stackable snaps, connect one end of all of the ground cords to one of the studs on the field-service mat.
6. Connect any AC-powered test equipment and/or the unit under repair to one of the outlets of the GFCI in the short extension cord.
7. Plug the modified three-prong plug of the mat's ground cord into the other receptacle of the GFCI.
8. If the unit under repair is unplugged from a power source, connect the alligator clip of the mat-to-equipment ground to a good ground on the frame of the unit.
9. Slip the wrist strap cuff onto your wrist, and tighten or adjust it.
10. Finally, snap the resistor end of the pretested cuff-to-ground cord onto the cuff.
Barring other unimpeded paths to ground, this system now provides at least 2 MOhms of resistance to ground form both the operator and the unit under repair, and at least 1 MOhm of resistance to ground between the mat and ground. With common resistance to ground from both the operator and the unit under repair, the operator and the unit are equipotential.

Summary

While even the system described above may not reduce residual voltage to zero in all cases, it is probably the best alternative for the field-service person who must work in various locations. As devices become more sensitive, this system may become mandatory in order to reduce residual voltages as low as possible.

References

1. MIL-STS-454, Standard, "General Requirements for Electronic Equipment."
2. Webster's New Riverside Dictionary, 1984.
3. NAVSEA SE 003-AA-TRN-TRN-010 Electrostatic Discharge Training Manual.
4. DOD-HDBK-263, Electrostatic Discharge Control Handbook.
5. FED-STD-101, Method 4046, "Static Decay Test Procedure".
6. MIL-B-81705, "Barrier Material, Static-Protective."
7. MIL-STD-883, Method 3015, "ESD Test Circuit Waveform."