SAFE GROUNDING OF STATIC-CONTROLLED WORKSTATIONSAlthough 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.
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 sensitiv parts, but can present a shock Nhazard 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. Figure 1. Earth grounding in a typical 120-V, 60-Hz electrical system |
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... An 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 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 |
Industrial GradeWriststraps We make our wriststraps 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 For the highest quality ground cords for mats, work surfaces, and equipment call ESD Systems. Ask for a sample. |
Figure 2. Separate ground systems present potential problems. Figure 3. How current flow through a worksurface is generated. Figure 4. The relationship between equipment (or safety) ground and earth ground. completely due to related effects. Fig. 3 shows how current flow through the worksurface is generated. 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. |
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Figure 5. Grounding strategy for workstations without AC power. Figure 6. Block diagram of a ground-integrity monitor. Testing Is a Must Doing It Yourself |
Wriststrap Ground Item No. 13525 provides a secure ground at each work station, and comes complete with grounding wire and instructions. Quad Common Item No. 13575 is designed for use as either a multiple grounding block or as a common point ground cord. Grounds Mats, Item No. 41193 is a wriststrap tester that also grounds your mat. Call us if you would like to see one. |
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 common-ground point.
Continuity of less than 1 S can be measured with the wiring 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.
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ESD Systems Makes Grounding of Work Surfaces and Floor Mats Fast and Easy 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 . Installing push and clinch snap for use with multi layer or soft conductive mats Item No. 12102 Our common point ground cords and testers allow for “common point grounding”.
Our ground cords are designed to hook directly onto any nearby “green wire” utility 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. |
