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Question

Question about Grounding an Optical Table to Reduce ESD Hazard.

My question is about grounding an optical table for ESD-reduction purposes. The table I’m talking about is stainless steel, and I don’t necessarily want to make it an ESD work surface: I just want to reduce the chances of an ESD incident with any sensitive items I might happen to be using on the table. I’m wondering if it’s okay if I simply ground the table top with a wire to the center screw of an electrical outlet, which would make the table top a hard ground, or should I put a 1-Mohm resistor in series with the wire I use?

Argument for the resistor:

Prevents electrocution in event of accidental contact with a live wire.

Arguments for the simple wire:

1) The above-mentioned electrocution-risk reduction is insignificant, as many of the pieces of equipment in use on the table have direct connections between ground and their outer cases, which pose just as much an electrocution danger in event of accidental contact with a live wire as with a simple connection to ground of the table top.

2) With the use of a resistor, any residual charge stored on a person’s body may not be completely discharged by touching the table momentarily before touching a sensitive object on the table. What I mean is that if a person carrying a residual charge approaches the table and touches the table top, thinking it immediately discharges him, and then he picks up a sensitive object, if the delay between table touch and object pickup is short enough, he may still have enough of a residual charge, which may damage the object. The answer many people give is to make it a point to wait at least a second or two between touching the table top and picking up any sensitive object, which would allow time for any residual charge to be discharged, even through the resistor. That would work if everyone always remembers to wait a second or two between touches, but leaving the resistor off would make the whole system more idiot proof, by making it far less critical to remember to wait between touches.

My thinking here is that the use of a resistor may reduce electrocution risk, but it’s such a minimal benefit in this case, because that wouldn’t do anything to reduce the risk posed by the countless other pieces of equipment on the table whose cases are hard grounded. None of the objects (including the table top) is continuously touched, as with a heel grounder, so it doesn’t make any sense to worry about soft grounding the table top, when no less than ten things on the table are hard grounded. In addition to it seeming silly, as I just explained, the inclusion of a resistor would lengthen the discharge time of a person touching the table top upon approach, so that he would have to wait a longer time between touching the top and touching a sensitive object, as compared to the case of the table top being hard grounded.

The notion of the 1 Mohm resistor seems quite sensible when applied to continuously contacted surfaces like wrist straps, ESD work surfaces, or heel grounders, because any residual charge is continuously bled off in those cases, so it’s highly unlikely that enough charge would remain to cause damage at any point. And, since they’re continuously contacted, these surfaces would definitely play a part in any accidental contact with a live wire.

On the other hand, the optical table top I’m talking about is NOT a continuously contacted surface. Researchers would simply touch it for an instant as they approached before picking up an ESD-sensitive object. In this case, any residual charge on the person is NOT continuously bled off, but is rather dumped out all in one shot as he touches the table right before picking up an ESD-sensitive object. This makes the discharge rate more critical than with a continuously contacted surface, like a wrist strap or heel grounder. The only possible benefit to a resistor would be a reduced risk of electrocution. But, since the table top is not a continuously contacted surface, and it is in fact littered by pieces of equipment whose outer cases are hard grounded, the risk reduction would be minimal.

Thank you very much.

Answer

A very low resistance surface, like stainless steel, can be an ESD worksurface meeting the ANSI/ESD S20.20 Table 3 Worksurface required limit of < 1 x 10E9 ohms resistance to ground. However, the ESD Handbook notes a Charged Device Model concern with conductive Worksurfaces.

Per ESD Handbook TR20.20 section 5.3.1.7 Electrical Considerations

“The most important functional consideration for worksurfaces is the resistance from the top of the surface to the groundable point. This establishes the resistance of the primary path to ground for items placed on the surface. When worksurface materials are being selected, consideration should be given to possible Charged Device Model (CDM) damage to ESD sensitive products. If CDM damage is a concern then setting a lower resistance limit for the worksurface should be considered. Typically, the lower limit for these types of worksurfaces is 1x10E6 ohms.”

The ESD Association recommends no resistor or “Hard ground” for worksurfaces. Per ANSI/ESD S6.1 Grounding - Recommended Practice Section 4.1.2

“In regard to a resistor between the common point ground terminal and a worksurface, floor mat, or shelving:

· where provisions are not made and not required for this resistor, a direct connection is fully acceptable and recommended;

· provisions may be made to include a resistor where it may be required for a purpose other than ESD;

· conductive worksurfaces such as stainless steel shall be hard ground connected.”

We can provide ESD control expertise; safety questions should be directed to your safety officer.

Although about wrist straps, you might find the following information helpful. Per ANSI/ESD-S1.1 Current-Limiting Resistance “A resistance value incorporated in series with the wrist strap’s electrical path to ground. This resistance limits electrical current that could pass through the ground cord in the event of inadvertent user contact with electrical potential.”

ESD Systems personnel grounding products are Underwriters Laboratory listed. UL Notes: “This product is not recommended for use on equipment with operating voltage exceeding 250 VAC.

CAUTION: The ESD Series is for electrostatic control. It will not reduce or increase your risk of receiving electric shock when using or working on electrical equipment. Follow the same precautions you would use without wrist straps, including:

• Make certain that equipment having a grounding type plug is properly grounded.

• Make certain that you are not in contact with grounded objects other than through the ESD Series.”

 
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