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Ground Trolleys in Series With Cords Having 1 Megohm Resistor

I am confused on ESD connections. I understand that you cannot clip a cord from a wrist strap to the edge of an ESD protective mat. I know that the cord for wrist straps has 1 megohm of resistance and that the point to connect the wrist strap cord should have less than 1 ohm to ground.

I have people using 2 meter high metal rolling trolleys. I believe the preferred method is to connect each trolley directly to a common ground point. Can several trolleys, say 3 to 5 be connected in series with 1 megohm cords with the final cord connected to a ground point or earth bonding point? From what I have read this would only mean about 5 megohms to ground at the furthest cart. I read somewhere something about staying under 35 megohms to ground. Now I cannot find that.

The compliance verification limit per EN 61340-5-1 for wrist strap system is less than 3,5 x 10^7 ohms listed in Personnel grounding requirements Table 2. The required limit per ANSI/ESD S20.20 for both working surfaces and mobile equipment (trolleys) is less than 1 x 10^9 ohms resistance to ground listed in EPA requirements Table 3.

It does not do much good to have an operator test their wrist strap using an integrated wrist strap tester as describe in IEC 61340-5-1 Clause A.1, but then work where the matting resistance is added resistance in the path-to-ground.

Although not recommended you could attach the cord for the wrist band to the matting material, and daisy chain the mats and/or trolleys if you could demonstrate that you reliably meet the required limits.

Per CLC/TR 61340-5-2:2008 User guide Wrist Strap subclause “NOTE Many wrist strap users have been observed to clip the wrist cord to the edge of an ESD protective mat. This process is not recommended as it can increase the total system resistance to ground to over the 3.5 x 10^7 ohms limit required by IEC 61340-5-1.”

Regarding daisy-chaining “All working surfaces need to be capable of being grounded. EPA grounding of instruments or surfaces by chaining, or by placing items in series should not be used, since in the event of a broken connection the risk of floating items and ESD damage will be unnecessarily increased.” [EN 61340-5-2 clause 5.2.2 Working surfaces and storage racks] Note that both of these recommendations are not requirements.

Regarding ground cords, the grounding conductors (wires) from wrist straps, working surfaces, flooring or floor mats, tools, fixtures, storage units, carts, chairs, garments and other ESD technical elements may or may not contain added resistance. Where added resistance is not present, a direct connection from the ESD technical element to the common ground point or common connection point is acceptable and recommended.

However, “A nominal 1 megohm resistor is commonly used in wrist straps and to ground work surfaces. In the event of an operator touching an energized conductor, for the normal mains electricity supplies this resistor will limit the current flowing through the person to less than 0,5 mA.” [User Guide IEC 61340-5-2-1999 subclause 5.1.1]

Charleswater sells ground cords with and without current limiting resistors – Click Here


Enhancing Profits with Effective ESD Control

Our thanks to Conformity Magazine Published in December 2004 issue

Accurate process evaluation provides real answers

Provided by the ESD Association
by Stephen Halperin, in collaboration with Ron Gibson

“We need to spend HOW MUCH?”
Recently, a company experienced several large losses due to electrostatic discharge (ESD) and had a very unhappy customer on their hands. The manufacturing vice president now faced a substantial expenditure for new ESD loss prevention equipment. The company’s first step had been to hire an ESD consultant who recommended the purchase of several thousands of dollars in ionization equipment and monitoring instruments for several of the company’s facilities.

The troubled VP read the report several times looking for justification of the expense. However, the report did not define how the recommended equipment would meet the VP’s specific needs. Other than describing how ionization reduced electrostatic charge after it is generated and that the instruments could confirm that a discharge occurred, the report did not identify the actual cause of the process problem. No ESD measurements were described. There were no details related to cause of product loss, device sensitivity concerns, value issues, process and handling details, examination and description of existing controls, or rationale for how the recommended tools would solve the problem in question. The report was clearly based on the consultant observing the process of a single manufacturing environment. In effect, the report made a purchasing recommendation based on a “blanket” opinion, not on facts specific to the needs of the company or their customer. Such an approach typically makes a bad situation worse. While the recommended tools may have been very useful for investigating a process or for solving defined problems, they are expensive Band Aids“ when used in undefined problem situations.

Today’s electronic manufacturing environment demands that minimal ESD controls be in place to provide fundamental protection for electrostatic discharge sensitive (ESDS) devices. When basic ESD controls are employed and losses still occur, manufacturing and quality managers face more difficult problems., In assessing the problem, companies struggle with a variety of major questions concerning a specialized technology, while having minimal information and available skills. To avoid the risk of making the wrong investment decision without solving the initial problem, management needs a way to select and implement the most effective ESD controls that fit their financial situation, solve their specific problems, and provide a respectable return on their investment.

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Reducing Floor Maintenance Costs while Improving ESD Performance

by Rick Cardinale, Bird Electronic

Bird Electronic, founded in 1942 by J. Raymond Bird, soon became a leader in radio frequency instrumentation. Today, Bird also has moved into digital instrumentation test equipment.

With the development of digital instrumentation came the increased need for controls to prevent ESD events. Improving ESD protection has been an ongoing process since the late 1980s. In 1997, the company determined that an automated PCB production line would be installed and that the entire manufacturing area should be protected against ESD.

This decision led to an evaluation of ESD protective flooring. In 1998, 20,000 square feet of conductive floor tile were installed in the main production area. To help brighten the area, white tile was selected. The floor resistance measured less than 1.0 × 10^6 ohms.

High-Cost Maintenance

A bright, high-gloss appearance was part of the selection criterion for the floor. While the electrical properties were unchanging, by 1999, the floor was starting to dull. It was being maintained like a regular tile floor. No waxes or finishes were used; however, the tile manufacturer did recommend using buffing pads.

After consulting with the tile manufacturer and the installer, maintenance was increased to sweeping clean and damp mopping two times per week and buffing once per month. Monthly floor maintenance was $1,700 per month, a $20,400 annual expenditure.

In late 1999, the maintenance schedule was modified to add more buffing since this was the only way to keep the floor shiny. The floor now was swept and damp mopped weekly and buffed twice per month. The floor was clean and shiny, but the cost went up 41% to $2,400 per month, a $28,800 annual expenditure.

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Use ESD Control Products Correctly Or You Can Do More Harm Than Good

A Comprehensive Program Is Required For Effective ESD Control

With most companies pressured by global competition, effective ESD control can be a key to improving productivity, quality, and customer satisfaction. It is a pity that many companies buy ESD protective products or equipment and then misuse them, often causing more harm than good.

Electronic components that are electrostatic discharge sensitive (ESDS) must be protected throughout the entire manufacturing cycle. According to ANSI/ESD S20.20, the ESD Association’s standard for the development of an Electrostatic Discharge Control Program, safeguards are required during activities that “manufacture, process, assemble, install, package, label, service, test, inspect or otherwise handle electrical or electronic parts, assemblies and equipment susceptible to damage by electrostatic discharges.”

If ESD latent defects occur during this manufacturing and product cycle, it can be most frustrating and costly. Latent defects in components by definition will not be detected so products will pass normal inspections. ESD damage is the hidden enemy; electrostatic charges cannot be seen, typically discharges less than 3,000 volts cannot be felt, and latent defects cannot be detected through normal quality control procedures.

Manufacturing facilities should be as diligent with their ESD control program as hospital operating rooms are in implementing sterilization procedures. Damage caused by invisible and undetectable events occurs in medicine where people can experience infection or even death from viruses or bacteria. In hospitals, the defense against this invisible threat is extensive contamination control procedures including sterilization.

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ESD Control Programme Periodic Verification

by Fred Tenzer and Gene Felder, Desco Industries, Inc.

Some practical advice for implementing
ESD control periodic checks
Want to accomplish something important? A familiar formula is write a plan, select the specifications, and then periodically test to verify that the plan is being implemented according to the test
results. This is basically the requirements of an ESD control program, per the ESD Association standard, ANSI/ESD S20.20. This important standard, entitled Development of an Electrostatic Discharge Control Program, covers the requirements necessary to design, establish, implement, and maintain an ESD control program to protect electrical or electronic parts, assemblies and equipment susceptible to ESD damage.
S20.20 is a process document, and provides ESD control plan guidance; one of its requirements is having a “compliance verification plan” as a component of the ESD control plan. Per S20.20, paragraph 6.1.3., Compliance Verification Plan:
“A Compliance Verification Plan shall be established to ensure the organization’s compliance with the requirements of the Plan. Formal audits or certifications shall be conducted in accordance with a Compliance Verification Plan that identifies the requirements to be verified, and the frequency at which those verifications must occur. Test equipment shall be selected to make measurements of appropriate properties of the technical requirements that are incorporated into the ESD program plan.”
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