Design Con 2015

ESD Garments Cover Up

-February 26, 2002

From the ESD advertising supplement
to Test & Measurement World, March 2001.

ESD Garments Cover Up


By integrating conductors in their fabric, manufacturers of the latest ESD garments create effective ways to dissipate potentially destructive static charges.
By John H. Mayer

» New advances in materials often improve performance or reliability, but these characteristics can sometimes run counter to the needs of people who must control electrostatic discharge (ESD) events.

Synthetic materials used in clothing offer an excellent example. Over the years, manufacturers have made more use of synthetic materials in everyday garments, and in the process they improved durability and reduced cost and weight. Yet these same advances have made modern clothing a prime source of charge potential. When you factor in the increased use of rubber or rubber-like soled shoes and reduced humidity due to air conditioning and heating systems, everyday clothing and factory garments can create high static potentials and may cause destructive ESD events.

That same trend in new materials has driven the development of static-control lab coats and other garments that can help control ESD events. This apparel provides a “shield” between the charge generator—a person’s clothing—and the sensitive products he or she handles. Early in the development of protective clothing, manufacturers relied primarily on steel fibers as conductors in static-control garments. But the industry quickly recognized that as the tiny steel fibers worked their way out of the fabric, they posed a contamination and health hazard.

Today, manufacturers use a variety of fabrics in their ESD-control apparel. Most product lines divide into two basic types: knits and wovens. In the former, manufacturers knit into the fabric a grid pattern of a carbon-suffused monofilament nylon. The actual pattern depends upon the needed level of protection. The conductive carbon controls the static charge and helps drain it from the user. Carbon content can range from 1% to 15% of the fabric’s weight.

Woven materials include cotton and polyester blended with carbon to form a single material. Densely woven fabrics prove especially effective in clean-room environments because they offer excellent contamination control.

Most manufacturers supply jackets, coats, and other apparel in a variety of material blends ranging from polyester to cotton. Some materials are lighter weight and designed for maximum comfort while heavyweight products help protect against solder splashes, harsh chemicals, or extreme abrasion.

Important considerations

Color, style, and fabric weight are important considerations when you select any piece of clothing, but you should consider a few additional features for ESD-control garments. You’ll need to know whether your application requires a grounded or non-grounded ESD garment. A grounded ESD garment provides a high level of electrical continuity across all fabric panels, from the sleeves and collar to the main body panels. This construction lets the wearer use a single grounding point, usually located at the hip. From that point, usually a standard snap similar to those used with wrist straps, a conductor runs to ground. Typically the user would wear a separate wrist strap to ground his or her body. But some garment manufacturers let users connect a wrist strap directly to a garment’s sleeve hem. In this case, a standard ground cord provides a single ground path from the hip of the garment, thus freeing the user’s arms from long ground-cord connections.

Manufacturers also offer non-grounded garments for static-shielding applications. This type of apparel does not offer as high a level of electrical continuity between various panels or garment parts. But it can provide a barrier between the static charges generated on a person’s clothing and static-sensitive components. The carbon web built into these garments dissipates charges evenly across the entire garment surface and into the surrounding atmosphere through a process known as corona discharge. Performance depends on temperature and relative humidity. When you work with highly sensitive components, you should use a grounded garment.

For those applications where grounding is critical, vendors also offer garments compatible with dual-loop ground monitors. This setup ensures that as soon as the path connecting ground to the garment or the body of a technician breaks, a monitor beeps and a supervisor can rectify the problem. These monitoring systems provide an effective way to limit losses due to ESD when people work with highly sensitive components.

The ESD Association’s ESD STM 2.1-1997 (Ref. 1,2) defines the standard test method employed to evaluate the resistance of garments used to control ESD. The document defines procedures available to measure the electrical resistance of a garment. It does not, however, address issues such as electrical resistance through a person, or in combination with a person connected to ground.
TMW0301SESDFIG1.gif (13768 bytes)
Figure 1. In a sleeve-to-sleeve test you hang the garment from insulating connections and measure the resistance through the entire garment. Prior to testing, you must precondition the garment. (Courtesy ESD Association)

ESD STM 2.1-1997 outlines two test methods you can use to measure electrical resistance in a garment: the sleeve-to-sleeve method and the point-to-point method. The sleeve-to-sleeve test (Fig. 1) measures the electrical resistance across the seams of a garment. For static-shielding garments, the less resistance across the seams and fabric, the more evenly the garment will dissipate charge imbalances. In the case of grounded garments, the continuity of the garment as a whole demonstrates the manufacturer’s ability to electrically connect all the panels in the garment. The test results show the capability of the entire garment to dissipate static charge through a single connection. Without a good connection between all panels, a coat grounded at both sleeves could still present a source of charge potential in the material that covers a wearer’s chest.

To run a sleeve-to-sleeve test, you hang a garment by its sleeve from electrically isolated clamps. Typically the clamps are made of stainless steel or a similar conductor and they measure 2x1 in. To ensure the integrity of the test, you must precondition each sample garment in a closed environmental test chamber at 12% ±3% relative humidity and at 23ºC ±3ºC for at least 48 hours prior to the test. You connect the positive voltage lead to one clamp and the negative voltage (sensor) lead to the other clamp. Then apply a test voltage of 100 V for a maximum of 15 s or until the megohmmeter reading becomes stable. Test results should range between 1x105 and 1x1011 ohms for an acceptable garment. Any readings below 1x105 ohms indicate the garment can present an ESD hazard.

TMW0301SESDFIG2.gif (16422 bytes)
Figure 2. The point-to-point tests measure the resistance from various parts of a garment to other points. You must check all the paths, from every cloth panel to every other cloth panel. (Courtesy ESD Association)

The point-to-point test (Fig. 2) measures the electrical resistance between any two points on a garment. This test calls for the use of electrodes that conform to ANSI/EOS/ESD S.4.1. That document defines the appropriate electrodes as two 2.27 kg (5 lb) cylindrical devices with a diameter of 63.5 mm (2.5 in.). Each electrode must have electrically conductive contacts with a Shore-A (IRHD) durometer hardness of 50-70. Resistance between the two electrodes must be &100 kohms when measured at 10 V on a metallic surface.

In the point-to-point test, you place the garment on a smooth, flat insulating surface with a surface resistance greater than 1x1013 ohms. Put the two electrodes on different panels in the garment and apply a test voltage of 100 V for 15 seconds, or until the megohmmeter reading becomes stable. You must repeat the test procedure for all combinations of garment panels. For acceptable garments, the test readings should fall within the same range as noted above for the sleeve-to-sleeve test.

If you can’t perform the test yourself, ask the garment manufacturer to verify that an independent lab ran the tests, and those tests met the standards outlined in the ESD STM2.1-1997 document. Many manufacturers will comply with your request.

In addition to requiring garments that dissipate static charges now, you want garments that will still dissipate charge after many uses and many washings. Typically a garment’s performance degrades over time, but most manufacturers will guarantee a specific performance over a minimum number of washings. You need to determine whether those specifications meet your needs, and then plan accordingly. T&MW

References:
1. ESD Association Standard Test Method for the Protection of Electrostatic Discharge Susceptible Items—Garments, ESD STM2.1-1997. ESD Association, Rome, NY. www.esdeos.org.

2. Baumgartner, G., “Consideration for Developing ESD Garment Specifications” ESD Association Technical Report, ESD TR 05-00. ESD Association, Rome, NY. www.esdeos.org.

Copyright 2001, Test & Measurement World. Published by Cahners Business Information, Newton, MA.

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