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According to S20.20, either personnel to ground resistance should be less than 35 MΩ or the footwear-floor combination shall generate less than 100 V. The essence of S20.20 is to protect devices to 100 V.

If the footwear-floor combination does not generate more than 100 V, the system is adequate for use even if the body resistance to ground is greater than 35 MΩ. This allows for a great deal of ESD control flexibility, particularly since many organizations use ESD control footwear and floors but few offer a combined resistance to ground in the 1-MΩ to 35-MΩ range.

If body voltage is the primary control element, the document requires that a proper measurement in accordance with ESD STM 97.2-1999 Floor Materials and Footwear—Voltage Measurement in Combination With a Person confirms this level of performance.

Measuring Body-Voltage Generation
Measuring body-voltage generation is not difficult and, with practice and simple calculations, offers an insight to projecting typical body voltages that could be experienced in a given facility. The procedures also are useful for evaluating footwear, floors, or both to obtain the best combination for the lowest voltage generation prior to purchase.

Measurement setup requires a charge plate monitor (CPM); an X-Y recording device; a long, high-quality test lead; and a few accessories. In effect, the operator will make several specific walking steps while connected to the CPM. The recorder documents the operator's generated walking voltage for later analysis.

There are several instrument options for this measurement. You might use a standard 20-pF CPM or a portable system suitable for the measurement. Recording devices may be a strip-chart recorder, an X-Y plotter, or an analog-to-digital converter connected to a computer.

The six-step pattern compensates for the fact that everyone walks differently. Repetitive patterns provide relative data that can be statistically assessed to approximate the range of probable voltage generation experienced with any combination of footwear and floors.

The walking pattern commences once the system is zeroed to its ground reference, then repeated approximately 20 times. Starting at the upper left corner of a 4-ft floor segment, the pattern begins with the operator stepping backwards with the left foot and basically continuing in a smooth, circuitous pattern back to the starting point, then pausing before commencing the next pattern.

Each walking pattern is identified, and maximum and minimum absolute values are defined. Once the values are defined, there are three approaches to evaluating the data:

1. ESD STM 97.2-1999 simply requires that the three maximum values be identified. If the peak values are less than 100 V, the footwear-floor combination generally is considered acceptable, particularly for protecting devices having HBM voltage thresholds well above 100 V.

While the standard's working group confirmed that various operators would obtain similar results, this approach does not mathematically account for the differences in operator capacitance or project probable body-voltage generation that might be anticipated by others. This approach also does not address how you would assess the data for protecting ESD devices having HBM sensitivities well below 100 V.

2. Stephen Halperin & Associates developed a test method in the mid 1980s to assess performance of new ESD control floors and footwear. To this end, a Gaussian distribution data analysis was conducted to ascertain the percent probability of equaling or exceeding any given walking or standing body voltage with ESD footwear and flooring products.

Ultimately, the materials combination was evaluated by calculating the maximum probable voltage generated beyond which there is a zero-percent probability that additional voltage could be generated under the test conditions. This proved far too complicated for the typical plant audit environment and required additional training in data analysis for test operators.

3. To simplify the data analysis in the ESD audit environment, identification of the maximum and minimum data points can be used to calculate the probable body voltage generation ranges during walking and standing. This analysis is illustrated in Figure 3.

by Stephen A. Halperin, Stephen Halperin & Associates