In Craig’s debut Guest Blog for Q Source, he explains the importance of testing and monitoring of ESD grounding devices.
|Transforming Technologies PDT 800K Test Station|
The most common and reliable method for grounding operators is the wrist strap and coil cord combination. The conductive materials inside the wrist band contact the skin and transfer the voltage from the operator through the cord and to ground. When worn correctly, and tested or monitored, wrist straps are the preferred method for grounding people.
When grounding a person through a wrist strap, ANSI/ESD S20.20 requires the resistance of the person to equipment ground to be less than 3.5 x 107 ohm (35 meg ohm). Typically, the resistance to ground of a person properly connected with a wrist strap is just over 1 meg ohm (the value of the resistor in the cord assembly).
Wrist strap and coil cord assemblies need to be tested or monitored regularly. Some companies chose to test the wrist strap system one or more times per day at central testing stations (for example, the PDT 800K Test Station). Other companies opt to continuously monitor (using a device such as the CM1602 Constant Monitor) the wrist strap connection so that product is not handled if the grounding system fails.
|Transforming Technologies CM1602 Constant Monitor|
The requirements for grounding a person through flooring and footwear have changed in the ANSI/ESD S20.20 2014 Standard. Previous versions of ANSI/ESD S20.20 provided you with two options. If the resistance from the person through the footwear, through the floor and to ground was less than 35 megohm, no further testing was necessary. If the resistance was greater than 35 megohm, you could still meet the requirements of the standard if the resistance was less than 1.0 x 109 ohms and you can demonstrate that the operators generate less than 100 volts. The latest version of S20.20 (ANSI/ESD S20.20-2014) requires the resistance test and the walking voltage test for qualifying flooring and footwear materials together. While many flooring and footwear options may pass the resistance test, it has been discovered that some combinations of floor materials and foot grounding materials do not work well together.
To test the walking voltage generation of flooring and footwear systems you use ANSI/ESD STM 97.2 Floor Materials and Footwear - Voltage Measurement in Combination with a Person. This test involves recording the voltage of a person while they perform a repetitive walking pattern. To record the voltage you can use a charge plate monitor with data recording capability, or a similar instrument with the same specifications listed in the test method.
|Body voltage measurement test set-up.|
The new software “Digilloscope”
can mark the five highest
peaks automatically and calculate
the average value.
The WT5000 records the voltage on the operator while they are performing the walking steps called out in the standard. Once several repetitions of the pattern are complete, the software will provide the peak voltage and average value. The data is used to confirm compliance to standards and to determine the lowest possible voltage device (Human Body Model Discharge Threshold) that can be handled in the facility.
Thanks for that very important information about ESD control and testing, Craig. We look forward to future Guest Blogs to learn from your expertise.
For information about Transforming Technologies and their ESD control/monitoring products, please visit QSource.com. You may also contact us via email or phone at 800-966-6020 and one of our associates will be happy to assist you.
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