ESD Standards

Question

Our company currently adheres to the same ESD standards as the ones you recommend on your website: The total system resistance of the person, footwear, walking surface and ground must be less than or equal to 3.5 x 107 or 35 meg-ohms maximum.

However, at a recent ESD conference, attendees were told that an ESD floor can have a maximum resistance of 1Ghom (1 billion ohms), as long as the voltage generated in the walking test does not exceed 100V.

Could you please provide your input to the pros and cons for this seemingly excessive resistance to ground?

Answer

From a purely technical perspective, a floor’s ESD properties can be evaluated:

1) by measuring the floor’s resistance to ground; 
2) by measuring the total resistance ground of the system including the person and the floor;
3) by using a walking test to measure voltage generation on a person; 
4) or by testing some combination of the above.

According to the recommendations of ANSI/ESD S20.20, it is acceptable to install a floor with a resistance to ground (RTG) that is ≤1.0 X10E9. But, to comply with ESD standards, the floor must not allow body voltage generation over 100V.

A static control floor with high RTG values up to 1Gohm may—or may not—prevent 100V generation. For example, despite its higher resistance values, rubber flooring, used in conjunction with ESD footwear, will usually prevent high body voltages. Vinyl in the higher resistance range, on the other hand, will not reduce body voltage spikes to below 100V.

In order to certify that a highly resistive floor is ANSI/ESD S.20.20 compliant, the ESD program manager must test the body voltage generation caused by the interaction between the floor, the person walking across the floor, and the footwear. Bear in mind that a floor’s voltage generation characteristics are affected by multiple variables, including maintenance procedures and humidity—even spills or contamination can have an effect—so the amount of body voltage a floor generates can change overnight. For that reason, body voltage tests must be monitored regularly and must also be conducted during routine auditing.

ANSI/ESD S.20.20 also recommends a “system resistance” of ﻛ 35 meg-ohms. Simple, easy-to-implement system resistance tests measure the entire static control system: the ESD footwear; the person wearing ESD footwear; and the floor’s resistance to ground. Significant research has proven that it is mathematically impossible for a floor with a system resistance below 35 meg-ohms to generate more than 100V of static electricity. Verifying readings below 35 megohms using this simple test eliminates the need for testing body voltage.

Test Methods Must Be Reliable When Used In Active Factory Environments

Weighing the technical advantages of one test method over another is only part of the answer. One must also consider which test is simplest, less expensive and most practical and provides the most reliable date when used in an active factory environment.

Complicated tests requiring the use of complex equipment and procedures should be performed by people with special skill or knowledge and are best suited for use in laboratory environments. The test methods that work best in active factory environments are those that use simple equipment, rely on simple procedures, and can be performed by less skilled individuals. Scientific validity and reliability (Reliability refers to the consistency of a measure. A test is considered reliable if we get the same result repeatedly.) become skewed when a test method relies too heavily upon a test method that relies upon too much human intervention.

Body voltage tests are expensive, time consuming, highly dependent on environmental conditions and, because of all the variables, require a much larger statistical sampling to gather meaningful data. Remember: the mere act of connecting people to a body voltage meter and recording data is neither scientifically reliable nor statistically valid. Because Body voltage tests should conducted by a skilled individual and should be performed exactly the same way, using a specific type of body movement each time the test is performed, ESD program managers usually find it unrealistic and cost prohibitive to monitor body voltage on a regular basis.

Body voltage tests, like system resistance tests, check the whole system, but voltage tests are tougher to duplicate because of the increased influence of multiple variables. If a tester wears a slip resistant shoe, for instance, the way the tester moves her feet will provide different data than the same test performed with smoother shoe soles. The tester can drag his or her feet, generating greater than normal voltage; walk in a way that the heel straps make poor contact with the floor; shuffle while lifting the shoe heels slightly above the floor; or even jump off the floor, breaking contact altogether. The official instructions describe a specific walking patter that is to be used for the test; unfortunately most people fail to follow the instructions, and, even when they do, the methods used by different people will always differ slightly.

Conclusion

For this reason, I highly recommend sticking with the 35 meg-ohms upper limit system resistance limit. The test is simple and inexpensive. One lead of the ohm-meter is connected to ground and the second lead is connected to a probe held by a test subject standing still. The test subject simply holds a the probe  and presses a button, and the meter tests the resistance between the tester and ground. Because a floor with a total system resistance of below 35 meg-ohms cannot generate body voltages above 100V, this simple test provides fast, inexpensive and reliable verification of the entire esd flooring/footwear system.

Body Voltage 35 Meg-ohm System Resistance  
Instrument Charge Plate Monitor Ohm Meter
Unit of Measure Volts/kV Ohms/Meg-Ohms
Test Method ANSI/ESD STM97.2-1999 ANSI/ESD STM97.1-1999
  Knowledgeable Tester Anyone can perform test
  Complex equipment Simple equipment
  Affected by variables No Interpretation Necessary
  Voltages can spike Measurements are consistent


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