ESD Systems - Questions And Answers

In an earlier question about the testing method for floor resistivity (# 154) you stated the industry does not use 500 Volts for testing. You also referenced ANSI/ESD-S7.1-1994 Standard as saying 100 Volts for the floor test. We had our floor tested by an outsidelaboratory and they used 3 voltages (10, 100, and 500). We also have a standard (ASTM Designation: F 150 - 72 (Reapproved 1985)) that states needing a Megohmmeter having a nominal open-circuit voltage of 500V and their procedure says "apply the prescribed voltage", but they never spell out the prescribed voltage. I have the following questions: 1. What is the "prescribed voltage" for this ASTM standard? (I believe 500 volts), 2. What is the significance/reason for the 3 different voltages? (evolution), 3. Which voltage should be used? (100 volts), 4. Not having seen major differences (less than 300k ohm) in our floor resistance regardless of the voltage used, is the test voltage really that important? (yes, to be consistent, for safety and the ability to conduct at lower voltages is now important) - Anonymous

Specific answers are below, appended to your questions. The ASTM Designation: F 150 - 72 (Reapproved 1985) standard is 15 years old. A lot has changed in 15 years in both materials used to help control ESD, the sensitivity of the devices being protected and the standards that are driven from advancements in understanding the problem. Our understanding of the control of ESD has been refined and the current international body having to do with ESD control standards is the ESD Association located in Rome, NY. Their web address is: http://esda.org/ I would highly recommend using their ANSI/ESD-S20.20-1999, the ESD Association Standard for the Development of an Electrostatic Discharge Control Program for – Protection of Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically Initiated Explosive Devices). As floor materials advanced, our understanding of ESD control evolved and the ESD susceptible devices became more sensitive, the requirement for a floor to remove charge from mobile conductors became more important. If a class 0 device is susceptible to less than 250 volts HBM, then if a floor can not conduct current until it has a pressure of 500 volts, it is not adequate for protecting this device. According to the ANSI ESD S7.1, the floors surface resistance (RTT) should be < 1x10⁹ ohms using the ESD S4.1 probes with the megohmeter set to 100 volts. This standard was last updated over 6 years ago. A newer standard ESD STM97.1 and ESD STM97.2 incorporates the floor and footwear as a system in the measurement: For the Resistance, ESD STM 97.1, < 35x10⁶ ohm (still at 100 Volt pressure) For Voltage generation, ESD STM 97.2, < 100 Volts

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