Q: What should be the Minimum ESD earth resistance and voltage?
A: RTG and PTP resistance needs to be between 2.5e4 and 1e06 Ω for conductive and from 1e06 to 1e09 Ω per ANSI/ESD S20.20-1999 via ANSI/ESD S7.1-2005 or the combination of a person wearing ESD footwear and standing on an ESD floor should have a resistance to ground of < 3.5e7 Ω per ANSI/ESD STM97.1.
That’s the resistance part of your question. What should the voltage be? That’s up to you. Go to your design engineers and find out what the most sensitive electronic device is and design a floor that creates no more than ½ of that voltage threshold. At minimum, no ESD flooring system in combination with ESD footwear should create more than 100V per ANSI/ESD STM97.2. I like a floor that generates less than +/- 15 volts. I have more to learn about this testing as I have recently acquired the equipment necessary to measure Body Voltage Tests on all floorings that I install and/or test.