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Materials are either conductors or non-conductors. Non-conductive materials are also called insulators and have a resistance of over 1 x 1011 ohms. Dissipative material refers to a range of conductive material; greater than 1 x 104 ohms and less than 1011 ohms which has the ESD control benefit of more slowly removing electrostatic charges to ground. For ESD control purposes conductive refers to material less than 104 ohms except for an ESD Floor which has a special definition. Per ESD STM7.1-2001 Definitions: “Conductive Flooring Material: A floor material that has a resistance to ground of less then 1.0 x 106 ohms.”
The minimum recommended technical requirement of ANSI/ESD S20.20 for ESD worksurfaces and floors is less than 1 x 109 ohms (see Table 1). The reason for this is that if a worksurface material resistance is too high (>1.0 X 109 ohms RTT and/or RTG) it will not provide a surface that is sufficiently “conductive” to provide an effective surface to “ground” conductive or dissipative items or materials placed on the surface.
However, typically, we recommend dissipative ESD Mats for worksurfaces, specifically 106 to 108 ohms. This is in accordance with ESD Association ESD S4.1 and ESD-ADV53.1 Electrical Requirements, "Worksurfaces - Resistance: Between 1 x 106 ohms and 1 x 109 ohms". If the ESD worksurface is too conductive (<1.0 X 106 ohms RTT and/or RTG), it can discharge charged ESDS devices too quickly and possibly cause device damage via the Charged Device Model (CDM).
Typically, we recommend conductive mats for floors; most of the ESD Systems.com Statfree® conductive mats are 103 to 105 ohms. Since dirt and other contaminant are typically insulators, the flooring will measure less than 109 ohms throughout the maintenance cycle.
The ESD Association has empirical data demonstrating that if the path-to-ground is less than 3.5 x 107 ohms, the charge on a person will be less than 100 volts.
For Personnel Ground, the minimum recommended technical requirement range of ANSI/ESD S20.20 Table 1 for Flooring – Footwear System is less than < 35 X 106 ohms tested per ESD STM 97.1 Floor Materials and Footwear-Resistance Measurement in Combination with a Person.
This is the resistance in series of the person + ESD Footwear + ESD Floor. So if the Floor by itself is 3.49 x 107 ohms, it is mathematically impossible to pass this test. Therefore, we recommend ESD floor mats 103 to 105 ohms.
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