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Question Why does the ESD Association recommend a "hard ground" for ESD protective flooring and highly conductive worksurfaces? Could a "soft ground" be used in these applications for safety reasons?
Answer

You may use a ground cord which includes a current limiting resistor. We offer ground cords with and without 1 megohm current limiting resistor. We use black wire for the ground cords having a resistor, and use green wire with yellow stripe for the ground cords with no resistor.

 

The ESD Association grounding standard is ANSI/ESD S6.1. Per its section 5.3.3 ESD Technical Element Conductors “The grounding conductors (wires) from wrist straps, worksurfaces, flooring or floor mats, tools, fixtures, storage units, carts, chairs, garments and other ESD technical elements may or may not contain added resistance. Where added resistance is not present, a direct connection from the ESD technical element to the common point ground or common connection point is acceptable and recommended.

Note: Manufacturers may add resistance to the grounding conductors for purposes other than ESD (e.g. current limiting). Added resistance is acceptable for the purposes of controlling ESD provided electrostatic accumulation does not exceed specific EPA requirements. The typical added resistance in grounding conductors is 1 megohm, although other values may be specified.”

 

Regarding “very conductive worksurfaces”, the ANSI/ESD S20.20 required limit for worksurfaces is less than 1 x 10^9 ohms with no lower limit. So a very conductive worksurface does meet ANSI/ESD S20.20. However, the worksurface standard is ANSI/ESD S4.1 and it recommends a lower limit of 1 x 10^6ohms.

 

Per ANSI/ESD S4.1-2006 section 8.0 RESISTANCE GUIDELINES

“Due to a wide variety of application for worksurfaces, specific requirements that could be broadly applied are difficult to determine. However, the following set of guidelines can be used as a starting point for establishing local requirements for the resistance of worksurfaces:

  • Resistance-to-groundable point 1 x 10^6 to 1 x 10^9 ohms
  • Resistance from point to point >/= 1 megohm

These guidelines represent a range of resistance that has generally been proven to provide protection in the manufacturing environment. They also are loosely based on models for the effects of ESD on devices. It should be noted that worksurfaces with point-to-point resistances below 1 megohm may be a hazard where exposed voltage may contact the user or the worksurface.”

 
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