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Question

I am confused on ESD connections. I understand that you cannot clip a cord from a wrist strap to the edge of an ESD protective mat. I know that the common cord for wrist straps has 1 megohm of resistance and that the point to connect the wrist strap cord should have less than 1 ohm to ground.

I have people using 6' high metal rolling carts. Can several carts, say 3 to 5 be connected in series with 1 megohm cords with the final cord connected to a ground point? I believe the preferred method is each cart to ground. From what I have read this would only mean about 5 megohms to ground at the furthest cart. I read somewhere something about staying under 35 megohms to ground. Now I cannot find that.

Answer

The required limit per ANSI/ESD S20.20 for wrist strap system is less than 3.5 x 10^7 ohms listed in Personnel Grounding Table 2. The required limit per ANSI/ESD S20.20 for both worksurfaces and mobile equipment (carts) is less than 1 x 10^9 ohms resistance to ground listed in EPA ESD Control Items Table 3.

 

Although not recommended you could attach the wrist strap cord to the matting material, and daisy chain the mats and/or carts if you could demonstrate that you reliably meet the required limits.

 

Per ESD Handbook ESD TR20.20 wrist strap section 5.3.2.2.2 “NOTE: Many wrist strap users clip the wrist cord to the edge of an ESD protective mat. This process is not recommended as it can increase the total system resistance to ground to over the 35-megohm limit required by ANSI/ESD S20.20.”

 

I could not find mention of daisy-chaining in ESD Association documents, but the European ESD User Guide includes this: “All working surfaces need to be capable of being grounded. EPA grounding of instruments or surfaces by chaining, or by placing items in series should not be used, since in the event of a broken connection the risk of floating items and ESD damage will be unnecessarily increased.” [EN 61340-5-2 clause 5.2.2 Working surfaces and storage racks] It says “should” not “shall”.

 

Regarding ground cords, the ESD Association Grounding standard ANSI/ESD S6.1 recommends no current limiting resistor, but a current limiting resistor can be included for non-ESD control purposes.

 

“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.” [Grounding ANSI/ESD S6.1-2009 section 5.3.3 ESD Technical Element Conductors]

 

Desco Industries sells ground cords with and without current limiting resistors.

 
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