ESD Systems - Questions And Answers

Our prepacacked mat kits include a No Resistor Common Point Ground Cord.
We have a customer asking why they would opt for a resisted ground cord.
Can you give us the function of a resistor in a ground cord connection from mat to ground. Is this in case of current going from the ground point back to the mat?

Good question.
The common point ground is typically located on the mat at a snap connector. From this point, you’d tie the mat to ground via a ground cord to building or power ground. The ESD Association’s standard ANSI/ESD S6.1 recommends all ground paths from mats and floors to have a resistance of less than 1 ohm:
Electrical requirements
Criteria
 The resistance of the conductor from the groundable point of the work surface, chair, wrist strap, walking surface or other items to the common point ground shall not be greater than 1.0 ohm. Where a resistor is used in the circuit, the total resistance includes the value of the resistor;
 The impedance of the equipment grounding conductor from the common point ground to the neutral bond at the main service equipment shall not be greater than 1.0 ohm. See figure 5 and figure 6;
 The resistance of the conductor from the common point ground to the equipment grounding conductor shall not be greater than 1.0 ohm.

We do sell ground cords with 1 megohm resistors in series with the ground path for customers who spec in resistorized ground cords. When is comes to personal grounding such as wrist straps or foot grounders, UL demands that they have a minimum 1 megohm resistor built in series to the ground path as close to the body as possible for safety reasons only (since you are grounding the body). ANSI/ESD S6.1 says that it is optional to have a 1 megohm resistor in series with the ground path for non personal grounded devices such as mats, shelves, carts, floors, etc.
So. The only reason to opt for a resistorized ground cord for non-personal grounding is for Additional safety concerns.....say even though a person is wearing a grounded resistorized wrist strap, if they lean on a conductive (< 10^4 ohms) grounded mat (< 1 ohm per ANSI/ESD S6.1 and ANSI/ESD S20.20) then they run a risk of having current travel through them through the mat to ground bypassing their wrist strap if they were to touch exposed live power.

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