Questions And Answers

It is best practice to have all elements at the ESD workstation be attached to that workstation’s common point ground, and then have one ground cord connect to equipment ground. If that cord is disconnected then all the elements would be connected together or at equipotential where no discharge can occur since all elements would have any electrostatic charge equally distributed.

The mat or worksurface could have more than one ground cord, but that typically is not necessary. Two or more common point grounds could be used, but it would be recommended to connect one to the other and then one ground cord to equipment ground.

The ANSI/ESD S20.20 requirement is that the resistance to ground be < 1 x 10^9 ohms for the mat (with lower limit recommendation of 1 x 10^6 ohms), and that personnel resistance to ground be less than 3.5 x 10^7 ohms.

Per ESD Handbook ESD TR20.20-2008 Grounding / Bonding Systems section 5.1.3 Basic Grounding Requirements

“The first step in ensuring that everything in an EPA [ESD Protected Area] is at the same electrical potential is to ground all conductive components of the work area (i.e., worksurfaces, people, equipment, etc.) to the same electrical ground point. This point is called the common point ground and is defined by ANSI/ESD S6.1 as "a system or method for connecting two or more grounding conductors to the same electrical potential.

A work area equipped with materials and equipment to limit electrostatic voltages is called an ESD protective workstation. Every element to be grounded in an ESD protective workstation is attached or connected electrically to the same common point.

The next step in completing the ground circuit is to connect the common point ground to the AC equipment ground or an auxiliary ground, as defined in ANSI/ESD S6.1. The AC equipment ground is the preferred ground connection because all electrical equipment (e.g., soldering irons,' scopes, etc.) at the workstation is already connected to this ground. Connecting the ESD protective products to the AC equipment ground brings all components of the workstation to the same electrical potential.

When neither AC equipment or auxiliary grounds are available, an equipotential bonding system may need to be used. In this situation, all of the items in the system are bonded together so that the charge that resides on the elements will be shared equally and therefore there will be no potential difference between the items. Once this step has been completed it is safe to handle ESD sensitive parts without inducing damage.”