Questions And Answers

The unit of measurement of an electrostatic charge is coulomb, and coulomb meter might be able to be used. An indication of an electrostatic field can be measured in volts using a static field meter. However, most required technical limits in ESD control are surface resistance in ohms, in part, because measuring resistance is easier.

Electrons will flow across or through a material having resistance in the conductive/dissipative range less than 1 x 10^11 ohms while insulative materials prevent or limit the flow of electrons across or through its volume. Most users would measure the resistance of the various ESD control items to verify that there is an electrical connection between all items. If so, an electrostatic charge would be equally shared among all the items. This is referred to as equipotential bonding.

Per ESD Handbook ESD TR20.20 5.1.3 Basic Grounding Requirement section 5.1.3 “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 bonded together an ESD event will NOT occur” between those items.

Ideally, both the person and the ESD cart are grounded to the same common point ground. Grounding works great and if all elements are grounded, then not only would the potential be equal, the charge would be very low approaching zero.

I would set the meter to the 10 volt test voltage, and have the grounded person hold one five pound electrode with the electrode’s bottom conductive pad resting on the open palm of the person’s hand. Resistance measurements could then be made to various parts of the grounded cart, placing the other 5 pound electrode on each cart shelf mat worksurface, or using a bulldog clip to attach to other parts of the cart.

Note: per ANSI/ESD S20.20 Table 3, the required limit for Mobile Equipment working surfaces is less than 1 x 10^9 ohms.