Questions And Answers

That is correct per ANSI/ESD S20.20-2007 Table 1. Grounding / Equipotential Bonding Requirements, the required limit for Equipment Grounding Conductor is <1.0 ohm impedance per ANSI/ESD S6.1.

Per ESD Handbook ESD TR20.20-2008 section 5.1.5 “Verifying the Ground

To verify that a ground system is properly connected, the following equipment is needed:

1. A DC ohmmeter capable of measuring resistance from 0.1 ohm through 1 megohm ± 20% with a minimum open circuit voltage of 1.5 volts.

2. An AC equipment ground impedance meter or indicator, with a measurement range up to 1 ohm, which will also verify wiring configuration of the outlet under test.”

Most users will not have access to equipment to measure impedance. While AC Impedance takes into account a number of factors, using the DC ohmmeter should be within a few percent of what the AC impedance is. Most users therefore just use a DC ohmmeter.

Also in section 5.1.5: “If the resistance in the system test is not within specification, test the individual components of the ground system. Start with the connection from the groundable point of the item (i.e., wrist strap, mat, etc.) to the common point ground. Use the ohmmeter for this measurement. Connect one test lead from the ohmmeter to the groundable point. Connect the other lead from the ohmmeter to the common point ground. Measure and record the values obtained. Compare them to the specifications. Be sure to compensate for the value of any resistors used in the ground cord from the device to the common point ground. Although the resistor is not visible, most commercially available ground cords have a 1 megohm current limiting resistor molded into the end that is closest to the user.

VERIFYING THE CONNECTION BETWEEN THE COMMON POINT AND EQUIPMENT GROUNDING CONDUCTOR-The second measurement is from the common point ground to the equipment grounding conductor. This measurement is done with the ohmmeter. Again, record the values and compare them to the specifications. Because this conductor does not have a built-in resistor, the resistance should be less than the listed 1 ohm. If the value is higher than 1 ohm, check the connections and test again. If the test values are repeated, verify the design or configuration of the grounding component or system with the manufacturer of the equipment to determine the correct value.

VERIFYING THE CONNECTION BETWEEN EQUIPMENT GROUNDING CONDUCTOR AND MAIN SERVICE ENTRANCE NEUTRAL BOND-The final test is an impedance measurement from the equipment grounding conductor to the neutral bond point at the main(s) service equipment At the same time, the receptacle wiring should be checked to verify correct wiring configuration. Commercially available equipment is usually used for this test and it is best to follow the manufacturers' instructions for the equipment operation.”

The resistance of the individual components has to be only a portion of 1 ohm as this is resistance in series and the resistance measurement added together need to be < 1ohm to meet the required limit of ANSI/ESD S20.20-2007. Oddly, this is the first time this question has come up and a measurement of 2 or 3 ohms is not unusual. One should identify the component with the highest resistance and see if there are poor or loose connections that can be repaired or replaced.

A direct attachment to the ground of an AC electrical outlet may reduce the resistance. A power strip or attachment to the screw holding a cover plate are often sources of a couple ohm resistance.