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05 Dec

Ground Rod-to-earth resistance

Q: What should be the required Ground Rod-to-earth resistance? is it less than 2ohms or less than 25ohms?

A: I am going to consult my document on Grounding-For the Protection of Electrostatics Discharge Susceptible Items (ANSI/ESD S6.1-2005), an ESD Association standard.  I am assuming that your inquiry is to provide the bonding and grounding for the prevention of ESD in an EPA (ESD Protected Area).

From the Main(s) service equipment or AC Mains, you have the Hot or Black conductor (Let’s assume AC Single Phase 120v) from the Circuit breaker panel, then you have the Neutral or white conductor coming from the Neutral bus, then you have the Equipment grounding conductor or green conductor.  The black or Hot conductor comes from a circuit breaker and goes to an AC outlet receptacle.  The white or Neutral conductor comes from a neutral bus which is bonded to an earth grounding electrode and goes to an AC outlet receptacle.  The green conductor or equipment grounding conductor comes from a ground bus and is bonded to the metal chassis or conduit.  The ground bus is then bonded to the Neutral bus.  The common point ground or bus bar is connected or bonded to this ground connection, as is various other ESD technical elements (the grounding conductors or wires from wrist straps, worksurfaces, flooring or floor mats, tools, fixtures, storage units, carts, chairs, garments, etc). 

The impedance of the equipment grounding conductor or receptacle ground to the common point ground or ESD technical element shall not be greater than 1 ohm (Ω).  I see no mention of 2 ohms (Ω) in this document, although it is noted that the ground resistance values objectives vary from industry to industry.  The telecommunications industry has often used 5 ohms or less as their value for grounding and bonding.  The goal in grounding resistance values is to achieve the lowest ground resistance value possible.  The National Electrical Code defines a ground as: “a conductive connection, whether intentional or accidental between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth.”  The purpose of a ground besides the protection of people and equipment is to provide a safe path for the dissipation of Fault Currents, Lightning Strikes, Static Discharges, EMI and RFI signals and Interference.

The reference to 25 ohms refers to facilities with AC Equipment Ground and Auxiliary Ground (A separate supplemental grounding conductor for use other than general equipment grounding) per ANSI/ESD S6.1-2005 6.3.2.  The auxiliary ground shall be bonded to the AC equipment ground when possible.  The AC equipment and the ESD technical elements might be at different potentials.  The auxiliary ground needs to be bonded to the equipment ground to ensure that there is no difference in electrical potential between the two systems.