Ground%20Resistance%20-%20It's%20Not%20What%20You%20Think%20(12-30-99)
 

The 1999 National Electrical Code specifies that the grounding of metal parts of the electrical system to the earth (earth grounding) is to protect persons and property by limiting voltage on the premise wiring from lightning or other high voltage surges [250-2(a)], Figure 12-1.

Author’s Comment: The resistance of the earth ground has no effect on the performance of “equipment (safety) grounding.”

Touch Potential

Section 250-2(b) of the NEC states that the grounding of metal parts of the electrical system to the earth is intended to help reduce touch potential on the metal parts. However, this is not a true statement for systems that operate at less than 600 volts. For example, if a metal pole supplied with a 120 volt branch circuit had a phase-to-ground fault, the circuit protection device would not open. The metal pole would have a touch potential of 120 volts and the fact that the pole was grounded to the earth would not reduce this touch potential, Figure 12-2

Author’s Comment: See the last paragraph of this article to help you better understand the above statement.

Sensitive Electronic Equipment Performance

Other reasons (not covered by the NEC) to have a “good earth ground” include the reduction of EMI – electromagnetic interference, RFI – radio frequency interference, static electricity and to improve the performance of transient voltage and lightning protection devices thereby improving the reliability and performance of sensitive electronic equipment. In addition, Chapter 8 of the NEC requires all communication systems that enter a building or structure to be grounded to the electrical system ground (single point ground), Figure 12-3.

Earth Ground Resistance

The earth’s ground resistance impacts the effectiveness of shunting high voltage surges from lightning and other sources to the earth. The generally accepted practice is to have the earth’s ground resistance not exceed 25 ohms. However, to protect communication systems (cell phone sites) and sensitive electronic installations (computers), the voltage dissipation capability of the earth (earth ground resistance) might be required to be less than 3 ohms, and in some cases less than 1 ohm.

WARNING: Failure to provide a low resistive earth ground can result in the migration of voltage from lightning or line surges onto the premise wiring, which can cause electric shock and/or fire within the building or structure.

Factors that significantly impact the earth ground resistance include the contact resistance of the grounding conductor to the electrode, the type of electrode used and the resistivity of the soil.

Author’s Comment: The National Electrical Grounding Research Project sponsored by the research foundation of the NFPA is a study on the performance characteristics of many different types of electrodes located in various climates and soils. For more information on this project you may contact the technical director Mr. Lindsey at tcl@co.clark.nv.us or research foundation personnel at NFPAresfdn@nfpa.org.

Soil Resistivity

The soil resistivity is dependent on the salt, mineral and moisture content of the soil. The “best earth ground” will be achieved when the grounding electrode system is in contact with corrosive soil that has permanent moisture. This can be accomplished by driving ground rods deep into the earth’s permanent water table, and/or by installing electrodes underneath a building or in direct contact with concrete.

Author’s Comment: Because the earth’s resistivity is not homogenous, soil resistivity testing by the use of the Wenner (4-pole) method should be done to determine the best location for the installation of the grounding electrode. For more information on soil resistivity testing, see www.leminstruments.com, or www.aemc.com.

The National Electrical Code

The NEC does not contain any requirement as to the maximum earth ground resistance permitted for a grounding electrode or the grounding electrode system. However, the NEC does have specific requirements for the installation of the grounding electrode and the grounding electrode conductor. These rules are contained in Sections 250-50 through 250-70 and the following is a short summary of each.

Section 250-50 Grounding Electrode System

The earth ground resistance can be reduced by installing multiple grounding electrodes (see list below) and bonding them together so that they are in parallel to each other, Figure 12-4.

1.      Metal underground water pipe in direct contact with the earth for 10 feet, supplemented by a “made electrode.”

2.      Metal frame of the building or structure that is bonded to another electrode.

3.      Electrically conductive foundation or footer steel not less than ½-in. diameter and not less than a total of 20 feet in length.

4.      A No. 2 conductor completely encircling the building or structure installed at a depth of not less than 2½ feet.

Section 250-52 Made Electrode (Ground Rod)

Where none of the electrodes listed in Section 250-50 are available, then a “made electrode” consisting of ½ inch copper clad or 5/8th inch galvanized (or larger) rod driven 8 feet vertically in the soil may be used. But if the ground resistance of a single “ground rod” is greater than 25 ohms, then a second “ground rod” must be installed so that is no closer than 6 feet, and both ground rods must be bonded together with a No. 6 wire [250-56], Figure 12-5.

Author’s Comment: The NEC does not require more than two “ground rods” to be installed, even if the total resistance of the two “ground rods” exceeds 25 ohms [250-56]. In addition, two “ground rods” bonded together do not significantly reduce the ground resistance unless they are separated by at least by 20 feet.

The diameter of a “ground rod” has an insignificant reduction in lowering the ground resistance, but by doubling the vertical length of soil contact, the ground resistance can be reduced by 40%.

250-62. Grounding Electrode Conductor Material

The grounding electrode conductor (the conductor that is connected to the grounding electrode) can be solid or stranded, insulated, covered or bare. This conductor must be resistant to any corrosive condition existing at the installation, and for practical purposes, the conductor should be copper. Aluminum conductors can be used if installed in accordance with Section 250-64(b), Figure 12-6.

Author’s Comment: The NEC does not have any color code requirement for the grounding electrode conductor, but the generally accepted practice is to color the terminations green.

250-64. Grounding Electrode Conductor Installation

The grounding electrode conductor must be installed in one continuous length unless spliced by a listed irreversible compression-type fitting or by exothermic welding.

250-66. Sizing Grounding Electrode Conductor

The grounding electrode conductor must be sized in accordance with Section 250-66. The conductor to a “made electrode” is not required to be larger than No. 6, and for concrete encased electrodes a No. 4 is sufficient.

250-68. Grounding Electrode Conductor Connection

The termination of the grounding electrode conductor must be accessible, unless the electrode termination is buried or encased in concrete.

250-70. Methods of Grounding Conductor Connection to Electrodes

The grounding electrode conductor must terminate to the grounding electrode by the use of listed pressure connectors or clamps, or exothermic welding. Where buried, the termination fittings must be listed and marked for direct soil burial (DB).

Author’s Comment: Be sure to review all of the NEC requirements for the grounding electrode system and the grounding electrode conductor contained in Sections 250-50 through 250-70, including the exceptions and the fine print notes.

Measuring the Ground Resistance
The 3-pole fall of potential method is used to measure the ground resistance of a grounding electrode by the use of a ground resistance tester (groundohmer or a groundometer) in accordance with the manufacturer’s instructions. This method determines the ground resistance by using the Ohm’s Law formula, R=E/I. This method uses the voltage difference between the grounding electrode and a driven (P) otential test stake and the current between the grounding electrode and a second driven (C) urrent test stake.

Author’ Comment: The test stakes are made of rod stock, ¼ to ½ inch diameter, 14 to 24 inches long, driven into the earth about 2/3rd of their length.

The distance and alignment between the grounding electrode and the (P) otential and (C) urrent test stakes are extremely important to the validity of the resistance measurements. For an 8-foot ground rod, the commonly accepted practice is to space the current (C) urrent stake from the grounding electrode 80 feet from a ground rod. The (P) otential test stake is positioned (in a straight line) between the grounding electrode and the (C) urrent test stake at 62% of the distance that the (C) urrent test stake is located from the grounding electrode.

Example: When measuring the resistance of an 8-foot ground rod, the (C) urrent test stake should be 80 feet from the ground rod and the (P) otential test stake should be (80 feet x .62) 50 feet from the ground rod. If the voltage between the ground rod and the (P) otential test stake is 30 volts and the current between the ground rod and the (C) urrent test stake is 2 amperes, then the ground resistance (according to the 3-pole fall of potential method) would be equal to, Figure 7:

Ground Resistance = E/I

E (Voltage between electrode and P) = 30 volts

I = (Current between electrode and C) = 2 amperes

Ground Resistance = 30 volts/2 amperes

Ground Resistance = 15 ohms

CAUTION

The three-pole fall of potential method must be performed on a grounding electrode system that is isolated from the electrical utility system ground. If the grounding electrode system to be tested is connected to the electrical utility ground (by the neutral), then the total resistance indicated on the meter will be the total resistance of all electrodes that are bonded together, including the electric utility grounding electrodes. Naturally, this value would be very low and wrong.

To measure the ground resistance of grounding electrodes that are not isolated from the electric utility (such as at industrial facilities, commercial building, cell phone sites, broadcast antennas, data centers or telephone central offices) a ground tester with “Selective Test” capability must be used. For more information about this subject, contact a manufacturer that supplies ground resistance testers.
Validity of Test

To test the accuracy of the measured ground resistance, the (P) otential and (C) urrent test stakes must be located outside the “voltage spheres of influence” of each other. This is accomplished by moving the (P) otential test stake 3 feet closer and then further away from the ground rod, each time taking fresh resistance measurements. If the measured ground resistance remains constant, then ground resistance results are correct.

If there is a significant change in the ground resistance reading (30%), then the distance between the ground rod and the (P) otential and (C) urrent test stakes must be increased (maintaining the 62% ratio separation) until the measured ground resistance value remains constant.

WARNING:

The earth’s ground resistance as determined by the fall of potential method is the resistance of the earth between the grounding electrode and voltage test stake. It is not the impedance of the ground fault return path. The earth resistance value is irrelevant as it relates to fault current flow and touch potential. It cannot be used to determine phase-to-ground fault current flow required to open the circuit overcurrent protection device, nor can the measured resistance value be used to determine touch potential on metal parts because of a phase-to-ground fault.

Interestingly, some electric utilities determine that they have a good ground if they blow a 4 ampere fuse when it is in series with 120 volt power supply and a driven ground rod. The logic is that if R = E/I, then the earth ground resistance would be R = 120 volts/4 amperes, R = 30 ohms. Sorry, it doesn’t work that way.

 

Mike Holt’s Experiments

I tested the ground resistance of a single 5/8th ground rod installed vertically 8 feet in the earth at my workshop. The soil was very moist due to many days of rain and the results were as follow:

 At 47 feet, the resistance was 16.2 ohms.

At 50 feet, the resistance was 16.6 ohms.

At 53 feet the resistance was 17.1 ohms.

Parallel Electrodes – I tested the ground resistance for two 8-foot ground rods bonded together with a No. 6 solid spaced 7 feet apart, and the earth resistance was reduced less than 5%.

One final thing, I was curious to see if I attached an energized (120 volt) wire directly from a 1-pole 15-ampere breaker to one of the parallel ground rods (the ground rods were isolated from the electrical system) if my circuit breaker would trip? The results:
  • One ampere of fault current flowed from the 120 volt source through the grounding electrode at my workshop, through the earth to the power supply (utility transformer).

  • The impedance of the ground fault return path was, Figure 12-8:
    Z = E/I, E = 120 volts, I = 1 ampere
    Z = 120 volts/1 ampere
    Z = 120 ohms

  • The touch potential from the ground rod was 120 volts to the earth because the 1-pole 15-ampere breaker did not open and clear the fault. The fault could not be cleared (open breaker) because the impedance of the fault current return path to the power supply (earth between the workshop electrode and the utility electrode) was so great (120 ohms) that it did not allow sufficient fault current to open the protection device.

Author’s Comment: Do not apply voltage to a ground system unless you really know what you’re doing, you’re qualified, and proper safety procedures are followed.

The above material was written by Mike Holt, www.mikeholt.com and it was published in Power Quality Magazine.

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