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DEEP EARTH GROUNDING VERSUS SHALLOW EARTH GROUNDING

by

Martin D. Conroy and Paul G. Richard

Contents abstract

objectives

forward

Basic Grounding Requirements

Grounding Electrode System Components

introduction

DISCUSSION

Figure 1. Sample ground rod resistance: ohms versus depth.

Figure 2. Average resistance graph.

Figure 3. Scattered plot chart.

Figure 4. Year comparison graph.

CASE STUDY 1

Photo 1. Ground rod enclosure.

CASE STUDY 2

CASE STUDY 3

INSTALLATION METHOD

Photo 2. Ground rod and coupler

Photo 3. Ground rod driving machine

Photo 4. Drive-on spline rod coupler.

Figure 5. Dry rod resistance graph.

CONCLUSIONS

REFERENCES

ABSTRACT Low resistance earth grounding is essential for safety and protection of sensitive electronic equipment. It is the basis for any facility’s power quality assurance program.

This paper presents the advantages of deep driven electrodes over shallow (10 feet or less) electrodes. This paper will demonstrate that deep driven electrodes provide low earth resistance, are economical to install, maintain low resistance over lime, are maintenance free, and do not have environmental concerns. This paper utilizes field data taken from over 140 deep driven electrodes installed over a 5-year period in several states. A discussion includes the development of the equipment, materials, and process used to install and test deep driven made electrodes. The process includes a new technique of injecting bentonite into the coupler void to maintain full rod contact of the total length. Several site reports are presented and discussed. This paper would be of value to anyone responsible for specifying, installing or testing low resistance ground systems.

OBJECTIVES The objectives of this paper are to:

Determine the electrode depths required to achieve low resistance values Determine if standard 8 to10-foot ground rods meet minimum code requirements Evaluate the stability of shallow electrodes Present a new process for installing deep-driven ground rods

FORWARD Confusing standards, different philosophies, and conflicting opinions have plagued the field of grounding for many years. The majority of these issues deal with the how and why of grounding and bonding in electrical, computer, and communications systems. Little information and discussion has been focused on the earth resistance of the grounding electrode system. Most plans and specifications give little direction for the installation and testing of a grounding electrode system and many merely state “ground per the NEC.” One noted publication on grounding1 stated that engineers who write such specifications are “not assuming their full responsibility for safety” and are leaving the installation of “effective” grounding to chance! Based on power quality site surveys done by the authors, 90-95% of all facilities inspected lack an effective grounding system. In addition, none of the facilities inspected had ever tested the ground resistance of their electrode system.

Effective earth grounding is essential for grounded AC and DC electrical equipment and distribution systems. Effective grounding provides the level of safety required to protect personnel and equipment from shock and fire hazard. The understanding and evaluation of a facility ground system should be part of any power quality assurance program.

In order to understand earth grounding and test procedures, it is necessary to review why grounding is important. The following list gives some of the basic requirements of an effective ground system.

Basic Grounding Requirements

Limits voltage in a electrical distribution system to definite fixed values Limits voltage to within insulation ratings Provides a more stable system with a minimum of transient over voltage and electrical noise Provides a path to ground in fault conditions for quick isolation of equipment with operation of ground fault protection Provides grounding of all conductive enclosures that may be touched by personnel, thereby eliminating shock hazards Reduces static electricity that may be generated within facilities Provides protection from large electrical disturbances (such as lightning) by creating a low resistive path to earth

A ground system must meet NEC (National Electrical Code) Article 250 requirements. The NEC2 defines grounded as:

“Connected to earth or to some connecting body that serves in place of the earth”

and effectively grounded as:

“Intentionally connected to earth through a ground connection or connections of sufficiently low impedance and having sufficient current-carrying capacity to prevent the buildup of voltages that may result in undue hazard to connected equipment or to persons.”

Grounding an electrical system to earth is done by bonding appropriate components of the distribution system to the grounding electrode system. This system is specified in NEC 250-81 & 83 and includes a combination of available items listed in the following list:

Grounding Electrode System Components

Metal water pipe 10’ in-earth metal frame of building Concrete-encased electrode Ground ring Rod and pipe electrodes Plate electrodes

The NEC does not specify a maximum earth resistance for the grounding electrode system required under Article 250-81. The only place that does specify earth resistance is under Article 250-84, for “made” (rod, pipe, and plate) electrodes. Here the NEC specifies a resistance to ground of 25 Ohms or less for a single electrode. If the electrode does not meet 25 Ohms, it must be supplemented by one additional electrode. However the combination of the two electrodes does not have to meet the 25 ohm requirement! One can only speculate that the writers of the NEC are assuming the combination of items listed in the preceding list will meet the 25 Ohms or less standard. For power quality concerns this assumption leaves the grounding resistance to chance.

According to the IEEE Green Book,3 the grounding electrode resistance of large electrical substations should be I Ohm or less. For commercial and industrial substations the recommended ground resistance is 2-5 Ohms or less. This low resistance is required due to the high potential to earth of the electrical system. Many equipment vendors and communication companies require ground systems of less than 3 Ohms resistance.

With modern construction methods and materials, it is becoming more difficult to achieve a low resistive ground system. Many municipalities are insulating metallic water mains for corrosion protection, or are switching to non-metallic water pipes. Building steel can only be used when effectively grounded.4  At most facilities, it is not. Concrete encased electrodes (Ufer grounds) are not common in many regions. Ring grounds and plate electrodes are rarely used due to their high installation cost. An untested 8 to 10-foot ground rod is the typical “made” electrode for most facilities.

For many sites that have minimal or missing grounding systems, installing a new grounding electrode system is cost prohibitive or impractical. It was for this reason that a process was developed to install deep driven ground rods as a low-cost, effective solution.

INTRODUCTION Starting in 1986 a study was done to determine the most effective method of installing low resistive earth grounding. Various grounding methods and materials were evaluated. The majority of the standard methods were rejected for practicality or cost reasons. New methods of using chemical rods and soil enhancement materials looked promising but left unanswered questions as to environmental impact and liabilities. When questioned about the “secret” chemical composition of one vendor’s product, a response was given that the item was EPA approved to be placed in a landfill. The problem is landfills do not require low resistive grounding! One state environmental engineer cautioned against using chemical soil enhancements near municipal water supplies. He was concerned about ground water contamination from the chemicals.

Based on the study it was determined that deep-driven ground rods would offer the best solution for low resistive earth grounding, if full rod contact could be maintained. In 1988, a new process was developed for installing deep driven ground rods. This process overcame the problems associated with installing deep ground rods.

This paper evaluates the field data taken from 140 deep-driven ground rods installed between May 1988 and July 1993. The ground rods were installed in 6 states with the majority done in Nebraska. Ground rod depths ranged from 15 to 90 feet. All resistance measurements were done with the three point fall-of-potential method using a Biddle Megger, Model No. 250220-1, Null-Balance Earth Tester.

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