DEEP EARTH GROUNDING VERSUS SHALLOW EARTH GROUNDING
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CASE STUDY 1
This case involved installation of a deep-driven ground system for a new telemarketing and reservation center. The facility built in early 1991 is a three story 60,000 square-foot building located near the top of a hill. Design of the building included a poured concrete foundation with steel support columns bolted to concrete footings. No grounding electrode system was specified in the construction documents. During construction of the building the metal water main was tested for earth resistance before it was connected to the interior piping. The water pipe tested over 10 Ohms resistance. A 10-foot ground rod was installed and tested at 45 Ohms resistance. A lightning risk assessment rated the facility in the moderate to severe category.5
To address safety and protection concerns, a new electrode system was proposed and installed. The new grounding electrode system consisted of both a ring ground and deep driven ground rods. A total of 4 rods, 70-78 feet deep, were installed, one on each corner of the building. Average resistance of the 4 rods was 1.57 Ohms and when tied together tested below 1 Ohm. A ring was formed by burying a #2 bare annealed copper conductor around the perimeter of the building. Each of the 4 deep-driven ground rods were connected to the ring ground with a bolted type connector and covered with a fiberglass enclosure (Photo 1). This provided capability for periodic disconnecting and testing of each electrode.
Photo 1. Ground rod enclosure.
The building steel was bonded at each corner column and at alternating columns to the ring ground by an exothermic connection. The ring ground was connected to the main electrical service and water main. Additional systems connected to the ground included the telephone lightning protection, phone system, standby generator, computer room raised floor, and power protection equipment.
It is not possible to compare before and after results since this is a new facility. However some general observations can be made. The facility has shown a history of trouble free operations with no known loss or damage of equipment from power or lightning related disturbances. It is interesting to note that early 1993 had unusual weather with many electrical/lightning storms. Local computer and telecommunication vendors have had record peaks in service calls and equipment failures in the same locale as the facility.
CASE STUDY 2
This case involved an existing facility located in a semiarid mountain region. The 40,000 square foot one story building was originally designed for commercial office use. Approximately 30,000 square foot was leased and remodeled for a telemarketing company. The facility had a history of equipment problems and failures as well as complaints by employees of electrical shock. The company was experiencing a 200% annual failure rate with their 300 computer terminals. Other problems included data communications errors and equipment damage.
A power quality survey and electrical inspection found several power and grounding problems at the facility. Among the most serious problems were violations of the NEC, including improper grounding and a lack of a grounding electrode system. The interior metal water piping was used as the main grounding electrode. However it was found that the metal pipe ran only 5 feet underground where it was converted to plastic. The building steel was not effectively grounded and no other grounding electrode was installed.
A power quality implementation plan was developed to address both safety and functionality of the electrical distribution system. This plan included electrical modifications and upgrading of the grounding electrode system. Local electrical contractors stated that earth grounding was very difficult in the region due to the poor resistance of the soil and difficulty of driving ground rods. They suggested a chemical ground rod as a solution. This type of rod reduces electrode resistance by leaching chemicals (electrolytic salts) into the surrounding soil. The client rejected the chemical rods for both maintenance and environmental concerns.
A deep-driven electrode system was selected as the best solution for this site. To overcome the difficulty of driving through the hard soil, pilot holes were bored for the rods. Two 60-foot deep by 4-inch diameter test holes were drilled at 70-foot intervals. The first 30 feet consisted of a sand and gravel layer, the last 30 feet was shale. According to ANSJ/IEEE standards,6 the resistance of sand and gravel soil ranges from
15,800-135,000 Ohms/cm. The resistance of shale ranges from 4060-16,300 Ohms/cm. The lower shale layer provides approximately a 10 times reduction in resistance as compared to the upper layer.
The test holes were filled with hydrated sodium bentonite into which the ground rod(s) were driven. Both rods consisted of 6 each 3/4 inch by 10 foot copper clad rods with drive on couplers. Final resistance of the two rods was 0.88 and 0.48 Ohms respectively.
As a general statement the facility has experienced a dramatic reduction in equipment failures and communication errors. From the client’s perspective the facility has become one of their most trouble free sites.
CASE STUDY 3
This study involves a military computer facility that was located in a converted aircraft plant. A dedicated substation with a 13,800 volt primary and 480/277 volt secondary was provided for the facility. The facility’s power protection system included parallel redundant static UPS and backup diesel generators. The specifications called for the grounding electrode system to be 3 Ohms or less ground resistance. The grounding electrode system consisted of 6 3/4 inch by 10-foot ground rods installed through the basement floor of the building. All 6 ground rods were installed within 6 inches of each other and bolted to a copper ground bar. The electrical substation utilized the same ground system. Design of the facility precluded using building steel, water pipes or ring grounds as grounding electrodes.
The site was plagued with computer hardware problems that the vendor blamed on power and grounding. The ground rod system was tested by facility personnel and measured 0.0 Ohms. A power quality survey revealed that the ground testing had been done incorrectly and that there was a safety hazard. Standard earth resistance testing methods require that the ground rods be disconnected during the test to prevent false readings.
Two 70-foot deep ground rods were installed at 90-foot intervals to augment the existing system. The earth resistance tested at 1.1 and 0.8 Ohms respectively. The new rods were connected to the existing ground bar to provide the facility earth ground. The 6 old rods were then disconnected and tested at 27-32 Ohms resistance.
After installation of the deep driven ground rods the computer service vendor reported fewer problems with the hardware. This case illustrates the problem of relying on improper ground resistance testing. The original design of installing ground rods adjacent to each other violates the NEC requirement of 6-foot minimum spacing.7 As a general rule ground rods should be spaced an interval that is not less than their depth. The poor resistance of the original ground system created a safety hazard to both personnel and equipment. A ground fault on the primary of the substation could have caused excessive voltage potential in the facility ground system.