GROUNDING ISSUES

Site Protection Through Proper Grounding, Bonding and Design Practices

Electronics and microprocessor-based equipment have become an integral part of our everyday lives. As technology advances and electronic equipment becomes faster, it also becomes more sensitive to the hazards created by poor grounding and bonding practices. Technology has advanced the speed of electronics by reducing the distance between components and lowering the voltage required to operate the system. While this increases the speed of the equipment, it also makes the equipment more vulnerable to damage at a lower voltage difference, thereby making the grounding and bonding of the equipment much more important than it was during the days of glass tube, high voltage equipment.

Ask any power quality expert today about equipment grounding and they will tell you, "poor grounding practices are second only to improper wiring as the leading cause of equipment malfunction." The standards for equipment performance mandate the installation and maintenance of a reliable, low resistance path to ground. Most electronic equipment today operates at a very low voltage and is often subjected to potentially lethal hazards, many of which are generated by the operation of the equipment itself.

A grounding system that is properly designed, and properly installed, will insure the operation of critical equipment from the hazards of transients, harmonics, and the most ultimate of transients, the lightning strike. A properly designed low resistance grounding system also provides the additional, and most important benefit -- personnel safety.

Earth Grounding
As with any construction project, it is always best to work from the ground up, so the ground connection is typically where one should start with a grounding system design. A ground electrode is "a conductor or group of conductors in intimate contact with the earth for the purpose of providing a connection with the soil". This definition does not mention or refer to any actual ohm resistance value of the electrode. It is simply just a physical connection of the ground electrode to the earth itself. The resistance value is determined by the resistivity of the soil in which the ground electrodes are buried. Fault currents must pass through the soil to the assumed earth potential of zero ohms.

When an object is grounded, it is forced to assume the same zero potential as the earth. If the potential of the grounded object is different than the earth, electrical current will pass through the grounding electrode connection until the object's and the earth's potential is equalized. The ground electrode is the grounding connection between the equipment and the earth. The measured resistance in ohms determines how quickly and at what potential the energy is equalized between the equipment and the earth. Therefore, proper grounding is necessary to maintain an equal potential between the equipment and the earth.

Soil Resistivity
The soil upon which an equipment site is constructed is the dynamic conductor for all fault currents from the equipment, whether they are natural or man-made. It is very similar to the chassis ground in a vehicle. All grounds, no matter how they are routed, eventually reference back to the earth itself. And just as different types of metals can be better types of conductors than other metals, so too can different types of soils. Most soils naturally contain varying amounts of electrolytes that conduct electricity. As a result, the addition of moisture will enhance or reduce the conductive properties of that soil. As a general rule, soil with a higher moisture content will have a lower electrical resistance.

Temperature, like moisture, can have a significant impact on resistivity. The soil resistance rises dramatically upon reaching the freezing point. Upon reaching 32 degrees Fahrenheit, any moisture in the soil will begin to freeze, and upon freezing, the soil resistance will increase by almost three times its normal value.

Frozen soil can have a detrimental effect on clay- or cement-based backfill materials that rely on water as their primary conductor. A carbon-based backfill material used to encase the grounding electrode will offer the advantage of year-round, all-weather low resistance ground system.

Soil Resistivity Measurements Every construction project begins on paper. Without some type of plan or design you won't know where to start or when you're finished. A good grounding system design should begin with a soil resistivity test to determine exactly what you have to work with. A 4-point ground meter is utilized to determine the conductivity of the soil and provide a basis for the beginning of the ground system.

The 4-point resistivity test, traditionally measured in ohms per centimeter, requires the user to place four equally spaced test probes in the ground at various locations within the site area to determine the actual soil resistance. Testing is performed at various spacing from five to forty foot intervals to simulate the resistance values at different depths in the soil from five to forty feet. To get a thorough test reading of the site, testing should be performed at several different locations within the site. The resistance value data obtained from the testing will then be used in the design and installation of a low resistance grounding system.

Soil with higher resistance readings will require more grounding electrodes and a more comprehensive grounding system to compensate for the poor soil. Soil values can range from 500 ohms/cm with high percentages of electrolytes, to over 1 million ohms/cm in sandy dry soil. This resistance will directly affect the overall impedance and performance of the site grounding system.

Tower and Transmission Line Grounding
Most communication sites have some type of tower or structure used to support the antennae and transmission lines which make the site function. These structures, while being highly susceptible to the most damaging of all surges, lightning, are also usually the most overlooked in the design process. Design engineers have historically tended to over-design the surge suppression area of a communications site while under-designing the grounding system.

In other words, instead of helping the lightning get into the ground, where it wants to go in the first place, they over specify products to contain the lightning damage once it has already entered the equipment shelter. Lightning is a very high frequency event. In laymen's terms this means it is an extremely fast current. Since lightning is a high frequency event, the impedance of the paths to ground, as well as the ground resistance, is very important in designing an effective grounding system.

Sharp turns or bends in grounding conductors will act as a choke to lightning transients and will be viewed as a high resistance path by the lightning energy. The actual geometry of how the conductors are installed plays a key role in the functionality of the system. The conductor leading to the counterpoise in the photo above is a perfect example of this problem.

Single Point Grounding
During a lightning strike, the ground potential at the point of the strike changes rapidly and can cause a difference of potential between ground reference points.

The design of the internal equipment shelter grounding system, and how it is connected to the earth grounding system, is crucial to the protection of sensitive equipment and to the safety of personnel located inside the shelter. System design has changed dramatically over the past couple of decades from the old grounding specifications requiring grounding at all corners of the equipment room and usually several places in between. With the new technology in communications equipment over the past two decades, it has been learned the hard way that even a small amount of difference in ground potentials can cause equalization directly through the equipment.

Single-Point Ground Referencing is a system design that allows all of the equipment to reference ground potential at only one point, thereby eliminating ground loops and potential differences. Single-Point Ground Referencing is obtained simply by bonding all of the internal equipment together, and taking it to ground at one single point. This design eliminates the possibility of any ground potential differences or transients equalizing through the equipment. Any change in ground potential does not damage the equipment. It is the difference in potential that equalizes through the equipment that causes damage.

Post Installation Testing
Once a ground system has been designed and installed, it is then a very good idea to verify that the system will work properly. This is accomplished by the use of a three-point fall-of-potential ground resistance test. This post installation test is performed by placing two test probes into the ground within the site area and testing from those probes back to the grounding system. The distance from the test instrument to these probes is determined by the size of the facility that is being tested. This traditionally is five times the diagonal distance of the grounding system that has been installed.

It is imperative that this test be performed before tying into any other ground source; for example, the power company ground or telephone company ground. The reason for testing before it is tied into any other ground is to verify that your system has met the designed ground resistance value without influence from any outside sources. If the test is performed after the power is connected, the Clamp-On ground resistance tester can be utilized. This involves clamping onto the power neutral between the transformer and the site ground. The user must be aware that a 0.7 ohm reading indicates a continuity loop and is not a ground resistance reading.

Low Resistance Grounding System Design: A Site Survey
The design process for a grounding system should begin with a site survey of the installation area including a complete survey of the existing AC power, Telecom, TVSS, and UPS systems and all of their associated bonding and grounding. Remember to include a survey of any other services that may be special to your specific site, such as, cable tv, direct data links from adjoining buildings, intercom systems, security systems, etc.

A proper site survey must also include soil resistivity analysis at several depths, relevant site plans, topography analysis, and a boring core sample, if available.

The site survey will show any physical barriers such as rock, high resistivity soil, power lines or any other variables that could affect the earth-ground resistance in the installation area. Once this information is obtained, an effective and properly designed grounding system can be installed.

Conclusion
A properly designed, low-resistance grounding and bonding system is a major component of a well-protected and efficient facility. A properly designed and installed grounding system is an integral part of any site and should be designed and purchased with the same research and consideration as any other critical piece of equipment.