PROTECTION AND CONTROL

Grounding for Lightning Protection: Building a Solid Foundation

The best foundation for any lightning or electrical protection system is an effective outside grounding system. Without a well-designed outside system, all other components of a protection system including inside bonding and surge protection, cannot function properly. All electrical infrastructure, whether it is located overhead or below grade, is constantly subjected to direct or indirect lightning potentials which will induce surge currents on the conductors and or the cable sheaths. Grounding systems provide the means by which to dissipate harmful surges to earth, thus preventing these surges from entering and damaging sensitive equipment. With the advent of todayc microprocessors and sensitive electronics, facility managers are finding that conventional grounding systems offer inadequate protection. Grounding systems designed today must have lower resistance values, higher energy dissipation characteristics, and must fit within the tight confines of right of ways or property limits.

There are four basic steps involved in obtaining an effective grounding system:

1. Soil Resistivity Testing
2. System Design
3. System Installation
4. System Testing

The Wenner method, shown in Figure 1, involves placing 4 probes in the earth at equal spacing. The probes are connected with wires to the ground resistance test set. The test set passes a known amount of current through the outer two probes and measures the voltage drop between the inner two probes. Using ohms law it will output a resistance value, which can then be converted to a resistivity value using the equation:

r = 2paR where:
a = spacing between probes
R = resistance value measured by the test set

Soil resistivity values will vary depending on the soil type (see Figure 2), temperature (see Figure 3) and moisture content (see Figure 4). As a result, it is important to obtain enough data to allow engineers to design a system that will maintain a consistent resistance value throughout the seasons. Typically, data is collected to depths of one to ten meters with additional testing required for difficult sites.

System Design
Once the data is collected, the second step is the design of the grounding system. Building a grounding system without a design is like throwing darts in the dark. Doing the design upfront ensures the grounding system will achieve the target resistance value.

The basic formula used for the design of a grounding system is:
R = r x f
Resistance = Soil Resistivity x Function based on electrode type, size, and shape

Typically, the target resistance is dictated by company standards. Less than 5 ohms is a common value used in the telecommunication industry. Soil resistivity is a given based on site conditions and "f" is a function based on the shape, size, type and layout of the electrode. A good design engineer will ensure that the components of the grounding system are configured to achieve the desired resistance value throughout all the seasons.

Some basic formulas that are used to determine electrode resistance can be found in the IEEE -"Green Book"IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems, Chapter 4, Table 13.

Traditional methods involved in grounding include ground rods, copper wire, and ground plates. All of these techniques can be effective in certain circumstances, however, they have their limitations in more resistive and more difficult sites. A more recent advancement for solving grounding problems in difficult as well as easy sites is the use of conductive concrete products. These products consist of special blends of carbon and cement to form a conductive grounding material. The conductive concrete is used as a backfill around traditional rods and wire to offer greater surface area to the electrodes, thus lowering resistance, impedance, and offering higher capacitance. Conductive concrete allows engineers to design a system to achieve the desired results in even the m'st difficult soil conditions, where traditional systems just don't work.

System Installation
Once the design is finalized, Step 3 may begin. System installation may involve excavation equipment, drilling rigs, or simple shovels and ground rod drivers. For safety reasons, grounding systems are usually buried approximately 30"deep, depending on local codes. Care must be taken to ensure that the system is installed exactly as described in the design. It is advisable to have the engineer or a supervisor onsite during installation to ensure construction is carried out according to the prescribed design.

Conductive concrete electrodes are m'st commonly installed in a horizontal configuration to take advantage of the horizontal nature of lightning dissipation.

Begin by digging the trenches 0.5 meters wide and approximately 0.6 meters deep. Smooth the bottom of the trench. When the trench is complete, the bare copper wire is laid straight down the middle. The conductive concrete is installed dry and it absorbs moisture from the surrounding soil and sets up to 17MPa. Spread the conductive concrete evenly to a thickness of 4 cm. over the copper electrode and to the width of the trench. Back fill the length of trench by hand for the first few centimeters of depth then finish back filling the trench with the backhoe.

In a vertical electrode application the steps are different. First drill the hole to the designed depth and diameter. Next place the wire or ground rods into the hole. Finally mix the conductive concrete with water into a slurry and pump into the hole. Care should be taken to fill the hole from the bottom up to displace any water or mud.

Connections of wire-to-wire, wire-to-rod, or wire-to-plate in the system below grade should be made using exothermic welds. An exothermic weld provides a molecular bond between the two materials and eliminates the potential for the connection to weaken due to corr'sion, lo'sening or any other problem associated with mechanical or compression connections. Exothermic welds are fast and c'st effective, however ,,extra care must be taken to ensure that they are done properly and safely.

System Testing
Finally, testing of the grounding system, Step 4, is important to determine whether ground resistance targets are met. Typically grounding professionals should be called to perform the ground resistance tests as each test must take into account on-site conditions, and it is very easy to get erroneous data. Testing can be accomplished using clamp-on resistance testers, fall of potential methods, or by simply calculating the probable resistance. Detailed procedures for accurate testing can be found in ANSI IEEE Standard 81.

3-Pole Ground Resistance testing (commonly referred to as Fall of Potential testing as illustrated in Figure 6) is used to measure the resistance value of a grounding system or ground electrode in-situ. In order to get accurate results, the electrode or system being tested must be isolated from all other grounding sources including the AC mains neutral. This test works by passing current between the grounding system and an outer probe. An inner probe is then moved to various locations and the test set measures the voltage drop and converts that number to a resistance value. The resistance values are plotted for the different locations and should form a curve with a flat region, which represents the actual resistance value of the grounding system. Fall of potential testing is very accurate for new installations or th'se which are completely isolated from any other ground source.

The clamp on method is a much faster technique to use, however it requires a very good understanding of the system under test, in order to obtain accurate readings. This test may be conducted while the grounding system is still attached to the ac power neutral or other grounding sources.

For difficult sites there are other measuring techniques that grounding professionals may use to determine the on site ground resistance value.

Conclusions
An often misunderstood component of a lightning or electrical protection system is the grounding system. Without addressing grounding these protection systems are inadequate. To build an effective grounding system, accurate site specific soil resistivity data must be collected. With this information an appropriate grounding system can be designed. Conductive concrete enhanced systems are often a main feature of an effective grounding system design. These systems offer lower resistance values and higher dissipation characteristics required to protect sensitive electronics. Finally, system testing is essential to demonstrate that the targeted resistance values are met. A professional who is qualified to test and design grounding systems should be used to ensure that the system meets the requirements of each particular location. Lawrence Arcand is the manager of engineering at SAE Inc. Grounding Systems. He has designed and installed grounding systems throughout North America. Visit the SAE website at www.saeinc.com. ET