By Lawrence Arcand and Wendy Sirola
Although we know grounding is important for personnel safety, as well as lightning and surge protection, this topic is not very well understood. With the proliferation of sensitive electronic equipment, grounding is taking on an increasingly important role. In fact, many equipment manufacturers now associate warranty requirements with proper grounding. With this in mind, here are some important tips for ensuring effective building grounding.
Acquire the Proper Expertise
The entire grounding process including the design, installation and associated testing should be supervised by a professional engineer or equivalently qualified individual with grounding expertise. In the last decade a number of new products have been developed to help lower ground resistance, improve capacitance, and prolong system life. Grounding professionals familiar with these improvements can help utilize them in your grounding projects.
Gather Accurate Soil Resistivity Data
Soil resistivity is a physical property unique to every soil. Prior to designing a grounding system, soil resistivity testing must be completed to determine the electrical properties of the soil at and around the site.
The Four-Probe Wenner method is most frequently used for measuring soil resistivity. This method measures the average soil resistivity down to a depth equal to the distance between the probes. By varying the distance between the probes a soil resistivity profile can be produced for use in the design of a grounding system.
The testing location must be chosen carefully. If the site has multiple soil types or complex geology, several tests may be required. Perform the testing as close as possible to the location where the grounding system will be installed. Avoid testing in areas with overhead wires, buried metallics and areas that have been influenced by salting or fertilizer.
Soil resistivities can range dramatically even on adjacent sites. As a result, several series of measurements may be required. For sites with high soil resistivity or where a low grounding system resistance target is required you may need to perform additional tests with larger spacings. In general, the more data gathered the more efficient the grounding system design.
Inside Grounding Should Be Configured As Single Point
Inside grounding involves the electrical bonding of all internal metallics in a building to the outside grounding system. Each facility should have a master ground bar (MGB) that serves as a hub with all grounding for the building being either directly or indirectly bonded to it.
All metallics including the water pipe, building structural steel, electrical equipment and the power ground must be bonded together and tied to the MGB. All critical electronic equipment should be configured such that there is only one path to ground. The equipment is electrically isolated from the building with the exception of one bond directly to the MGB. This is referred to as single point grounding. Intermediate ground bars (IGB) may be used to provide an intermediate ground point which is ultimately bonded to the MGB as shown above.
Outside Grounding Should Address Lightning
Lightning storms can produce peak currents up to 400,000 amps. It is important that the grounding system is configured properly and is sufficiently robust to handle the intense energy contained in a lightning strike.
A well designed outside grounding system is an essential part of any lightning protection plan. Outside grounding must provide a low impedance path to allow lightning energy to dissipate into the earth without causing any damage to the protected structure and its contents.
It is crucial to bond all outside grounding together to create an equipotential grounding system. Once complete, the outside grounding is then bonded to the inside at the MGB. This ensures that everything on-site is at the same electrical potential thus reducing dangerous and damaging voltage gradients.
Generally, each grounding system should consist of a ground ring installed around the perimeter of the building and a means by which to lower ground resistance to meet the target resistance value. The ground resistance can be lowered by using ground rods, and vertical or horizontal enhanced electrodes.
At the Sandia Research Lab, a U.S. Department of Energy laboratory, scientists studying triggered lightning have documented a constant characteristic. Lightning tends to move horizontally along the earth once it makes contact, as shown above. This research, coupled with ample evidence in nature, clearly indicates that lightning dissipates horizontally. As a result, grounding systems designed with horizontal electrodes are best suited to deal with lightningÕs horizontal dissipation characteristics. Vertically oriented grounding systems, for example ground rods, are not as effective for lightning protection.
Grounding Electrode Enhancement
At many locations traditional grounding systems alone will not meet your grounding system requirements. Traditional systems may not be able to achieve your resistance targets, may not be able to dissipate high rise time surges, and may be subject to corrosion.
The use of a conductive concrete backfill product is an effective way to solve these problems. Conductive concrete increases the electrode surface area and contact with soil, thus decreasing the overall system resistance. A grounding system that is designed with a capacitive nature will aid in the dissipation of high rise time surges that are potentially damaging to sensitive electronic equipment. A grounding system's capacitance can be dramatically increased with the use of ground enhancing conductive concrete.
Depending on the soil conditions at your location, corrosion of the grounding system may be an important consideration. Protection of the grounding system against corrosion through the use of exothermic connection techniques, and encasement in conductive concrete will help increase the effective life of your system. It is important to choose a product that will endure years of burial in the earth.
Surge Protection Is Crucial
Surge protection should be installed at the main AC service entrance and on any service panel feeding critical equipment. Surge protection should also be installed on incoming twisted pair telephone lines, data lines and co-axial cables entering your facility.
The majority of surges are not caused by direct lightning strikes to the facility but are either carried in on utility lines, or generated internally. Surges can be caused by direct strikes on utility poles, strikes to nearby buildings, by energy induced on entering conductors, or by the normal operation of high load equipment. The use of surge protection devices can reduce or eliminate the effect of these surges. All surge protection devices must be connected to a low impedance building ground in order to function properly.
For the ultimate protection on twisted pair and data lines, fiberoptic cables or optical isolating devices provide a complete break in the conductor and eliminate the possibility of current flow due to voltage gradients. This is particularly critical in campus arrangements, where multiple locations or buildings are bonded via twisted pair telephone or data cables.
Measure Grounding System Resistance To Confirm Results
Following construction, the grounding system must be measured to ensure compliance with the target resistance value. Testing must be completed before the electrical neutral is connected. To obtain accurate readings, the grounding system must be isolated from all other grounding systems including adjacent buildings and the power system ground.
The resistance value of a grounding system can be measured by using a variety of techniques. The most common techniques are the fall of potential and clamp-on methods. The key to success with any technique is a good understanding of the testing method, and following proper testing procedures.
Summary
For effective grounding well-trained and equipped staff must measure on site soil resistivity and determine an appropriate design for the facility. The outside grounding system should have a horizontal component to deal with lightning. The use of conductive concrete can help create a low impedance, capacitive, and robust outside grounding system. Critical electronic equipment should be installed in a single point configuration and guarded with surge protection or isolating devices. Proper measurement and inspection of the grounding system is critical to ensure system performance.
Grounding is no longer a 'Black Art' or 'Voodoo Science'. By following simple, proven, scientific principles you can ensure an effective grounding system is installed at your facility.
Lawrence Arcand and Wendy Sirola are with SAE Inc. Grounding Systems.