Bv earth ground testing

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Bv earth ground testing

  1. 1. 1<br />Ideal Test & Measurement<br />Earth Resistance Testing<br />
  2. 2. 2<br />Integrity of the Ground<br />Why test Ground Electrodes?<br />It is essential to meet NEC, OSHA, and other electrical safety requirements. <br />The Three Major reasons for Ground Resistance are;<br />To provide Zero reference for the electrical service<br />Provide a low resistance path to protect from electrical faults<br />Protect equipment against static electricity and protect against frame potential for personnel safety<br />
  3. 3. 3<br />Integrity of the Ground<br />The Three key components of a single ground Electrode are;<br />Resistance between the Ground Electrode and Clamp<br />Resistance of contact between soil and rod<br />Resistance of Concentric Shells of earth<br />
  4. 4. 4<br />Integrity of the Ground<br />Effect of the Soil on Electrode resistance <br />Soil conditions, and soil types<br />Sandy soils have higher resistance than clay or loam<br />Moist soils are better than dryer conditions <br />Environmental moisture and temperature Is it rainy season or the middle of a hot dry summer? Is it winter and there is perma frost? Conditions change throughout the year and have a significant impact on the readings. <br />
  5. 5. 5<br />Test Methods<br />Ground Electrode<br />Typically measured only at installation<br />In truth very seldom tested by installers. <br />Only one out of 10 say they test single point ground Electrodes, in an informal survey<br />Methods of testing the ground electrode<br />Fall of potential test (FOP) also known as the 3 point and or the 62% tests.<br />Ground resistance clamps<br />
  6. 6. 6<br />Theory<br />Fall of Potential, 3-point measurement<br />Fall of potential instrument measures resistance through Ohms Law. It uses two measuring circuits, voltage and current, to establish Resistance. <br />The circuit is established through the soil by proper placement of a potential and a current probe which are both connected to a common point, which in a 3-point measurement is the Ground Electrode. <br />The unit injects a known current between the C electrode and ground rod. This causes a voltage to develop between the P electrode and ground rod. Using Ohms law, the resistance is calculated.<br />
  7. 7. 7<br />Fall of Potential, 3-Point Test<br />
  8. 8. 8<br />Fall of Potential Testing<br />Proper Probe Placement<br />The goal is to place the auxiliary potential electrode far enough outside the effective resistance areas of both the ground electrode and auxiliary current electrode<br />
  9. 9. 9<br />Fall of Potential Testing<br />Insufficient Placement of Potential probe<br />If the potential electrode is to close it is said to be inside the “sphere of influence” of the ground electrode and the resistance measurements will vary. <br />
  10. 10. 10<br />Fall of Potential Testing<br />Insufficient spacing of Current Probe<br />If the current probe is placed too close, its electrical field will overlap the ground electrode field, causing the potential probe to make a measurement in an electrical field of overlapping area of influences. <br />
  11. 11. 11<br />Fall of Potential Testing<br />The ground electrode must be isolated from the electrical system <br />It is important in doing this test that the probes are in a straight line<br />To make certain that the measurement is not in the sphere of influence make 3 measurements <br />First at 62%<br />Second at 52% <br />Third at 72% <br />If there is a large difference in readings then adjust spacing of the probes. You are expecting 3 similar readings.<br />
  12. 12. 12<br />Fall of Potential Testing<br />Note: In the diagram to the right, the bottom graph depicts a plot as if a reading were taken every 10 feet from the rod to the furthest electrode and then carefully graphed. This would be accomplished by moving the P electrode. When properly tested and plotted, the graph will show the classic plateau as seen. Averaging three readings from within the plateau will give you the most accurate indication of resistance. If the plot reveals only a slope, then the electrodes are improperly spaced and the test should be redone.<br />
  13. 13. 13<br />Fall of Potential Testing<br />For diameters of ½” reduce distance by 10%. <br />For Diameters of 2” increase distance by 10%. <br />A rule of thumb is to place the potential probe about 10 times the length of the ground electrode away for the test <br />
  14. 14. 14<br />Ground Resistance Clamps<br />Advantage is that the ground electrode doesn’t need to be disconnected from the electrical system.<br />The jaws of the clamp must be placed in the electrical path of the systems grounding conductor or around the rod itself.<br />
  15. 15. 15<br />Ground Resistance Clamps<br />If voltage (E) is applied to any measured grounding probe R(X) through a special transformer , current (I) flowing through the circuit can be measured , thereby establishing an equation which can be used to calculate R(x) <br />E/I=RX. If E is kept constant, then through the use of a CT, current can be detected and the resistance of the ground electrode circuit may be calculated. <br />Make sure you have a clean, clear connection of the clamp around the ground.<br />Select and measure the current first, if current is > 5 amps, resistance can’t be measured using the ground clamp tester. Note that high currents present on the ground are dangerous and need to be corrected.<br />
  16. 16. 16<br />Earth Testers<br />Ideal Model 61-796<br />Self powered 8AA’s - no need for hand crank<br />Low power consumption<br />Compact and lightweight<br />Quick battery check<br /> Integral carrying case<br />Water resistant case.<br />Includes complete set of 100 foot leads and electrodes.<br />
  17. 17. 17<br />Earth Testers<br />Ideal 61-920 Ground resistance Clamp <br />Ground Resistance <br />Ground Leakage Current <br />Auto ranging <br />Audible indication < 40 ohms <br />Open jaw indication <br />Data hold with 99 point memory<br />
  18. 18. Which Tester is Best?<br />It is important to realize that each method has strengths and weaknesses<br />Ask the customer if they need a Fall of Potential or 3 point tester with electrodes. Their specs may require this. The 61-796 is less expensive but more time consuming. If they are testing just a few newly installed rods, this is a good choice.<br />If they have many rods to test, and they are already connected to the system, then a better choice is the clamp on 61-920.<br />In a number of cases, they will buy both and use the clamp on meter wherever they can and then the FOP method for everything else.<br />Most customers already know which tester they are looking for. It is a good opportunity to make them aware of both before they make a decision.<br />18<br />
  19. 19. Questions: True or False<br />For a FOP test, the ground rod can remain attached to the ground conductor.<br />For an 8 foot long rod, the P electrode would be placed 50 feet from the ground.<br />Good grounding soils contain equal amounts of rock and sand.<br />The FOP method uses measured current and OHM’s law to calculate the resistance.<br />Electrode placement and spacing has a minor effect on the calculated resistance.<br />Using the resistance clamp always gives the correct resistance.<br />If plotting the measured FOP results, a gentle slope will result from all of the readings.<br />If necessary, simply disconnect the ground rod to facilitate an FOP test.<br />A rule of thumb for the spacing of the electrodes is to place the C or furthest electrode at the end of the longest test lead included with the kit.<br />In shallow soil conditions, it is accepted that you may have to cut the ground rod if it cannot be driven any deeper.<br />19<br />
  20. 20. Answers<br />For a FOP test, the ground rod can remain attached to the ground conductor. FALSE Failure to disconnect the rod will result in an artificially low reading. <br />For an 8 foot long rod, the P electrode would be placed 50 feet from the ground. TRUE 62% of 80 feet is 49.6 feet.<br />Good grounding soils contain equal amounts of rock and sand. FALSE Rock and sand are both poor conductors of electricity. You are looking for moist, loamy soils for low resistance.<br />The FOP method uses measured current and OHM’s law to calculate the resistance. FALSE The tester injects a current and measures the resultant voltage.<br />Electrode placement and spacing has a minor effect on the calculated resistance. FALSE The placement of the electrodes is very important. Bring a tape measure and place them in a straight line for consistency.<br />Using the resistance clamp always gives the correct resistance. FALSE It is a far simpler and easier method, but if a loop is present, you will measure the resistance of the loop and not the ground rod.<br />20<br />
  21. 21. Answers<br />If plotting the measured FOP results, a gentle slope will result from all of the readings. FALSE Correctly measured and plotted, you will observe a definite plateau on the graph with definite slopes on either end.<br />If necessary, simply disconnect the ground rod to facilitate an FOP test. FALSE NEVER disconnect a ground rod from an energized system as potentially lethal voltage and current may be present.<br />A rule of thumb for the spacing of the electrodes is to place the C or furthest electrode at the end of the longest test lead included with the kit. FALSE Although some lead sets are 100 feet long and you may be working with a 10 foot rod, always be sure of the length of the rod, multiply that times 10, and place the C electrode that distance from the ground rod.<br />In shallow soil conditions, it is accepted that you may have to cut the ground rod if it cannot be driven any deeper. FALSE NEVER cut a ground rod. It may be necessary to dig a trench and lay the rod into it and cover it with special ground enhancing materials such as bentonite clay.<br />21<br />

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