EARTH GROUNDING RESISTANCE Principles, testing methods and applications DIAGNOSE intermittent electrical problems AVOID unnecessary downtime LEARN earth ground safety principles
Why ground, why test? Why ground? Poor grounding not only contributes to unnecessary downtime, but a lack of good grounding is also dangerous and increases the risk of equipment failure. Without an effective grounding system, we could be exposed to the risk of electric shock, not to mention instrumentation errors, harmonic distortion issues, power factor problems and a host of possible intermittent dilemmas.
Table of contents What is a good ground resistance value? There is a good deal of confusion as to what constitutes a good ground and what the ground resistance value needs to be. Ideally a ground should be of zero ohms resistance. There is not one standard ground resistance threshold that is recognized by all agencies. However, the NFPA and IEEE have recommended a ground resistance value of 5.0 ohms or less.
Grounding basics Components of a ground electrode What affects the grounding resistance? • Ground conductor • Connection between the ground conductor and the ground electrode • Ground electrode First, the NEC code (1987, 250-83-3) requires a minimum ground electrode length of 2.5 meters (8.0 feet) to be in contact with soil. But, there are four variables that affect the ground resistance of a ground system: Locations of resistances 1. Length/depth of the ground electrode 2.
Number of ground electrodes Another way to lower ground resistance is to use multiple ground electrodes. In this design, more than one electrode is driven into the ground and connected in parallel to lower the resistance. For additional electrodes to be effective, the spacing of additional rods need to be at least equal to the depth of the driven rod. Without proper spacing of the ground electrodes, their spheres of influence will intersect and the resistance will not be lowered.
What are the methods of earth ground testing? There are four types of earth ground testing methods available: • Soil Resistivity (using stakes) • Fall-of-Potential (using stakes) • Selective (using 1 clamp and stakes) • Stakeless (using 2 clamps only) Soil resistivity measurement Why determine the soil resistivity? Soil Resistivity is most necessary when determining the design of the grounding system for new installations (green field applications) to meet your ground resistance requirements.
How do I measure soil resistance? To test soil resistivity, connect the ground tester as shown below. As you can see, four earth ground stakes are positioned in the soil in a straight line, equidistant from one another. The distance between earth ground stakes should be at least three times greater than the stake depth. So if the depth of each ground stake is one foot (30 centimeters), make sure the distance between stakes is greater than three feet (91 centimeters).
What are the methods of earth ground testing? Law (V = IR), the tester automatically calculates the resistance of the earth electrode. Connect the ground tester as shown in the picture. Press START and read out the RE (resistance) value. This is the actual value of the ground electrode under test. If this ground electrode is in parallel or series with other ground rods, the RE value is the total value of all resistances.
Selective measurement Selective testing is very similar to the Fall-of-Potential testing, providing all the same measurements, but in a much safer and easier way. This is because with Selective testing, the earth electrode of interest does not need to be disconnected from its connection to the site! The technician does not have to endanger himself by disconnecting ground, nor endanger other personnel or electrical equipment inside a nongrounded structure.
What are the methods of earth ground testing? Stakeless measurement NO FFO The Fluke 1625-2 earth ground tester is able to measure earth ground loop resistances for multigrounded systems using only current clamps. This test technique eliminates the dangerous and time consuming activity of disconnecting parallel grounds, as well as the process of finding suitable locations for auxiliary ground stakes.
Ground impedance measurements When attempting to calculate possible short circuit currents in power plants and other high voltage/current situations, determining the complex grounding impedance is important since the impedance will be made up of inductive and capacitive elements. Because inductivity and resistivity are known in most cases, actual impedance can be determined using a complex computation.
Measuring ground resistance MG B N MG Ground field Water pipe Building steel The layout of a typical central office. At central offices When conducting a grounding audit of a central office there are three different measurements required. Before testing, locate the MGB (Master Ground Bar) within the central office to determine the type of grounding system that exists.
R SENSING TRANSFORME EI-162XCURRENT 1625-2 ADVANCED EARTH / GROUND TESTER GEO START TEST H/C2 DISPLAY MENU E Earth/Ground Resistance 300kΩ RA CHANGE ITEM 4 POLE 3 POLE 3 POLE 4 POLE S AC Rresistance 300kΩ R~ S/P2 H DC Resistance 3kΩ R— 2 POLE 2 POLE 4 POLE 3 POLE ES/P1 OFF E/C1 SELECT EI-162 AC INDUCING TRANSFO CURRENT RMER MG B CURREN RMER T EI-162 X SENSING TRANSFO To prove this, you need to perform a few additional tests on individual resistances.
More ground resistance applications Electrical substations A substation is a subsidiary station on a transmission and distribution system where voltage is normally transformed from a high value to low value. A typical substation will contain line termination structures, high-voltage switchgear, one or more power transformers, low-voltage switchgear, surge protection, controls, and metering.
For all applications, this is not a true ground resistance measurement because of the network ground. This is mainly a continuity test to verify that the site is grounded, that we have an electrical connection, and that the system can pass current. A typical setup at an electrical substation. 3-pole Fall-of-Potential measurement Second, we measure the resistance of the entire system via the 3-pole Fall-of-Potential method. Keep in mind the rules for stake setting.
Earth ground products Fluke 1625-2 Advanced GEO Earth Ground Tester Fluke 1623-2 Basic GEO Earth Ground Tester The most complete tester The Fluke 1623-2 and 1625-2 are distinctive earth ground testers that can perform all four types of earth ground measurement: • 3-and 4-pole Fall-of-Potential (using stakes) • 4-pole soil resistivity testing (using stakes) • Selective testing (using 1 clamp and stakes) • Stakeless testing (using 2 clamps only) The complete model kit comes with the Fluke 1623-2 or 16
