Basics of Ground Rod Testing

Name: Basics of Ground Rod Testing
Uploaded: 2017-07-11T02:10:59+00:00
Duration: PTM28S38
Channel: Abigayle Logan
Description: Basics of Ground Rod Testing

Basics of Ground Rod Testing
Objective is to give the attendee a basic understanding of Ground electrode testing using 3 pole tester and Clamp-On Ground testers. 1 Why test ground 2. What effects your ground measurements 3. Types of Ground test equipment 4. Methods of test. Basics of Ground Rod Testing

Before we get started this presentation is design to give you a basic understanding of Ground testing. For addition information it is recommended that you refer to the following Getting Down to Earth, Megger Guide for Measuring Earth Resistively, Ground Impedance, and earth Surface Potential -- IEEE Std 81 Guide for protection for Wire-line Communications facilities-- ANSI/IEEE Measurement of Resistance of Earth Electrode System Covering a Large Area– Dr. G. Tagg, IEEE procedure Vol. 116 and 117 Basics of Ground Rod Testing

Job Hazards Analysis It is the responsibility of the Customer, Contractor or Operator to access Job Hazards Potential Hazards Equipment Standards and Test Methods Basics of Ground Rod Testing

Integrity of the Ground
Why test Ground Electrode It is essential to meet NEC, OSHA, and others electrical safety requirements. Three major reasons for ground To provide Zero reference for the electrical service Provide a low resistance path to protect from electrical faults Protect equipment against static electricity and protect against frame potential for the operator safety The three point fall of potential method should be applied to a single electrode as per the NEC 250-??? NEC 250-?? resistance of man-made electrodes A single electrode consisting of a rod, pipe, or plate which dose not have resistance of 25 ohms or less shall have a second electrode of the types specified in or installed not less than 6’ apart. Telecommunication central offices often require 5 ohm or less while distribution substation require A< 1 ohm. Of course these appellations normal require buried grid systems to reach these desired resistances, we will only be discussing single electrode application typically use in residential applications. Basics of Ground Rod Testing

NEC Code requires a single ground electrode to have 25 Ohms or less resistance, and if not, be augmented by one additional spaced at least 6’ apart of any type specified in section or We are all for-mil-ur with the NEC code requirement for a single ground electrode to have a resistance of 25 ohms or less, and if the resistance is not >25 ohms to drive a second electrode at less 6 feet from the first. Depending on where you live in this country, that may not be enough. How would we know when so few actually measure ground resistance of an electrode. Basics of Ground Rod Testing

Three key components of the single ground Electrode Resistance between Ground Electrode and Clamp, Bonding in multi-rod applications Resistance of contact between soil and rod(s) Resistance of Concentric Shells of earth The three key elements to the earth ground electrode are Ground conductor from electrical distribution to the rod and its bonding points Contact resistance between the earth and the rod The resistance of “concentric shells” of earth Basics of Ground Rod Testing

Effect of the Soil on Electrode resistance Soil conditions, and soil types Sandy soil have higher resistance that clay or loam Seasonal Environmental conditions which effect both moisture and temperature Soil resistibility can very widely around the world and is effected by the seasonal environmental conditions of both ground moisture and temperature. Basics of Ground Rod Testing

Ground Electrode Typically measured only at installation In truth very seldom tested by installers. Only one out of 10 said they test single point ground Electrodes, in an informal survey Methods of testing the ground electrode Fall potential test, and Slope method Ground resistance clamps Basics of Ground Rod Testing

Test Equipment Fall of potential instrument measures resistance thought the Principles of Ohm’s Law Just like a Volt-Ohm meter or Multimeter. It uses two circuits, a Current Source circuit and a Voltage measurement circuit, then it calculates the unknown Resistance. Basics of Ground Rod Testing

Test Equipment 3 or 4 point measurement 4 wire measurement method the lead resistance is eliminated by using C1 and C2 leads to establish the current at the unknown resistance and measuring the voltage potential directly at the unknown via P1 and P2. Understanding the test A ground resistance tester uses two circuits to establish ground resistance One is a constant current circuit of a unique frequency that is separate of any possible utility or other existing currents. Second is the voltage potential circuit to measured the drop of the soil Ohms measurements normally use a two wire method to measure resistance, but when measuring low resistance like milli-ohms or micro-ohms the lead resistance can become a factor in the measurement. In the 4 wire measurement method the lead resistance is eliminated by using C1 and C2 leads to establish the current at the unknown resistance and measuring the voltage potential directly at the unknown via P1 and P2. In a three point test P1 and C1 are shorted together at the instruments and a “Short” lead is used to attach them to the ground electrode. Basics of Ground Rod Testing

Test Equipment 3 or 4 point measurement The 3- point test Shorts P1 and C1 together at the instruments and a “Short” lead is used to attach them to the ground electrode. Understanding the test A ground resistance tester uses two circuits to establish ground resistance One is a constant current circuit of a unique frequency that is separate of any possible utility or other existing currents. Second is the voltage potential circuit to measured the drop of the soil Ohms measurements normally use a two wire method to measure resistance, but when measuring low resistance like milli-ohms or micro-ohms the lead resistance can become a factor in the measurement. In the 4 wire measurement method the lead resistance is eliminated by using C1 and C2 leads to establish the current at the unknown resistance and measuring the voltage potential directly at the unknown via P1 and P2. In a three point test P1 and C1 are shorted together at the instruments and a “Short” lead is used to attach them to the ground electrode. Basics of Ground Rod Testing

Test Equipment Clamp-on-Resistance tester uses a new method for testing Ground resistance, Through inductance A current is induces through a loop and the return current is measured. Resistance is then calculated Nice and fast, but not always the best tool for the job. Basics of Ground Rod Testing

Test Equipment Clamp-On Ground resistance tester. If voltage (E) is applied to any measured grounding probe R(X) through a special transformer , current (I) flows through the circuit and can be measured , thereby establishing an equation which can be used to calculate R(x) 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 my be achieved. If voltage (E) is applied to any measured grounding probe R(X) through a special transformer , current (I) flows through the circuit, thereby establishing an equation which can be used to calculate R(x) Therefore, E/I=RX. If E is kept constant, then through the use of a CT we can measure current and a calculation for the resistance of the ground electrode circuit my be achieved. Before you began some key steps Make sure you have a clean, clear connection of the clamp around the ground. Select the current range measurement, if excessive current is > 20 amps ground resistance can’t be measured using the ground clamp tester. Basics of Ground Rod Testing

Fall of Potential Test The circuit is established through the soil by proper placement of a potential and current Auxiliary probes which are both connected to a common point,in a 3 point measurement to the Ground Electrode. Understanding the test A fall of potential ground resistance tester uses these two circuits to establish ground resistance through ohms law. One is a constant current circuit of a determined frequency that is separate of any possible excising current cause by utility or other sources. Second is a voltage potential measurement circuit to measure the voltage drop of the soil. Both are accomplished through a current potential probe or Auxiliary Current probe and a voltage potential measurement probe or Auxiliary probe Basics of Ground Rod Testing

Fall of Potential Short Cut Method.
The ground electrode must be isolated from the electrical system It is important that the probes are in a straight line and the potential Auxiliary probe is then place at intervals within the current path To make certain that the measurement is not in the sphere of influence make 3 measurements First at 62% Second at 52% Third at 72% If there is a large difference in readings then adjust spacing if the probes Note on the graph the flat area of the crave. If properly space there should be an area where the reading remain stable within a few ohms and that there is little change within a +/- 10% variances of the first reading at 62%. This is the method described by IEEE STD #81 as the recognized standard for earth testing The 62% method has been adapted after empirical data shows that the ohm value measured at 62% distance between the ground and remote current electrode is normally the systems ground resistance This method is only applied when your are testing a single ground electrode system. There is ample distance for the placement of the potential probes not to be in the sphere of influence of either the ground rods or the current auxiliary probe. The the placemen to the potential probe is in a straight line between the ground and current auxiliary probes. Basics of Ground Rod Testing

The auxiliary rods placements are important to making an accurate measurement. The sphere of influence or what some call the equal-potential planes results from the flow of current through the resistance of the earth and can be illustrated by equal-potential or spheres. The concentration of the voltage is greater at the remote Current probe and decreases as it approaches the ground rod were it exits ground. At some point there is a plateau. It is at that point were we find the resistance of the ground system. The goal is to place the auxiliary potential probe far enough outside the effective resistance areas of both the ground electrode and auxiliary current probe. It is very difficult to establish an absolute fixed value to this measurement. Some trial and error should be expected. The Effective resistance area is that distance from any electrode or probe that there is no noticeable effect of the surrounding earth to effect the resistance of the electrode or probe Basics of Ground Rod Testing

Insufficient Placement of Potential probe in relationship to the Current auxiliary rod. If the potential probe is to close it is said to be inside the “sphere of influence” of the Current rod and the resistance measurement will increase. Basics of Ground Rod Testing

Insufficient Placement of Potential probe in relationship to the ground rod under test If the potential probe is to close to the “sphere of influence” of the ground electrode then the resistance measurement will decrease approaching zero ohms. Basics of Ground Rod Testing

For diameters of ½” reduce distance by 10%. For Diameters of 2” increase distance by 10%. A rule of thumb is to place the potential probe 6 to 8 times the length of the ground electrode This rule of thumb has been stated as being 4 and as high as 10 so for this presentation 6 to 8 is a good average. Depth Driven Distance to P Distance to C 6 ft 45 ft 72 ft 8 ft 50 ft 80 ft 10 ft 55 ft 88 ft 12 ft 60 ft 96 ft 18 ft 71 ft 115 ft 20 ft 74ft 120 ft 30 ft 86 ft 140 ft This is not a full chart, but a good example of typical spacing of probes. Basics of Ground Rod Testing

Fall-of-Potential Test
The fall of potential test normally consist of taking measurements at defined intervals along a straight line between the electrode under test and the Auxiliary Current Test Probe. Most want at less 10 measurements. Basics of Ground Rod Testing

Fall of Potential Testing
Basics of Ground Rod Testing

Multi Electrode System Normally when two or three ground electrodes are driven they are connected in a straight line and the Short Cut or 62% method can be use but in multiple electrode networks this my not apply. The distance or spacing of the auxiliary electrodes are now based on the maximum grid distance or diagonal distance. For example the Diagonal distance in this square with 40’ maximum sides is 56.56’ Basics of Ground Rod Testing

Multiple Ground electrode Systems In this example our Grid diagonal is 56 feet our current auxiliary probe should be around 370 ft from the grid. The potential probe measurements should be measured at 230 ft., again at 193 ft and at 267 ft Multiple Electrode Systems Max Grid Distance Distance for Potential Distance to Current 8 ft 87 ft 140 ft 12 ft 105 ft 170 ft 16 ft 130 ft 120 ft 20 ft 136 ft 220 ft 40 ft 186 ft 300 ft 60 ft 230 ft 370 ft 100 ft 310 ft 500 ft 372 ft 600 ft 180 ft 424 ft 700 ft 200 ft 453 ft 730 ft Basics of Ground Rod Testing

Multiple Ground electrode Systems Although the 62% method has empirical data that show in most cases to give the correct ground resistance value, It is normally recommended that a minimum of 10 points be taking. Basics of Ground Rod Testing

Multiple Ground electrode Systems Reason One, the short method it doesn’t give us a total picture of the earth’s resistance in relationship to the grounding system. Two, in some cases we my not have adequate spacing or area to accurately perform the test. Basics of Ground Rod Testing

Multiple Ground electrode Systems Although it may not always be necessary to make a number of measurement to construct a graph of the reading, it is normally required by customer or regulating body. It is widely recommended because it provides valuable data for future reference. It also ensures that proper practice and measurements have been preformed. After the initial setup it only take a few extra minutes to perform the complete test. Basics of Ground Rod Testing

In this example two Test Sites were tested and the data plotted. Note that Test Site 2 doesn’t display a flat area of the measurement. There can be several reasons, but the most obvious is that the probes cannot be adequately spaced. Basics of Ground Rod Testing

Fall of Potential Slope Method
Insufficient spacing of Auxiliary Probes 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. In this case the slope method my be your answer. Note in the graph that if there is an overlap of fields of influences that it is almost impossible to get repeatable reading Basics of Ground Rod Testing

You have taking 10 readings and there is no area on the graph which would be considered flat Take the readings at 20%, 40% and 60 % of the total distance between the Ground rod and the current auxiliary rod using the potential rod, and call them R1, R2 and R3 We want to calculate the slope coefficient (μ) that shows the rate of change of the slope. μ =(R3-R2) / (R2-R1) Basics of Ground Rod Testing

Lets assume that the values for R1 = 2.1 , R2= 4.8 and R3 = 6.6 and the distance to the current auxiliary probe was 100 ft the value for mu (μ) would be 0.667 Looking up the value of (μ) from Tagg Slope Method Table will show a Slope Coefficient (dpt/dc) of Using the Tagg slope method you can determine at what distance (dpt/dc) to assign a ground resistance value. Basics of Ground Rod Testing

The value of (μ) should be between 0.40 and 1.60 If it is outside of this the reading are invalided and a new set of test data should be collected and plotted. You my want to change direction of the auxiliary probe and perform a test again. Comparing your data with the first graph. Basics of Ground Rod Testing

Test Equipment Clamp-On Ground resistance tester. Before we test the ground electrode resistance we need to make sure that we have a clean ground. Make sure you have a clean, clear connection of the clamp around the ground. Select and measure the current, if current is > 5 (this my very from manufacture) resistance can’t be measured using the ground clamp tester. If voltage (E) is applied to any measured grounding probe R(X) through a special transformer , current (I) flows through the circuit, thereby establishing an equation which can be used to calculate R(x) Therefore, E/I=RX. If E is kept constant, then through the use of a CT we can measure current and a calculation for the resistance of the ground electrode circuit my be achieved. Before you began some key steps Make sure you have a clean, clear connection of the clamp around the ground. Select the current range measurement, if excessive current is > 20 amps ground resistance can’t be measured using the ground clamp tester. Basics of Ground Rod Testing

Test Equipment Clamp-On Ground resistance tester. The Resistance of the total ground network is now measured. This is a loop tester and requires the utility return to act in simples terms as the aux rods. Clamp on ground resistance measurements are new to the market. Their advantage is that the ground electrode resistance can be measured without being disconnected from the electrical system. Basics of Ground Rod Testing

Test Equipment Clamp-On Ground resistance tester. If voltage (E) is applied to any measured grounding probe R(X) through a special transformer , current (I) flows through the circuit and can be measured , thereby establishing an equation which can be used to calculate R(x) 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 my be achieved. If voltage (E) is applied to any measured grounding probe R(X) through a special transformer , current (I) flows through the circuit, thereby establishing an equation which can be used to calculate R(x) Therefore, E/I=RX. If E is kept constant, then through the use of a CT we can measure current and a calculation for the resistance of the ground electrode circuit my be achieved. Before you began some key steps Make sure you have a clean, clear connection of the clamp around the ground. Select the current range measurement, if excessive current is > 20 amps ground resistance can’t be measured using the ground clamp tester. Basics of Ground Rod Testing

Clamp-On Ground Resistance Test
Advantage is that the ground electrode doesn’t need to be disconnected from the electrical system. The jaws of the clamp must be placed in the electrical path of the systems grounding wire to the ground rod Clamp on ground resistance measurements are new to the market. Their advantage is that the ground electrode resistance can be measured without being disconnected from the electrical system. Basics of Ground Rod Testing

Clamp-on-Ground Resistance
In the example we have a loop between Rx and R1 Loop is created from the rod through the electrical service around to R1, back around thought the earth This is a series circuit, so the resistance is the sum of the resistance of Rx+R1 Basics of Ground Rod Testing

In this example the Clamp-On Ground tester is connected to the power poles grounding conductor. The loop is between the ground conductor and the parallel path of the R1, R2, ---Rn back through the earth resistance. Clamp on ground resistance measurements are new to the market. Their advantage is that the ground electrode resistance can be measured without being disconnected from the electrical system. Basics of Ground Rod Testing

This is an example the Clamp-On Ground tester is being used to measure the ground resistance in a street light application Note in this application the tester is place between the ground conductor and the ground electrode. The loop is between the ground conductor and the parallel path of R1, R2 ---Rn back through the earth resistance. Clamp on ground resistance measurements are new to the market. Their advantage is that the ground electrode resistance can be measured without being disconnected from the electrical system. Basics of Ground Rod Testing

Earth Testers Ideal/Megger 3 pole Testers IDEAL/Megger 4 pole Testers Self powered - no need for hand crank Low power consumption Compact and lightweight Quick battery check Limited lifetime warranty Basics of Ground Rod Testing

Earth Testers Ideal Ground resistance Clamp Ground Resistance Ground Leakage Current Auto ranging Audible indication < 40 ohms Open jaw indication Data hold Basics of Ground Rod Testing

Basics of Ground Rod Testing

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