Verifying Ground Rod Installation


In Article " Ground Resistance Measurements " , I indicated that there are many Ground Impedance Measurement Methods as follows: 


  1. The 2-Point Method, 
  2. The 3-Point Method, 
  3. The Fall of Potential Method, 
  4. The 62% Rule, 
  5. The Ratio Method, 
  6. The Tag Slope Method, 
  7. The Intersecting Curve Method, 
  8. Staged Fault Tests, 
  9. Driving Point Impedance, 
  10. The SGM Method 


And I explained the most used method; The Fall of Potential Method.


Today, I will explain the following points:

  • The factors affecting Ground Resistance Measurement Reliability, 
  • Verifying Ground Rod Installation. 


You can review the following Articles For more information





3.3 Measurement Reliability





Factors Affecting Ground Impedance Measurement

Valid and reliable measurements of ground resistance are susceptible to errors originating from the following:

  1. Probe spacing
  2. Stray current
  3. Self- and mutual impedance
  4. Probe resistance






1- Probe Spacing

Probe spacing readings can vary considerably due to distance from the ground system being measured. It is therefore very important to place the measuring or current probe as close to the calculated measuring distance as possible. This is evident from the curves in Figure 2. Place pp too close to the system being measured and the resistance is too small. Place pp too far away from the system and the resistance is too high.






2- Stray Current

The conduction of current through the soil is electrolytic in nature, and back voltage can develop at the auxiliary electrodes. An easy way to eliminate these electrolytic effects is to use alternating test currents. If the test current is of power frequency, electrolysis is not eliminated and stray alternating current at power frequencies may influence the results. At higher frequencies, electrolysis is negligible.
The best current source for resistance or resistivity measurements is direct current that you periodically reverse, with a complete break in the circuit between reversals.






3- Self- and Mutual Impedance

At higher frequencies, the self- and mutual impedance of the leads are increased and errors may be introduced. If an impedance test is performed, the reactance component will be different from the 60 Hz value. Usually, a compromise using frequencies in the order of 80 Hz is considered adequate. Eliminate the error that is introduced due to the self- and mutual impedances by using direct current.






4- Probe Resistance

The current electrode resistance is in series with the power source and is one of the factors governing the testing current.  If this current is low, it may be necessary to obtain a higher current, resulting in lower current electrode resistance, by driving additional ground rods. It is a good practice to drive rods at an angle other than vertical in rocky soil, as inclined rods will slide over the top of a rock resulting in deeper penetration and better soil contact (less resistance).






Important!!!

When using instruments like the Vibraground and the Biddle Megger, such errors as listed above are eliminated because of their special circuit design.






3.4 Results Analysis




The following is a list of sample readings from a hypothetical ground resistivity measurement: 






Apparent Ground Impedance (ohms)
Distance From Potential Electrode To The Station Grid (meters)
1.1
25
1.3
50
1.4
75
1.45
100
1.45
125
1.50
150
1.50
175
1.50
200
1.55
225
1.60
250
1.65
275
1.9
300
2.4
325





Figure 1. Plotted resistance values 



Figure (1) shows a plot of these values. The curve tends to level out between 75 meters and 275 meters and relates to an ohmic reading of 1.5 Ω. This is the impedance value of the ground under test.





4- Verifying Ground Rod Installation




As an electrical inspector, your assessment of ground rods or electrodes starts with physical inspections with the station layout plan to determine correct placement, and size. You may also need to take resistance measurements at the time of inspection.




4.1 Resistance to Earth



Ground resistance is the overall resistance from the point of the grounding connection of a system or an item of equipment to the body of earth. The resistance of the grounding electrode is a component of this.

These are the characteristics of an electrode’s grounding metal electrode:

  • Contact resistance between the electrode and the soil, 
  • Resistance of the soil, from the electrode surface outward, in the geometry setup for the flow of current outward from the electrode to infinite earth 




Important!!!

The term electrode in this section primarily indicates driven rods, but can also include steel reinforcing bars in below-ground concrete, buried strips, cable or plates, grids, and or counterpoises.





4.2 Design Guidelines and Requirements




  • The most elaborate grounding system may not perform satisfactorily unless the connection of the system to earth is adequate for the particular installation. Earth connection is one of the most important parts of the whole grounding system and the most difficult to design. 
  • The connection to earth, or the electrode system, requires a sufficiently low resistance to permit prompt operation of the circuit protective devices if a ground fault occurs. This also provides the required safety from shock to personnel who may be near equipment frames, enclosures, conductors, or the electrodes themselves. Logically, the lower the resistance of the grounding system, the more adequately these meet those requirements. 
  • Some Codes requires grounding electrodes to be buried conductors constituting a direct metallic path and supplemented when necessary, by rods or groups of rods to obtain the required value of ground resistance. 
  • System ground resistances of less than 1Ω may be obtained by a number of individual connected electrodes. 





Criteria For Ground Rods And Wells

The following list is the criteria for ground rods and wells:
  • Ground wells are electrodes encased in sand or similar material and protected by PVC pipe above grade. They are designed to allow testing of the ground system (Figure 2). Placement of these is diagonally and within the substation. The number of these is determined by the design engineer. So if 2 were required, they would be placed at opposite corners, and if 4 were required, they would be placed at each corner.
  • Rods must be spaced a minimum of 6 feet (1.8 m) apart per the NEC requirements when multiple rod, pipe, or plate electrodes are installed. The paralleling effciency of rods longer than 8 feet (2.44 m) is improved by spacing greater than 6 feet (1.8 m). Spacing rods, etc., closer than this will not improve the resistance.
  • Rods must be a minimum of 5/8 inch (16 mm) in diameter and 8 feet (2.4 m) minimum in length, made of copper clad steel or similar metal. If the soil resistance is less than 70 Ω/m, they must be stainless steel.
  • Ground wells must be connected to the ground grid by approved mechanical fittings (i.e., bolted) which are accessible for inspection, servicing, and disconnection.
  • Aluminum electrodes are not permitted.
  • Electrodes should be flush, buried, or protected against physical damage.







Figure 2. Ground well design 




4.3 Measuring Rod Resistance




  • Check electrodes periodically or when you suspect they are defective. 
  • The method to measure individual rod or small grids resistance is similar to measuring system ground, with these differences: 
  1. The current probe distance of the test setup is 100 to 125 feet (30.5 to 38.1 m). It can be longer but, in most cases, it does not need to be. 
  2. An electrode has to be isolated (disconnected) before any measurements are performed. This is especially true if a test is being conducted on an individual rod at a residence. 
  • All other procedures are the same and can be followed in the order as they appear in the "Ground Resistance Measurement" section under "Ground Resistance" located in this course. 




In the next Article, I will explain How to Verify Ground Connection and Fence Ground Installation. Please, keep following.





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