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Measure Earth’s resistance using fall of potential method.

OBJECT: Measure Earth’s resistance using fall of potential method.

THEORY:

Introduction

The resistance to remote earth of the grounding system needs to be at a minimum in order to sustain its effectiveness.   A few of the components that make up this   resistance   are   the   physical   properties   of   the material used to make the electrode and conductor, all connections made, contact resistance between the electrode  and  the  soil,  and  the  soil  resistivity.     A complete  grounding  system  might  include  only  one earth electrode, an entire group of electrodes with a grounding grid,  or anything  in between  and  beyond. The  earth  electrodes   from  any  of  these  types   of systems can have their resistance to remote earth determined. Theoretically, the resistance to remote earth of an earth electrode can be calculated.   This calculation is based on the general resistance formula:

R = (r x L) / A

   where:R = resistance to remote earth (W)

r = soil resistivity (W-cm)

L = length of conducting path (cm)

A = crossectional area of path (cm)

 

 

Fig. 8.1 Single Electrode Measuring Method

Fig. 8.2 Single Electrode Measurement Setup

After applying a voltage at the designated location in the circuit, the current between the EUT and auxiliary probe is measured,  and the voltage between the EUT and  the  reference  probe  is  measured. Using  Ohm’s Law, the resistance can now be calculated.  This routine is completed by the earth tester.  It has been determined that the distance D be approximately 100 feet, and the distance between the EUT and reference probe be 0.62 times the value of D.   These values have been tested using the fall of potential method to give optimal. But  due  to  the  large  amounts  of uncertainty  in  all  properties  of  probes  and  soil,  the value  of D should  be varied  for  each  individual  test until reasonably consistent values appear; however, it is recommended that the value of D stay greater than 80 feet.    It  is  also  necessary  to  take  a  second  set  of readings at 90o  to the original  in case of interference from   overhead   power   lines   or   any   underground electrical equipment or metal objects.

Rs = 1 / [ (1/R1) + (1/R2) + (1/R3) + …(1/Rn) ]

where

Rs =  resistance to remote earth for entire grounding system (W)            

R1,2,3…  n   = resistance to remote earth for each individual electrode (W)

Fig. 8.3 fall of potential method

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Fig. 8.4 Waveform of fall of potential method

Working: First, the earth electrode of interest must be disconnected from its connection to the site. Second, the tester is connected to the earth electrode. Then, for the 3-pole Fall-of-Potential test, two earth stakes are placed in the soil in a direct line—away from the earth electrode. Normally, spacing of 20 meters (65 feet) is sufficient. For more detail on placing the stakes, see the next section.A known current is generated by the Fluke 1625 between the outer stake (auxiliary earth stake) and the earth electrode, while the drop in voltage potential is measured between the inner earth stake and the earth electrode. Using Ohm's 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. To achieve the highest degree of accuracy when performing a 3–pole ground resistance test, it is essential that the probe is placed outside the sphere of influence of the ground electrode under test and the auxiliary earth.If you do not get outside the sphere of influence, the effective areas of resistance will overlap and invalidate any measurements that you are taking. The table is a guide for appropriately setting the probe(inner stake) and auxiliary ground (outer stake).To test the accuracy of the results and to ensure that the ground stakes are outside the spheres of influence, reposition the inner stake (probe) 1 meter (3 feet) in either direction and take a fresh measurement. If there is a significant change in the reading (30 %), you need to increase the distance between the ground rod under test, the inner stake (probe) and the outer stake (auxiliary ground) until the measured values remain fairly constant.

 

Factors That Can Change Your Minimum Earth Resistance:

We will discuss later what value of earth resistance is considered low enough. You’ll see that there is no general rule usable for all cases. First, however, consider three factors that can change the earth electrode requirements from year to year:A plant or other electrical facility can expand in size. Also, new plants continue to be built larger and larger. Such changes create different needs in the earth electrode. What was formerly a suitably low earth resistance can become an obsolete “standard.”As facilities add more modern sensitive computer-controlled equipment, the problems of electrical noise is magnified. Noise that would not effect cruder, older equipment can cause daily problems with new equipment.As more nonmetallic pipes and conduits are installed underground, such installations become less and less dependable as effective, low resistance ground connections. In many locations, the water table is gradually falling. In a year or so, earth electrode systems that formerly were effective may end up in dry.

 

RESULT: A number of tests are made at different spaces of P and the resistance curve is plotted.

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