Earth Resistance using Fall-of-Potential Method

Introduction

 

What is Earth Resistivity

                      

            Earth resistivity is a measure to indicate how much the soil resists to conduct electrical current. It is measured in Ω cm (Ohm Centimeters). Measurement of earth resistivity are useful for finding the best location to pile earth electrodes. Resistivity of the earth depend on its temperature, soil type, seasonal variations, Moisture and dissolved salt levels. When the temperature increases decrease the resistivity of water.

                      

                       So, in this experiment we had to measure the earth resistivity using some equipment’s in several methods.

 

 

OBJECTIVE

                             

                              The purpose of this experiment is to determine the earth resistance of an earth electrode and to determine the resistivity of the earth.

 

APPARATUS

                             

                              1. Earth Tester

                       2. Galvanized iron pipe electrodes and 2 spikes

                       3. Wires

                       4. Tape

 

                       Earth resistivity is a very useful thing in all fields such as medical field, construction field and electrical field. Normally before every building construction, companies measure the soil resistance before they start construction. Also, there is a minimum value for a good soil resistance. So, they need to make sure the soil is acceptable with construction rules and regulations. There are advanced method of determining the resistivity but, in this experiment we are using a basic method. So, we are taking three values and take the average of it.

 

                       We will be explaining step by step at the discussion below.

 

 

 

Discussion

 

                  First, we need to know what earth resistivity depends on. So, as per the information in internet the factors are,

1.      Condition of soil

2.      Temperature of soil

3.      Moisture content of soil

4.      Size and spacing of earth electrodes

5.      Depth at which the electrode is embedded

6.      Material of conductor

                       We could not find different values for different soils, because we did the experiment on the same soil.

 

 

Earth Electrode Resistance

 

The resistance experienced by the current going to the earth electrode is called earth electrode resistance. It has main three components such as,

 

·         Resistance of the electrode itself and connections to it.

·         Contact resistance between the electrode and the soil adjacent to it.

·         Resistance of the surrounding earth.

Why we do a soil resistivity testing

Soil resistivity testing is the process of measuring a volume of soil to determine the conductivity of the soil. The resulting soil resistivity is expressed in ohmmeter or ohm-centimeter.

Soil resistivity testing is the single most critical factor in electrical grounding design. This is true when discussing simple electrical design, to dedicated low-resistance grounding systems, or to the far more complex issues involved in Ground Potential Rise Studies (GPR). Good soil models are the basis of all grounding designs and they are developed from accurate soil resistivity testing.

 

Soil Resistance Measuring Method

1.      Fall-of-potential method, or three-terminal test.

2.      Dead earth method (two-point test).

3.      Clamp-on test method.

Most accurate method – Fall-of-potential method

1.     Fall-of-potential method (Three terminal method)

To measure the earth ground resistance of a single ground electrode at E in Fig. 1, a current is forced to flow to E by driving auxiliary test probe H into the ground at a certain distance from electrode E and connecting probe H to a current source.

     

                                                                                                                                                                                                                                                                                                 From all the methods of measuring the earth resistance, fall of potential method is the most accurate method.

                       Step 1- Place Earth Electrode fully inserted

                       Step 2 – Place the current probe with a long distance

           Step 3 – Place the potential probe at the middle point between earth and current                probe and measure the soil resistance using the meter

           Step 4 - Replace the potential probe x meters close to earth and measure the soil resistance

           Step 5 – Replace the potential probe x meters close to the current probe and take measurements

           Step 6 – Take the average value of three measurements

                       There is a sub-method of fall-of-potential method called the 62 method. First 2 steps are same as the fall-of-potential method. Next steps,

                       Step 3 – Place the potential probe at a distance of 62% from the total distance and measure the resistance using the meter

                       Step 4 – Do the same step for 2 more different total distances

                       Step 5 – Take the average value of 3 measurements

 

2.     Dead Earth Method (two-point test)

                

                 The final simplified method is again one of limited reliability and should not be employed generally; that is the “Dead Earth” Method. This technique is quite popular because of its simplicity. Only two leads are used, one hooked to the test ground and one to a reference ground. This is essentially the same as using a multimeter.

 

                  When connecting to the metallic water pipe, there are some limitations regarding to the water pipe. They are,

1. The waterpipe system must be extensive enough to have a negligible resistance.
2. The waterpipe system must be metallic throughout, without any insulating couplings or flanges.
3. The earth electrode under test must be far enough away from the water-pipe system to be outside its sphere of influence. In some locations, your earth electrode may be so close to the water-pipe system that you cannot separate the two by the required distance for measurement by the two-terminal method.

      If conditions 1 and 2 above are met, you can connect to the water-pipe system and obtain a suitable earth electrode. Dead earth method is the simplest way to measure the earth resistance but not the most accurate method.

3.     Clamp-on test method

The clamp-on ground testing method, although it does not conform to IEEE 81, does provide the operator with the ability to make effective measurements under the right conditions. The clamp-on methodology is based on Ohm’s Law (R=V/I). A known voltage is applied to a complete circuit and the resulting current flow is measured. The resistance of the circuit can then be calculated. The clamp-on ground tester applies the signal and measures the current without a direct electrical connection. The clamp includes a transmit coil that applies the voltage and a receive coil that measures the current.

 

 

 

Observations

            We did the experiment using the fall-of-potential method which is most accurate. We did the same test using two total distances.

Test 01

Total distance – 19.2m

First measurement (middle point) – 200 ohms

2^nd measurement (middle point + 2m) – 200 ohms

3^rd measurement (middle point – 2m) – 200 ohms

Test 02

Total distance – 10m

1^st measurement (middle point) – 200 ohms

2^nd measurement (middle point +1m) – 200 ohms

3^rd measurement (middle point – 1m) – 200 ohms

Test 03

We did the same test by only inserting half of the earth electrode into soil.

Total distance – 10m

1^st measurement (middle point) – 440 ohms

2^nd measurement (middle point + 1m) – 440 ohms

3^rd measurement (middle point – 1m) – 435 ohms

 

 

Calculations

Test 01

Average value for earth resistance = (200 + 200 + 200)/3

                                                       = 200 ohms

Test 02

Average value for earth resistance = (200 + 200 + 200)/3

                                                       = 200 ohms

Test 03

Average value for earth resistance = (440 + 440 + 435)/3

                                                        = 438.33 ohms

 

Conclusion

          In this experiment we determined the earth resistance using the fall-of-potential method. We got an average value of 200 ohms. During this experiment we had a good idea about the factors of earth resistance and also about earth potential meters.