Earth faults create a high level of danger to any metallic object or person, which is why they require proper earthing. In addition, Earthing is designed to minimize the effects of transient voltage caused by a lightning strike. An earth electrode is driven into several places in order to make connections with the earth. Earth electrodes are metal pipes or conducting plates that are connected to the earth for measuring its resistance.
There are testers available for measuring earth resistivity, including a digital earth resistance tester . However, there are many factors that influence the earth’s resistance, such as the soil composition, temperature, moisture content, and electrode depth. We will examine all the factors affecting the earth’s resistivity in this blog.
Earth Resistivity Factors
In this section, we will take a look at the five factors that affect the earth’s resistivity.
Soil Types Affect Resistivity
The earth’s resistivity can be greatly influenced by the nature of the soil, whether it is predominantly clay or very sandy. Sometimes it is difficult to define exactly what is in the soil that is affecting the resistance. In terms of resistance, clay soils have a maximum resistance of 16,300 ohms-cm.
Moisture And Dissolved Salts Decrease Resistivity
The conductivity of current in the soil is largely electrolytic. Therefore, moisture and salt content in soil greatly affect its resistivity. There is a great deal of variation in how much water the soil contains, of course, depending on the weather, the depth of water, and the nature of the subsoil.
Soil is a good insulator when dry, so its resistance will be greater when it is dry. A dramatic reduction in resistivity occurs when the moisture content is 15%. Pure water is infinitely resistive. Dissolving naturally occurring salts in water lowers the earth’s resistivity.
Temperature effects on Earth Resistivity
There is little information on the effects of temperature, however, an increase in temperature decreases resistivity, based on two facts.
- It is mostly the water in the soil that determines the soil’s resistivity.
- The resistivity of water decreases with an increase in temperature. It is important to note that when water in the soil freezes, the resistivity rises considerably, and that is why ice has a high resistivity. As the temperature drops below freezing, the resistivity continues to increase.
Earth Resistivity And Seasonal Variations
Temperatures, moisture, and salt content have been a factor in the earth’s resistivity. Thus, it is not surprising that the resistance of soil will vary considerably with the seasons. It is especially true in climates with extreme temperature fluctuations, rainfall, droughts, and other seasonal variations. We can observe that the earth’s resistivity varies.
Determining Electrode Location
A low-resistance earth electrode relies on a low-resistance soil in an area where you can drive the electrodes and there are two ways to choose your location:
- While driving the rods, test their resistance and drive them in different locations to the depths necessary.
- Before driving ground rods, measure the earth’s resistivity. Calculate the number and length of rods needed.
How To Determine Electrode Location?
Layout a straight line 10 feet apart, covering the area, to get a low resistance electrode in an undesirable location. Measure the resistance R between stakes b and c along a line a-b-d-c, staking four stakes 10 feet apart but not more than 6 in deep. Shift the stakes along the line in question to points b-c-d-e, c-d-e-f, etc., and test until it has covered the entire line. Repeat this process until you have covered the entire area. Among the chosen depths of ten feet, the location with the lowest value for R has the lowest soil-specific resistance. This is likely to be the most reliable earth electrode location.
You could repeat the survey twenty feet apart and with stakes-placed twenty feet apart if you wanted the average earth resistivity to affect the results. This kind of survey is relatively quick and can pay off in terms of ensuring a good grounding system.
Effects Of Grain Size And Distribution
Granule size, distribution, and packing closeness are also contributory factors since they affect how moisture is held in the soil.
Effects Of Current Magnitude
Currents of significant magnitude and duration will cause soil to dry out considerably, increasing its resistance. Current flowing from ground electrodes into the soil surrounding them may affect soil resistivity. Thermal characteristics and moisture content of the soil will determine whether a given magnitude and duration of current will cause significant drying or not and thus increase soil resistivity.
Obstructions
Under a few feet of soil, there may be obstructions such as virgin rock. In this case, obstructions affect resistivity. In addition, resistance will be affected by obstructions like concrete structures near pits. There will be a high resistance value if the earth’s pits are nearby.
Way Ahead
That’s all about the factors affecting Earth’s Resistivity. Few other factors that affect the earth’s resistivity are Climatic condition, soil type, temperature, moisture, and dissolved salt. In addition to earth resistance testers, a ground fault locator is a helpful tool for detecting ground faults.
