As most underground metro stations have a limited space/ land area, the earthing system for the auxiliary sub station or the feeder post or the signalling and telecom are inside the station box area, basically 300-1000 mm below the station base level. And for an earthing system which is very close to humans, we should consider the touch and step voltages that will come into effect during the fault. (For more details read GPR and step-touch voltages) That’s why the preferred earthing is Copper electrode grid earthing (also known as copper earth mats)
For a good soil strata the soil resistivity comes out to be appx 11 ohm-m to 22 ohm-m. Soil resistivity helps us in getting the number of electrodes required in an earth mat.
Ra=R (1+λa/n) Where a= ρ/2 X 3.14 X R X S
where S= Distance between adjascent Rod
λ =Factor Given in Table
n= Number of Electrode
ρ=Resistivity of Soil (Ω Meter)
R=Resistance of Single Rod in Isolation (Ω)
SOIL RESISTIVITY TEST
Soil resistivity data is the key factor in designing a grounding system for a specific performance objective. All soils conduct electrical current, with some soils having good electrical conductivity i.e. poor resistivity while the majority has poor electrical conductivity i.e. high resistivity. The resistivity of soils varies widely throughout the world and changes dramatically, if soil strata/geological variation will encounter. Soil resistivity is primarily influenced by the type of soil (clay, shale, etc.) and its moisture content, the amount of electrolytes (minerals and dissolved salts) and very nominally by temperature. When designing a grounding system for a specific performance objective, it is necessary to accurately measure the soil resistivity of the site where the grounding is to be installed. Grounding system design is an engineering process removing guess work from earthing, leading to a more scientific approach to the grounding problem. The grounding process just gets a go from the very beginning which definitely results in cost saving, avoiding unnecessary expenditure. The best method for testing soil resistivity is the Four Point (Electrode) method. It uses a4-probe digital ground resistance meter. It requires inserting four probes into the test area. The spacing of the probes gives a particular system (configuration) (Fig.1).
The probes are installed in a straight line and spaced depending on the configuration being used. The four probe test meter establishes a constant current flow with the ground via the tester and the outer two probes. These two outer probes are the current probe designated as C1 (or A) and C2 (or B) which are connected to a constant voltage source and it is assumed that the current flowing through the medium is constant. The current flowing through the earth (a resistive material) develops a potential difference between the two points through which the same current is flowing. This potential difference resulting from the current flow is measured between the two inner probes. These two inner probes are known as the potential probes and designated as P1 (or M) and P2 (or N). The meter measures the current (i) flow between the outer two electrodes and the potential difference between the inner two electrodes. Both these data are taken off by the meter for processing and yield the value of resistance. This displayed resistance value is in ohms and is converted into the resistivity of the material which is expressed in ohm-meter. To convert from the displayed ohms to ohm-meter, the meter reading is multiplied by the geometric factor which is calculated from a formula which utilizes the various distances between the probes i.e. ‘a’ of a particular configuration. This resistivity is the resistance of a material that is one meter in length and one meter square in area and is comparable to the term specific resistance of a material. For determining the resistivity at different depths and in different directions multiplereadings are taken changing the probe spacing and the direction (generally 3 or 4directions are used) but keeping the central point of all measurements to be the same. This technique is known as Resistivity Sounding. The more data available to the design agency, the more accurately they will be able to design and predict the grounding system performance. Multiple data provided to the customer allows them a better approach to the process of most right selection of grounding system. A pipe or some underground structure could influence the readings. But the data gathered in various directions minimizes such effects. The more data available and used in the design provides more confidence in the outcome. Resistivity sounding involves gradually increasing the spacing between the current and/or potential electrodes to obtain deeper penetration. Under resistivity profiling, the electrode spacing is kept constant and the entire arrangement is moved along profile lines, to obtain lateral variation in subsurface resistivity. Electrical resistivity techniques are based on the principle that the resistivity varies depending on the material encountered. Resistivity can then be used to identify different geological units by their electrical properties. If a material’s resistivity value drops it could mean that the rock is water saturated and one can expect to find fractured bedrock. The variation in resistivity will correspond to a geological variation along an investigated line. The data are presented as profiles in which the spatial distribution of the electrical properties of the investigated material can be qualified.
SOIL RESISTIVITY TEST LOCATION
Soil resistivity testing should be conducted as close to the proposed grounding system as possible, taking into consideration the physical items that may cause erroneous readings. There are TWO (02) main reasons that may cause poor quality readings.
- Electrical interference causing unwanted signal noise to enter the meter.
- Metallic objects ‘short –cutting’ the electrical path from probe to probe. The rule of thumb here is that a clearance equal to the pin spacing should be maintained between the measurement traverse and any parallel buried metallic structures. Testing in the vicinity of the site in question is obviously important; however, it is not always practical due to unfeasible site conditions and other obstructions, as well as interference during data acquisition. The geology of the area also plays into the equation as dramatically different soil conditions may exist only a short distance away