Definition of Earthing
The Earthing is also known as Grounding. It is a process of connecting the metallic frame (i.e. noncurrent carrying part) of electrical equipment or some electrical part of the system (e.g. neutral point in a star connected system, one conductor of the secondary of a transformer, etc.) to earth.
Objectives of Earthing
Earthing is carried out to achieve the following objectives:
- It is used to save human life from the danger of shock death when it comes in contact with a charged frame due to any fault or leakage current.
- It maintains the line voltage constant.
- Earthing protects large buildings from atmospheric lightning.
- It protects all machines fed from Overhead lines from atmospheric lightning.
Necessity of Earthing
The below figure shows the case of an ungrounded metal enclosure. If the equipment is functioning correctly, nothing will happen when a person touches the metal enclosure.
But if the winding insulation becomes faulty, the resistance Re between the motor and enclosure drops low value. A person having a body resistance Rb would complete a path as shown in the below figure.
If Re is small (as usually the case when insulation occurs), the leakage current IL through the person’s body could be dangerously high. As a result, the person would get a severe electric shock which may be fatal.
The above problem may be solved by connecting the enclosure to the grounded neutral wire. Now leakage current IL flow from the motor, through the enclosure and straight back to the ground or earthing wire.
Type of Earthing
The name of different types of earthing is from the type of earth electrode used for earthing. We shall obtain information about the following three types of earthing.
- Plate earthing
- Pipe earthing
- Coil earthing
Methods used to Measure Earth Resistance
The value of earth resistance increases after some time. So, the resistance should be measured periodically and steps to decrease it should be taken if the resistance has found to be measured.
The following methods are used to measure earth resistance.
- Voltmeter-ammeter method
- Earth tester method
- Ohm meter method
- Earth loop tester method
The earth resistance can be measured by the fall of potential method. The circuit arrangement for the earth resistance measurement by the fall of potential method is shown in figure.
This set up consists of one earth electrode E and two auxiliary electrodes P and Q. E and Q are separated by a sufficiently large distance and are connected through a battery.
A current of I ampere flow between the E and Q through the ground. Now, another electrode P is inserted between the E and Q, and the voltage between E and P is measured with the help of a voltmeter.
We notice that when the electrode P is moved away from the electrode E the voltage reading voltmeter initially increases.
But when the electrode P is at an adequate distance from E, the voltage reading remains almost constant. But when electrode P approaches the electrode Q, the voltage again rises rapidly.
Since, in this whole experiment, the magnitude of current remains constant. Therefore, by using the relation (V/I), we obtain the following curve.
The resistance region at which the V/I remain almost constant is called earth resistance.
If the experiment is done by using the supply mains, we need to use isolating transformer between the supply main and experiment set up to eliminate the effect of residual earth currents.
For the single ground rod, the resistance of ground rod to earth is given by:
ρ = Average soil resistivity (ohm-meter)
L = Ground rod length (meter)
a = Ground rod radius (meter)
R = Resistance of ground rod to earth (ohm)
Earth Tester Method
In this method, the earth tester is employed. As shown in the above figure, terminals P and C of the earth tester are connected to voltage spikes Q and R respectively.
Earth terminal E of the earth tester is connected to the earth electrode P. The earth tester is an instrument to measure low resistance.
There is one small generator inside. About 500 V DC has generated. So, the current flows to earth through spike R and coil of the earth tester.
Due to this current, a force is generated due to which the coil deflects and remains stationary at some position.
A pointer is attached to the which moves over the scale. The scale is calibrated in Ohm so the pointer shows the resistance of the earth.
The ohmmeter is used in this method. First the suitable is selected and the pointer is adjusted to the zero position by shorting the probes of the meter.
One probe is joined to the earthing electrode and the other probe is joined to the nearby water pipe or other earthing terminal and the reading is taken.
Earth Loop Tester Method
In this method, the earth loop tester is employed. There are three terminals marked P, N, and E.
P terminal is connected to the phase wire of the supply, N terminal is connected to the neutral, and the E terminal is connected to the earth electrode.
Due to these connections, the primary winding of the transformer of the earth tester loop tester gets supply and the secondary winding will be connected across N and E. Thus, the circuit of the earth loop is completed through a, b, c, d, e, f, g, h, i, a.
The resistance of the earth loop can be calculated from the reading of voltmeter and ammeter. It should be remembered that this is the resistance of the earth loop and the earth electrode.
The method to be used depends upon the available equipment, the capacity of the installation, etc.
For the new installation, a 500V megger is used. Loop tester method is used to measure earth loop resistance between the substation and place of use. With the help of Ohmmeter quick measurement can be made.
1,008 total views, 1 views today