What is earthing and how is it measured? Why is earthing needed?
Definition of Earthing
Earthing is mainly called grounding. It is the process of attaching a metal frame (noncircuit layering part) to an earth or part of an electrical system, such as a neutral point during a star connected system, a conductor of a secondary of a Transformer, etc.
Objectives of Earthing
Earthing is meted out to attain the subsequent objectives:
- It is accustomed save human life from the danger of shock death when it comes involved with a charged frame because of any fault or leakage current.
- It maintains the road voltage constant.
- Earthing protects large buildings from atmospheric lightning.
- To protect from atmospheric lightning, overhead lines, machines are connected to earthing. Earthing is selected from different functions.
Why is earthing needed?
If the winding insulation will be defective. So the resistance value between the motor and the enclosure decreases. an individual having a body resistance Rb would complete a path as shown within the below figure.
If Re is tiny (as usually the case when insulation occurs), the leakage current IL through the person’s body can be dangerously high. As a result
The above problem could also be solved by connecting the enclosure to the grounded neutral wire. Now leakage current IL be due the motor, through the enclosure and straight back to the bottom or earthing wire.
Types of Earthing
The name of various kinds of earthing is from the sort of earth electrode used for earthing. We shall obtain information about the subsequent three styles of earthing.
- Plate earthing
- Pipe earthing
- Coil earthing
Methods of measuring the resistance of the Earth
The value of earth resistance increases after your 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 wont to measure earth resistance.
- Voltmeter-ammeter method
- Earth tester method
- Ohm meter method
- Earth loop tester method
The earth resistance is measured by the autumn of potential method. The circuit arrangement for the planet resistance measurement by the autumn of potential method is shown in figure. This founded consists of 1 earth electrode E and two auxiliary electrodes P and Q. E and Q are separated by a sufficiently large distance and are connected through electric battery.
A current of I ampere flow between the E and Q through the bottom. Now, another electrode P is inserted between the E and Q, and also the voltage between E and P is measured with the assistance of a voltmeter. We notice that when the electrode P is moved aloof 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, during this whole experiment, the magnitude of current remains constant. Therefore, by using the relation (V/I), we obtain the subsequent curve.
The resistance region at which the V/I remain almost constant is named earth resistance. If the experiment is completed by using the provision mains, we want to use isolating transformer between the provision main and experiment founded to eliminate the effect of residual earth currents.
For the only ground rod, the resistance of ground rod to earth is given by:
Where, 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 world tester is used. As shown within the above figure, terminals P and C of the world tester are connected to voltage spikes Q and R respectively. Earth terminal E of the world tester is connected to the world electrode P. the world tester is an instrument to live low resistance.
There is one small generator inside. About 500 V DC has generated. So, the present flows to earth through spike R and coil of the world tester. Due to this current, a force is generated thanks to which the coil deflects and remains stationary at some position. A pointer is attached to the which moves over the dimensions. the dimensions is calibrated in Ohm that the pointer shows the resistance of the planet.
The ohmmeter is employed during this method. First the acceptable is chosen and therefore the pointer is adjusted to the zero position by shorting the probes of the meter. One probe is joined to the earthing electrode and also the other probe is joined to the nearby hubbly-bubbly or other earthing terminal and also the reading is taken.
Earth Loop Tester Method
In this method, the planet loop tester is used. There are three terminals marked P, N, and E. P terminal is connected to the phase wire of the availability, N terminal is connected to the neutral, and therefore the E terminal is connected to the world electrode.
Due to these connections, the first winding of the transformer of the planet tester loop tester gets supply and therefore the secondary coil are connected across N and E. Thus, the circuit of the world loop is completed through a, b, c, d, e, f, g, h, i, a. The resistance of the planet loop may be calculated from the reading of voltmeter and ammeter. It should be remembered that this is often the resistance of the planet loop and therefore the earth electrode. The method to be used depends upon the available equipment, the capacity of the installation, etc.