In this paper, I will introduce the working principle of grounding resistance tester and the method of measuring grounding resistance. A megohm earth resistance tester or megohm meter can be used to measure the resistance of the earth.

Earth resistance tester working principle:

Megohm meter is essentially a direct-reading ohmmeter with a manual generator that provides the test current. The ohmmeter consists mainly of two coils (current coil and pressure coil) mounted at fixed angles to each other on a common axis. It has four terminals, P1, C1, P2 and C2. Its terminals P1 and C1 are short-circuited. This join point is a common point. So it has three external terminals, E (common point), P (P1), and C (C1).

In order to measure the ground resistance using a digital ground resistance tester, the E terminal of the ground electrode under test is connected, and the P and C terminals are connected to the auxiliary electrode through a resistive negligible connection.When the handle of the earth resistance tester rotates at a constant speed, it indicates the earth resistance directly on the dial or the calibrated dial.A set of readings is obtained by burying electrode P in various locations.First, it can be buried between the ground electrode and the current electrode C.Next, it should be driven 15 meters away from the grounding electrode on the other side of the current electrode C.It was then buried 15 meters from the current electrode C.The average of the three readings gives the resistance between the ground electrode and the soil.

The resistance between the ground electrode (i.e., plate or tube) and the soil does not remain constant due to changes in humidity conditions.For good and effective grounding, the grounding system should be tested from time to time and water added to increase the moisture content in the nearby soil.

The grounding resistance of the power station shall be less than 1 ohm.For substations, it should be less than 5 ohms.It should be noted that the grounding resistance should be as small as possible for two reasons:

In the event of a fault, when the metal frame is in contact with a live wire or phase wire, the current will flow through the ground connection, which will result in a potential difference between the metal frame and the ground.The potential difference should be very low because it will act as a person to touch the metal frame in such fault conditions.

In the event of a fault, a low ground resistance will cause a large current to flow through.High current will cause the fuse to melt in a short time, thus disconnecting the faulty equipment from the line, thus ensuring safety.

The meaning of grounding and grounding resistance:

It is important to provide grounding for electrical installations for the following reasons: 

All components of the electrical equipment, such as the housing of the machine, the housing circuit breaker, and the oil tank of the transformer must be connected to the ground electrode.This is done to protect the various parts of the installation and personnel from damage in case the system insulation fails at any time.

By connecting these components to an grounded electrode, a continuous low-resistance path can be used to allow leakage current to flow to the ground.This current makes the guard work, so if a fault occurs, the fault circuit is isolated. 

The grounding electrode ensures that normally dead equipment components do not reach dangerously high potential in the event of system overvoltages due to lightning discharges or other system failures.

In a three-phase circuit, the neutral point of the system is grounded to stabilize the potential of the circuit with respect to the ground.

The grounding electrode is only effective if it has a low resistance to the ground and carries a high current without deterioration. Since the current carried by the grounding electrode is difficult to measure, the resistance value of the grounding resistance is considered a sufficiently reliable indicator of its effectiveness.The resistance of the grounding electrode shall provide good protection and must be measured.The main factors on which the resistance of any grounded system depends are:

The shape and material of the grounding electrode used. 

Depth in the soil where the electrode is embedded.

The resistivity of the soil near the electrode of the resistance.

Soil resistivity is not constant, but varies from one type of soil to another. The presence of water in the soil affects the resistivity of its grounding electrodes not as a fixed factor but as a result of seasonal variations. This requires regular testing to see if the grounding system is still working.