2. Objective :
*- The purpose of this project is to use a proper method for measuring the ground resistance
with the help of Megger earth tester device.
3. What is grounding or earthing system ?
The purpose of earthing electrical equipment (providing a path for fault current to earth) is to protect the
equipment and who in touch with it from the electrical shock and also from the fault currents
Although the earthing or grounding in electrical system is always invisible by physically & by its function
but we can feel its importance only with the problems in electrical system with poor or no earthing
system.
The term ground is defined as a conducting connection by which a circuit or equipment is connected to the
earth
So , An earthing system or grounding system is circuitry which connects parts of the electric circuit with
the ground
Purpose of grounding :
4. is an electrical conductor used to
make contact with a part of the
circuit to the earth.
If a building is fully safely grounded, all of the
electrical system circuits, panels, and the
equipment of the system will be connected to a
metal grounding rod driven into the earth, or
sometimes buried in the earth.
Since the earth is able to absorb any amount of
dangerous current, the grounding rod protects
people from shocks and makes our homes and
offices safe.
Grounding Rod or electrode
5. What is ground testing?
Ground testing is the verification that resistance between the grounding system and earth
meet the purpose of providing a path for the fault current to flow so that it will
end danger and take rid of it through passing in this path to earth .
The path should have the minimum resistance as possible to ensure the most amount of
current flow in it.
6. Earthing Values
Ideally electrical engineers should maintain ground resistance at zero ohms.
The ground resistance value may vary depending upon soil conditions, but basically for low voltage system it
should be less than 1 ohm and for medium and high voltage system it should be in the range of 1 to 5 ohms.
Soil resistance values depend on soil composition, moisture, and temperature.
There are four basic variables which affect the resistance of the earth electrode:
1) The soil.
2) The length of the earth electrode.
3) The diameter of the earth electrode.
4) The depth in earth of the earth electrode.
7. Ground Resistance Measurements
The methods of measuring and testing the ground resistance as follows :
Two-point method
Three-point method
Fall-of-potential method
Ratio method
Four-point method
Touch potential measurements
Clamp-on method
The most common method for measuring ground resistance uses the fall-of-potential method.
8. Fall of potential method :
This method measures grounding electrode resistance based upon the principle of potential
drop across the resistance.
It also uses two auxiliary electrodes (one current rod and the other a potential rod) that are
placed at a sufficient distance from the test electrodes.
a current of known magnitude is passed through the electrode under test and one of the
auxiliary electrodes (current rod).
The drop in potential between the electrode under the test and the second auxiliary electrode
(potential rod) is measured.
The ratio of volt age drop (V) to the known current (I) will indicate the resistance of the
grounding circuit. Either a DC or AC voltage source may be used for con ducting this test.
10. Effect of distance between the electrode
under test and auxiliary electrode
(a) Low distance
(b) High distance
11. The readings of megger tester is not accurate
because of the distance between the main
electrode and current electrode is low (5m) such
that the real value of electrode under test cannot
be measured
Single electrode
It is found that under this condition ( i.e. the
distance between electrode under test and
auxiliary current electrode is 13 meter ) that this
distance give the accurate value of resistance
which is R=7.2Ω .
Practical results:
12. Multiple Ground
the readings was practical and
less resistance value than the
case of single electrode where
at this condition
we found that the resistance
of the electrode under test is
approximately equal to 5 Ω
13. Four electrodes in a straight line
At the same depth and equally spacing between them
14. Electrodes under test Auxiliary voltage electrode
Auxiliary current electrode A reading
