King Saud University
College of Engineering
Electrical Engineering Department
EE445
Section number
80717
Title
Exp.1: Resistance of
grounding arrangement
Date of submission
16/2/2022
Student name
Fahad Q
ID number
Instructor’s name
Dr. Abdulrahman AlOraini
OBJECTIVES:
- To measure the grounding resistance for rod and hemispherical electrodes
and to examine major factors that can influence the grounding resistance of
rods.
- To measure soil resistivity.
- To measure the ground resistance of typical grounding system (e.g. HV lab
grounding).
SUMMERY:
- We start by comparing the ground resistance between different mediums
(soils), and it is noticed that the treated soil has less resistance value that the
dry soil. Then we compared between different rod diameters and then
measure the resistance of them based on the depth of the rod. It is noticed
that there are many ways to decrees the ground resistance such as increasing
the rod diameter and making parallel combination of them. Lasty, we
measure the ground resistance of the HV lab.
QUESTIONS:
Q1) Why do we need to measure grounding resistance and soil resistivity?
- Measuring the soil resistivity and ground resistance is to design a grounding network.
The engineers need to consider that they might use low resistivity materials for personal
and equipment safety.
Q2) Using the measured value of grounding resistance for hemispheres, calculate
the resistivity of sand and clay samples:
-
For sand (water, salt and chemicals added):
𝑑
R=2 =
6.25
2
= 3.125 cm
𝑝= R π r = 1181* π * 0.03125= 115.944 Ω.m
-
For dry sand:
The ground resistance value can't be measured because it is higher than the ohmmeter
ability. So, we can guess that: Rg > 2 kΩ
Q3) Calculate the sand resistivity using the cylindrical rod equations for one case:
-
𝑝=
R 𝝅𝒅
4𝑑
[ln( )−1]
𝑟
=
450∗𝜋∗0.15
ln(
4∗0.15
)−1
0.024
= 95.57 Ω.m
Q4) Compare the above two values and comment on the reasons for differences, if
any:
-
The resistivity of the sample using hemispheres is greater than the resistivity of the
sample by using rod because of the difference between the calculation method. In the first
case the radius and depth of the hemispheres are neglected. But in the second case the
radius and depth are considered in the calculation.
Q5) In one figure plot variation of Rg against depth of rod for the three cases
studied. Give your comments on the effect of rod depth, rod diameter and number
of rods on Rg.
Fig.1: Ground resistance with different rods diameters and depths.
-
Fig.1 shows that grounded resistance will decrease when the rod depth, diameter or
number of rods increases.
Q7) Plot variation of R with X. Then find the value of grounding resistance Rg of
the HV lab.
Fig.2: Grounding resistance of HV lab.
-
From Fig.2, the grounding resistance for 63% of 40m is about 0.3 Ω
Q8) List possible problems of high grounding resistance.
-
The grounding resistance must be low because if there is a fault with a high current it will
have no other way to go through except the ground. High grounding resistance can create
many problems. It will cause damages in equipment and may causes injures.
Bonding resistance results (in µΩ):
•
•
•
Good: 72.9
Medium: 346.5
Bad: 1075.5
COMMENTS:
-
The fall of potential is a method to measure the resistance of the HV lab grounding
system.
There are two methods for measuring soil resistivity:
o Two-point method.
o Four-point method.
CONCLUSION:
-
The grounding resistance can be decreased by:
o Treating the soil by adding salt and water.
o Increasing the rod diameter.
o Increasing the rod depth.
o Making parallel combination of the rods.
-
The grounding resistance of the HV lab is about 0.3Ω.
The bonding resistance is considered “Good connection” if it is low.