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FACULTY OF ENGINEERING
LAB SHEET
EEL 3146
ELECTRIC POWER UTILIZATION AND
INSTALLATION
EIW 1 -Earth Resistance Measurement
EIW 2 -Design of Electrical Power Distribution Using Single Line
Diagram
*Note: On-the-spot evaluation is carried out during or at the end of the experiment.
Students are advised to read through this lab sheet before doing experiment. Your
performance, teamwork effort, and learning attitude will count towards the marks.
EEL 3146 Electric Power Utilization and Installation
Modified Oct 2013
EIW 1: Earth Resistance Measurement
Objectives
This experiment introduces the concept of the grounding system for Earth Resistance
measurement. The objectives of this experiment are:
 To examine different grounding methods.
 To evaluate the ground resistance using different grounding methods at various
locations.
Introduction
System grounding has been used since electrical power system began. However, many
companies and industrial plants have used system-grounding methods differently.
Grounding of many systems has been based upon past experience rather than engineering
analysis. This experiment explains how to measure ground resistance in order to maintain
the grounding system.
1.
Methods used for Ground Resistance Measurements
To maintain sufficiently low resistance values of grounding systems, their periodic testing
is required. The testing involves measurement to ensure that they do not exceed design
limits. The methods of measuring and testing the ground resistance are as follows:
 Two-point method
 Three-point method
 Fall of potential method
 Ratio method
 Four-point method
 Touch potential measurements
The measurement of ground resistances may only be accomplished with specially designed
test equipment. The most common method of measuring ground resistance uses the fall of
potential principle of alternating current of 50 Hz or some higher frequency circulating
between an auxiliary electrode and the ground electrode under the test. The reading will be
given in ohms and it represents the resistance of the ground electrode to the surrounding
earth.
1.1
Two-Point Method
This method may be used to measure the resistance of a single driven ground rod. It uses
an auxiliary ground rod whose resistance is either known or can be measured. The
resistance value of the auxiliary ground rod also must be very small compared to the
resistance of the driven ground rod so that the measured value can be assumed to be wholly
contributed by the ground rod. An example of where this test might be applicable is the
measurement of resistance of the single driven ground rod for a resistance or in congested
areas where finding room to drive the two auxiliary rods may be a problem. In this case the
municipal metallic water supply fine can be assumed as the auxiliary ground rod whose
resistance value is approximately 1 ohm or less. This value is quite small compared to the
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EEL 3146 Electric Power Utilization and Installation
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value of a single driven ground rod, whose value is in the order of 25 ohms. The reading
obtained is that of the two grounds in series. The lead resistances will also be measured
and should be deducted from the final measurements.
Two-Point Ground Resistance Measurement Method
1.2
Three-Point Method
This method is similar to the two-point method except it uses two auxiliary rods. To obtain
accurate values of resistance measurements, the resistance of the auxiliary electrodes
should be approximately equal to or less than that of the electrode under test.
The connections for the three-point method are shown in figure below.
Either alternating current of 50 Hz or direct current may be used for making this test. The
advantage of using alternating current is that it minimizes the effects of stray current on the
measurement readings. However, if stray current happen to be of the same frequency, error
will be introduced in the readings. The use of direct current for making this test will totally
eliminate the ac stray currents. However, stray direct current and formation of gas around
the electrode will introduce error in the readings when using direct current for this test. The
average of two readings will give an accurate test value. Apply currents only long enough
to take readings.
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EEL 3146 Electric Power Utilization and Installation
Three point test method and its equivalent circuit
The resistance value of the test electrode can be calculated as follows:
Let
Solving these three equations we have
Ry = R3 - Rz
Rx = R1 – Ry = R1 – R3 + Rz
Also Rx = R2 - Rz
From which
2 Rz =R1+R2 – R3 or Rx = (R1 + R2 – R3)/2
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1.3
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 potential rod) that are placed at a sufficient distance from the test electrodes: a
current of known magnitude is passed through the electrode under the 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 the voltage 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 conducting this test.
The following problems and errors may be encountered with this method:
Stray currents in earth may cause voltmeter readings to be either high or low. The
resistance of auxiliary electrode and electrical leads may introduce errors in the voltmeter
readings. This error can be minimized by using a voltmeter of high impedance value.
This method can be used with either a separate voltmeter and ammeter or a single
instrument which provides a reading directly in ohm. To measure the resistance of a
grounding electrode, a current electrode is placed at a suitable distance from the grounding
electrode under test as shown in figure. The potential difference between rods X and Y is
measured by a volt meter, and the current flow between rods X and Z is measured by an
ammeter. (Note: X, Y, and Z may be referred to as X, P , and C in a three point tester or
C1, P2 and C2 in a four point tester)
By ohms law E = R I or R = EA, we may obtain the ground electrode resistance R. If E=20
V, and I=1 A, then
R= E/I =20/1 = 20 Ohm.
Fall of potential method Ground resistance Instrument
2.
Equipment for measurement
2.1
Megger DET5/4R
The megger is used for testing or measuring earth resistance. The main features and the
controls of the instrument are shown in Figures below.
The instrument uses the four-terminal method of measurement. A reversing dc test current
is injected into the earth through terminals C1 and C2. The potential developed across the
earth is monitored with P1 and P2. A three-pole test is achieved by shorting terminals C1
and P1. At the beginning of test the control logic initiates a potential spike resistance
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check, monitoring the input over-range detector for the result. The input auto-ranges by the
control logic monitoring the output of the over range detector and switching the current
source to a lower range output. The instrument also auto-ranges if the high current loop
resistance for the present range.
Megger DET5/4R device
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The instrument measuring circuitry is connected to terminals P1 and P2. The voltage
limiter and input buffer prevent damage to the instrument and loading of the resistance
under test. Synchronous filtering and detection are used to recover the test signal from
noisy environments followed by filtering and conversion to a reading by the digital panel
meter.
The test signal frequency is produced by dividing the frequency of a crystal oscillator.
This is then passed through logic circuitry to produce the waveforms for synchronous
filtering and detection.
3.
Procedure
3.1
Position of Auxiliary Electrode on Measurement
The goal in precisely measuring the resistance to ground is to place the auxiliary current
electrode Z far enough from the ground electrode under test so that the auxiliary potential
electrode Y will be outside of the effective resistance areas (effective cylinder of earth) of
both ground electrode and the auxiliary current electrode. The best way to find out if the
auxiliary potential and Y is outside the effective resistance areas is to move it between X
and Z and to take a reading at each location. If the auxiliary potential rod Y is an effective
resistance area by displacing it the readings taken will vary noticeably in value. Under
these conditions, no exact value for resistance to ground may be determined.
Fall of potential method
On the other hand, the auxiliary potential rod Y is located outside the effective resistance
areas, as Y is moved back and forth the reading variation is minimal. The reading taken
will be relatively close to each other and are the best values for the resistance to ground of
the ground X.
.
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3.2
Fall-of-potential Method Using DET5/4R Megger
The connection diagrams are shown below. Insert the current test spike into the ground
some 30 to 50 meters away from the earth electrode to be tested.
Fall-of-potential method
Fall-of-potential method using a single lead to the earth electrode
Firmly connect this spike to the instrument terminal C2. Insert the potential test spike into
the ground midway between the current test spike and the earth electrode. Firmly connect
this spike to instrument terminal P2. All the three spikes should be in a straight line. Firmly
connect C1 and P1 instrument terminals to the earth electrode.
3.3
Three -Terminal Measurement
The procedure is same as for the four-terminal measurement except that the 3-pole pushbutton is used to operate the instrument. In both the methods take readings with potential
spike in the original location and also 3 meters away on either side. The average of these 3
readings is the earth resistance.
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3.4
Four -Terminal Measurement
Press and hold the 4-pole push button. After a short pause this will begin the test sequence
and include spike resistance and noise verification.
Check that adverse conditions like Rp, Rc and 'noise' LED'S with their alert warning
indicator are not showing. After a few seconds the display will stabilize. The reading given
on the display is the earth resistance. The push-button may be released at any time after the
instrument has commenced its test sequence. To switch the instrument 'off’ before the test
period has elapsed, press and immediately release either of the two push-buttons.
Exercises
1. Analyse the Earth Resistance measurement by using any of the above mentioned
methods for two different soil conditions.
2. Explain the nature of variation of earth resistance with distance between earth
electrodes.
3. Explain the factors affecting earth resistance of an earth electrode.
4. What can you conclude from this experiment?
(Submit report in hard copy)
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EEL 3146 Electric Power Utilization and Installation
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EIW 2: Design of Electrical Power Distribution Using Single Line
Diagram
Objectives
This experiment introduces the concept of Electrical Design in power distribution system.
The objectives of this experiment are:
 To examine the electrical power distribution of a power system.
 To construct the single line drawings using VISI0 software.
Introduction
Electrical distribution of power using single line diagram is being practiced in industries
and other places as a standard method for design, installation and trouble shooting.
Standard symbols are used to represent various elements in electrical installations. From a
given single line diagram, information are gathered and interpreted for further extending
the power to other places based on its capacity to carry current etc. Various tools are
available to draw the single line diagrams. In this experiment you will be using VISIO
software to draw single line diagram for electrical power distribution.
Electrical Design
Before starting to do the electrical design, information are to be gathered about loads to be
energized and the electrical supply availability regarding voltage, phase, and frequency.
Based on the details of load and supply system details, various data such as number of
circuits, load current, cable/wire capacity etc., are worked out and design will be made
using top down approach. Standard technical tables are available to know the current
carrying capacity of various conductors and number of wires that can be taken together in
conduit pipe etc. Some useful information used for design calculation can be seen from
Table l. More details regarding the cable size to be used, back up HRC fuse size, starter
relay ratings and full load Amps of various motors are given in Table 2.
1.
Procedure
1.1
Distribution of Power: Example 1
Consider two Motors of 5HP/3.75kW, 3-Phase, 415V, 50 Hz and one motor of
3HP/2.25kW, 3 Phase, 415V, 50Hz are to be connected and powered in industry where the
electrical supply system of 4l5V, 3-Phase, 50Hz is available. Design suitable electrical
distribution for connecting the motors.
From the Table 2;
The Full load current of a 5 HP/3.75 kW motor is 7.5 Amps.
If DOL starter is used, the O/L relay scale required is 6 - 10 Amps.
Typical back-up HRC fuse (Amp) required is 25 Amps.
Typical cable size required is – 2.5 mm2 Aluminum.
The Full load current of a 3 HP/225 kW motor is 5 Amps.
If DOL starter is used, the O/L relay scale required is 4 - 6.5 Amps.
Typical back-up HRC fuse (Amp) required is 20 Amps.
Typical cable size required is - 1.5/2.5 mm2 Aluminum.
Hence the total current to be carried from the main feeder, IL = 7.5 x 2 + 5 = 20 A.
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Considering a factor of safety for momentary overload and short circuits as 1.5 times, the
total current to be carried by main feeder is 30 Amps. The main feeder has to supply this
current through a MCB/TP&N switch having a capacity of 60 Amps with de-rated fuse of
30A. From Table 2, for carrying a current of 30A, 4-core 10 mm2 aluminum cable is to be
used. The single diagram with all the above details for such system is shown below.
Single line diagram
1.2
Distribution of Power: Example 2
Design suitable electrical power distribution using single line diagram for a house having 4
rooms with each room having 2 lights of 40W (each) and a fan of 60W. It also has one airconditioner of 2.5 kW-1 Ph, Water pump of l HP/0.75 kW-1 Ph and Heater of capacity 1.5
kW- 230V. The supply system available is 230 V-1 Phase - 50 Hz.
Considering one circuit line for each room,
Total lighting loads = [(2 x 40 + 60) x 4] /1000 = 0.56 kW or 0.14 kW/ circuit.
Current/ circuit = 0.6 A. Total power load = 2.5 + 0.75 + 1.5 = 4.75 kW.
Total load = 4.75 + 0.56 = 5.31 kW, (round off to 6 kW).
Total current = 6000/230 = 26 A.
Referring the Table 1 for various conductor sizes for rooms and power loads, a single line
diagram is drawn for the house showing the distribution of electrical power.
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EEL 3146 Electric Power Utilization and Installation
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You start VISIO software and open Electrical & electronic template. The main window
opens with a drawing page and one or more stencils.
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A - Tools on the toolbars are grouped in functions sets, such as the standard and text
toolbars. You can choose which toolbars you want to display, saving space on screen.
B - By default, stencils dock to the left of the drawing page. To bring a stencil to the front,
click that stencil's title bar which may be at the top or bottom of the stencil.
C - The drawing page opens with size, orientation, scale, grid and measurement
appropriate for the drawing page.
D - Stencils contain master shapes that you can drag to the drawing page.
F - The drawing page has a drawing grid, which you can use to align shapes.
1.3
Using VISIO software to create single line diagram
The VISIO Technical software version 5.0 is designed for engineers to produce several
engineering applications. It has got 2500 technical smart shape symbols to represent real
world objects.
In general steps you take in Visio to create a drawing:
1. Start a drawing by opening a template
A template sets up your drawing page and opens stencils that contain master shapes.
2. Add shapes to your new drawings by dragging them from a stencil to the drawing page
or drawing your own.
You can create additional shapes with the Visio drawing toots on the Standard toolbar, and
open additional stencils to choose from a wider variety of shapes.
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3. Arrange shapes to create the drawing you want.
On screen, the grid helps you align shapes, and the rulers give you precise control.
You can place guides and use the Align and Distribute commands for quick alignments, or
use the size & Position command to enter exact co-ordinates and dimensions.
4. Glue shapes together, so they stay together when you move them.
With glue, you can update your diagrams quickly, without selecting and dragging every
line.
5. Add text to your drawing.
You can add text to a shape by clicking a shape with the pointer tool, then typing. Or use
the text tool to type anywhere on a drawing.
6. Modify the look of shapes.
Formatting shapes is quick with the tools and style lists on the toolbars and the commands
on the Format menu. You can define your own style in addition to the ones that come with
the product.
1.4
Basing new drawings on template
You can quickly create the type of drawing you want when you start with a Visio template
(*.vst) file. A template includes everything you need to create a drawing, from the drawing
page to shapes and styles. Using template ensures consistency across your drawing files, so
you can focus on what goes on the page while the templates takes care of the rest.
When you start a drawing with a template, a Visio file opens that contains the following:




One or more stencils containing related shapes.
A blank drawing page using grid and measurement system that's appropriate for the
type of drawing you're creating, saving you the time it takes to set up pages.
A drawing page set up with the correct scale and page size. This applies to scaled
drawing types.
Styles for text, lines, and fill appropriate to the type of drawing you're creating,
saving you the time to takes t o define your own styles.
1.5
Saving drawing files
When you work on a drawing, you should save the drawing file frequently. When you
change a stencil or template, you should also save these files. You save a Visio drawing
file (*.vsd) under proper folder. Save the workspace when you save a drawing file. When
you next open the file, the stencil and drawing page windows will look just as they did
when you saved the file. You can save a workspace as part of a drawing, stencil, or
template (*.vst), or a separate file (*.vsw).
For more details visit Web page at http:lluurw.visio.com/support/visio_index.html
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Table -1
(Based on table 4DlA of IEE regulations)
Current carrying capacity (Amperes)
Ambient temperature 30º C
Conductor operating temperature 70º C
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Table -2
DOL starter details for Motors, 415 V, 3-phase, 50 Hz
Exercises
1. Design and construct the electrical power distribution single line diagram for the power
system that connecting the following: 3 Phase, 415 V, 50Hz motors using DOL starter and
a furnace. The supply system available is 3-Phase, 415 V, and 50Hz.
7.5 HP motor -2 peices, 2 HP motor -2 peices, 5kW furnace -1 peice.
2. Design and construct electrical power distribution single diagram for a house having the
following loads. The supply system available is Single Phase, 230 V, 50 Hz., 5 Rooms with each room having 4 lights 40W-each and 2 fans 60W-each, Air-conditioners 2.5 kW 2 peices, Heaters 1.5 kW-2 peices, Water pump 1HP-1 peice.
3. What can you conclude from this experiment?
(Answers to be submitted in hard copy using VISIO software)
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