Electrical Engg

1. Page No.1
Electrical Maintenance Lab Manual
Karnataka German Technical Training Institute
Gulbarga

2. Page No.2
Read the instructions carefully and follow up before you start working on the Electrical Lab
Safety Rules and Guidelines
General Rules
1. No food or beverages in lab.
2. "Horseplay"' is hazardous and will not be tolerated.
3. No tools, supplies, or any other items may be tossed from one person to another.
4. All laboratory aisles and exits must remain clear and unblocked.
5. Do not plug in or unplug electrical devices in the lab.
6. Do not tamper with electrical outlets or other installations in the lab. _
7. Do not move the equipment in the lab.
8. All electrical devices must be grounded before they are turned on. Ungrounded wiring and two-Wire '
extension cords are prohibited. Worn or frayed extension cords or those with broken connections or
used wiring must not be used.
9. Keep hands dry and secure loose items of jewellery.
10. No deviation from approved equipment operating procedures is permitted.
11. Breakage or malfunction of equipment must be reported to the lab instructor immediately.
12. No student may work alone in the laboratory at any time.
13. Always turn off the equipment before you leave the lab.
14. All equipment must be neatly returned and each workstation must be cleaned after the completion of
the lab experiment.
15. Casual visitors to the laboratory are to be discouraged and must have permission from the laboratory
director to enter.
16. Make sure to turn off the light and lock the door if you are the last one to leave.
Electrical Hazards .
Electricity is dangerous in that electric shocks can cause serious injuries and even death.
Electric shock is the main enemy in our lab, therefore it is important to understand the effects of
electrical currents on the human body and develop habits that will minimize the likelihood of an
electric shock.
Shock occurs when the body becomes a part of the circuit, with current entering the body
at one point and leaving at another. Shock normally occurs in one of three ways. The person must
come into contact with:
1. Both wires of the electrical circuit,
2. One wire of an energized circuit and the ground, or
3. A metallic part that has become "hot" by being in contact with an energized wire while the person
is also in contact with the ground.
The severity of the shock received when a" person becomes a part of an electrical circuit is
affected by three primary factors:
1. the amount of current flowing through the body (measured in amperes),
2. the path of the current through the body, and
3. The length of time the body is in the circuit.
Other factors that may affect the severity of shock are the frequency of the current, the
phase of the heart cycle when shock occurs, and the general health of the person prior to shock. The
effects from electric shock depend on the type of circuit and its voltage, resistance, amperage, and
pathway through the body and duration of the contact. Effects can range from a barely perceptible tingle
to immediate cardiac arrest.

3. Page No.3
Larger currents cause contractions of muscles close to the current path. A difference of
less than 100 milliamperes (mA) exists between a current that is barely perceptible and one that can kill.
Muscular contraction caused by electrical stimulation may not allow victims to free themselves from the
circuit. The current causing that condition is called freezing current or "let-go" current and it may be as
small as 6 mA for Women and 9 mA for men at the frequency of 60 Hz. Furthermore, increased duration of
exposure increases the danger. For example, a current of 100 mA for 3 seconds is equivalent to a current of
900 mA applied for 0.03 seconds in causing‘ fibrillation. The so-called low voltages can be extremely
dangerous, because the degree of injury is proportional to the length of time a body is in a circuit.
LOW VOLTAGE DOES NOT MEAN LOW HAZARD!
A particularly dangerous situation arises when the current path is through the chest cavity.
That occurs when the current enters the body through one hand and leaves it through the other one or
when the current enters a hand and leaves the foot. Currents in excess of l0 mA flowing through the chest
cavity can cause respiratory inhibition, ventricular fibrillation (loss of the rhythmic contractions of the
heart), and cardiac arrest. Higher currents can damage the nervous system and cause burns.
The body resistance is due primarily to the skin resistance (internal body resistance is less
than 1 kW ). Skin resistance depends on the surface moisture and pressure exerted. Dry skin has a
resistance of few MW . Moisture (e. g. due to perspiration) lowers that value significantly. Fingers
immersed in water bring the body resistance down to few kW . Metal items like jewellery further reduce
the resistance of the current path.
Electric current can flow only if a circuit is complete. In 120-volt distribution systems one
wire is the current source and the other is the ground wire. If a person or object contacts the circuit or
current path and forms an alternate route to ground, current will flow through the alternate path. All 120-
volt distribution systems have a separate wire intended to form a low-resistance, alternate route to
ground. The bodies of electrical devices are attached to this ground wire to ensure that inadvertent
contact between the circuit wires and the body of the devices do not create an electrical hazard for users.
Circuit ground and safety ground wires must be attached - to approved electrical grounding systems.
Electrical systems and equipment must meet local and national electrical codes and OSHA (Occupational
Safety and Health Administration) standards.
When working with electrical systems, the best protection is to turn off the electrical
power. If that is not possible, it is imperative to minimize the current that may accidentally occur and to
assure that the current path is not through the chest cavity. Rubber-soled shoes and a non-conducting dry
floor are desirable for eliminating the current path from a hand to the feet. Whenever working with
circuits under voltage, only one hand should be used, the other one should be kept in a pocket or behind
the back. This precaution prevents a person from touching parts of the circuit with two hands and thus
eliminates the possible current path through the chest cavity. A conscious effort is necessary since the
tendency to use both hands is very strong. It is also good practice to remove jewellery when working with
electrical circuits.
Thermal Hazards:
Take care when hot water is involved in the experiment. Make sure not to spill the hot
water and put the hot water bath in a safe position.

4. Page No.4
1. ELECTRICITY THEORY
1.1.What is Electricity?
Electricity is a general term that encompasses a variety of phenomena resulting from the
presence and flow of electric charge. These include many easy recognisable phenomena such as lightning
and static electricity, but in addition, less familiar concepts such as the electromagnetic field and
electromagnetic induction.
Figure 1.1
In general usage, the word 'electricity' is adequate to refer to a number of physical effects. However, in
scientific usage, the term is vague, and these related, but distinct, concepts are better identified by more
precise terms:
1. Electric charge : A property of some subatomic particles, which determines their electromagnetic
interactions. Electrically charged matter is influenced by, and produces, electromagnetic fields.
2. Electric current : A movement or flow of electrically charged particles, typically measured in
amperes.
3. Electric field : An influence produced by an electric charge on other charges in its vicinity.
4. Electric potential : The capacity of an electric field to do work, typically measured in volts.
5. Electromagnetism : A fundamental interaction between the electric field and the presence and
motion of electric charge.
1.2.Some facts about Electricity : How Electricity made?
Have you ever wondered where electricity comes from? You might be surprised to learn
that it comes from magnets.
In the early 1800s, Michael Faraday discovered “Electromagnetic Induction” – the
scientific way of saying that if he moved a magnet through a loop of wire, the wire would become
electrified.
In 1882, Thomas Edison opened the first full-scale power plant in New York City. Edison’s
electric generator was a bigger version of Faraday’s basic experiment – a big magnet rotates around a wire
to produce an electric current.

5. Page No.5
Today’s power plants are bigger and controlled by computers, but the basic process is still
the same as it was nearly 120 years ago.
1.3.Here’s how power plants make Electricity : Some examples to show how Power Plant make
electricity
Figure 1. 2 Coal Figure 1.3 Figure 1.4 Figure 1.5
dug up and sen The Trains The coal Inside the
trains and boats And Boats is burned Generator,
Deliver The to heat The steam
coal to the water to spins a big
Power make fan called a
Plant. steam. turbine.
The spinning turbine rotates a big magnet around a length of wire, creating a magnetic
field that electrifies the wire. The electric current flows through the wire and is pushed out through high-
voltage transformers.
1.4.More ways to develop Electricity :
Instead of using coal, some power plants use other ways to make Electricity :
Figure 1.6 Some Figure 1.7 Figure 1.8 Figure 1.9
power plants A Nuclear A wind farm A Hydro
burn natural Power Plant uses the wind PowerPlant
gas instead of splits Apart to spin the uses running
coal to make uranium To blades of the Or falling
Steam. release Heat turbine. water to spin
energy. the turbine.
1.5.How electricity gets to your home :
It's always there whenever you flip a switch or plug in a cord - but electricity has to travel a
long way to get to your house. In fact, the power plant where your electricity is made might be hundreds of
miles away.
All the poles and wires you see along the highway and in front of your house are called the Electrical
Transmission and Distribution System. Using hydroelectric power from Niagara Falls, scientist Nikola Tesla
designed the first full-scale electric system in the early 1900s.
Today, power plants all across the country are connected to each other hrough the electrical system
(sometimes called the “Power Grid”). If one power plant can’t produce enough electricity to run all the air
conditioners when it’s hot, another power plant can send some where it’s needed.

6. Page No.6
Electricity is made at a Power Plant by huge generators. Most Power plants use coal, but
some use natural gas, water or even wind.
Figure 1.10
The current is sent through Transformers to increase the voltage to push the power
long distances.
Figure 1.11
The electrical charge goes through high-voltage Transmission Lines that stretch across
the country.
Figure 1.12
It reaches a Substation, where the voltage is lowered so it can be sent on smaller power
lines.
Figure 1.13
It travels through distribution lines to your neighbourhood, where smaller pole- top
transformers reduce the voltage again to take the power safe to use in our homes.
Figure 1.14
It connects to your house through the service drop and passes through a meter that
measures how much our family uses.
Figure 1. 15
The electricity goes to the service panel in your basement or garage, where breakers or
fuses protect the wires inside your house from being overloaded.
Figure 1.16
The electricity travels through wires inside the walls to the outlets and switches all over
your house.
Figure 1.17

7. Page No.7
1.6.How we use electricity?
Can you name all the ways you use electricity at home? We use electricity almost every
minute from the time we get up in the morning until we go to bed at night.
Take a look at all the things we depend on each day that need electricity:
In the kitchen :
Figure 1.18 Refrigerators Figure 1.19 Dishwashers Figure 1.20 Stoves
In the family room :
Figure 1.21 Lamps Figure 1.22 Computer Figure 1.23 Air conditioning
In the basement or utility room:
Figure 1.24 Washer and Dryer Figure 1.25 Furnace Figure 1.26 Water heater
Outdoors:
Figure 1.27 Outdoor Figure 1.28 Figure 1.29 Pool heater
lighting Electric
lawn mower

8. Page No.8
1.7.Fun facts about electricity :
• Electricity travels at the speed of light - more than 186,000 miles per second.
• A spark of static electricity can measure up to three thousand (3,000) volts.
Figure 1.30
 A bolt of lightning can measure up to three million (3,000,000) volts – and it lasts less than one second.
 Electricity always tries to find the easiest path to the ground.
 Electricity can be made from wind, water, the sun and even animal manure.
 Burning coal is the most common way to produce electricity.
 One power plant can produce enough electricity for 180,000 homes.
 The first power plant – owned by Thomas Edison – opened in New York City in 1882.
 Thomas Edison didn’t invent the first light bulb – but he did invent one that stayed lit for more than a
few seconds.
 Thomas Edison invented more than 2,000 new products, including almost everything needed for us to
use electricity in our homes: switches, fuses, sockets and meters.
 Benjamin Franklin didn’t discover electricity – but he did prove that lightning is a form of electrical
energy.
1.8.Why Safety is necessary?
Safety is the job of each individual. You should be concerned not only with your own safety
but with the safety of others around you. This is especially true for persons employed in the electrical field.
Some general rules should be followed when working with electric equipment or circuits. We use
electricity to enhance our lifestyles. We use electricity to carry out a large range of everyday activities such
as cooking, cleaning, heating and lighting our homes, as well as running industry to manufacture goods for
our use and for export. Electricity is clean, quiet and invisible. The fact that it is invisible means, we often
take it for granted and its inherent dangers are not always immediately apparent. Every year in the world,
electricity kills or seriously injures a considerable number of people. Most of these injuries occur through a
lack of knowledge and understanding about electricity and its dangers.
General Safety Rules :
Figure 1.31

9. Page No.9
Confined Spaces :
A confined space is any space having a limited means of Entrance or Exit. These work
places can be very hazardous often containing atmosphere that are extremely harmful or deadly. Confined
spaces are very difficult to ventilate because of their limited openings. It is often necessary for a workman
to wear special clothing and use a different air supply to work there.
Some general rules are including the Following :
 Have a person stationed outside the confined space to watch a person or persons working inside the
outside person should stay in voice or visual contact with the inside workers at all times. He should
check air sample readings and monitor oxygen and explosive gas levels.
 The outside person should never enter the space, even in an emergency, but should contact the
proper emergency personnel. If he or she enters the space and becomes incapacitated, there would
be no one to call for help.
 Use only electric equipment and tools that are approved for the atmosphere found inside the confined
area. It may be necessary to obtain a burning permit to operate a tools that have open brushes and
that spark when they are operated.
 As a general rule, a person working in a confined space should wear a harness with a lanyard that
extends to the outside person, so the outside person could pull him or her to safety if necessary.
1.9.Lockout and Tagout Procedure :
Figure 1.32
Lockout and Tagout procedures are generally employed to prevent someone from
energizing a piece of equipment by mistake. This could apply to switches, circuit breakers, or valves. Most
of the companies have their own internal policies and procedures. A safety precaution tag should be
placed on the piece of equipment being serviced; some also require that the equipment be locked out with
padlock.
The person performing the work places the lock on the equipment and keeps the key in
his or her possession. Violating lockout and tagout procedure is considered an extremely serious offense in
most industries and often results in immediate termination of employment, as a general rule, there are no
first time warnings.
After locking out and tagging a piece of equipment, it should be tested to make certain
that it is truly de-energized before working on it. A simple three step procedure is generally recommended
for making certain that a piece of electric equipment is de-energized. A voltmeter that has a high enough
range to safely test the voltage is employed.
1. Test the voltmeter on a known energized circuit to make certain the voltmeter is working properly.
2. Test the circuit you intend to work on with the voltmeter to make sure that it is truly de-energized.
3. Test the voltmeter on a known energizes circuit to make sure that the voltmeter is still working
properly.

10. Page No.10
1.10.Protective clothing :
Maintenance and construction workers alike are usually required to wear certain article of
protective clothing, dictated by the environment of work area and the job being performed.
1.11.Head protection :
Some type of head protection is required on almost any work site. A typical electrician’s
hard hat, made of non-conducting plastic, is shown in Figure 32.
Figure 1.33
1.12.Eye protection :
Eye protection is another piece of safety gear required on almost all work sites. Eye
protection can come in different forms. Common safety glasses may or may not be prescription glasses,
but almost all provide side protection. Sometime a full face shield may be required.
Figure 1.34
1.13.Hearing protection :
The need of hearing protection is based on the ambient sound level of the work site or the
industrial location. Workers are usually required to wear some type of hearing protection when working in
a certain areas, usually in the form of earplugs & earmuffs.
Figure 1.35
1.14.Never work on an energized circuit :
When possible use the following three-step check to make certain that power is turned off :
1. Test the meter on a known live circuit to make sure that the meter is operating.
2. Test the circuit that is to become the de-energized circuit with the meter.
3. Test the meter on a known live circuit again to make certain the meter is still operating.
Install a warning tag at the point of disconnection so the people will not restore power to
the circuit. If possible, use a lock to prevent anyone from tuning the power back on.

11. Page No.11
1.15.Think :
Think of all the ruled concerning safety, this one is probably most important. No amount
of safeguarding or idiot proofing a piece of equipment can protect a person as well as taking time to think
before acting. Many technicians have been killed by supposedly “dead” circuits. Do not depend on circuit
beakers, fuses or someone else to open the circuit. Test it yourself before you touch it. If you are working
on high voltage equipment, use insulated gloves and meter probes to measure the voltage being tested.
Think before you touch something that could cast your life.
1.16.Avoid horseplay :
Jokes and horseplay have a time and place but not when someone is working on electric
circuit or a piece of moving machinery. Do not be the cause of someone’s injured or killed or do not let
someone else be the cause to your being injured or killed.
1.17.Do not work alone :
This is especially true when working on a hazardous location or on a live circuit. Have
someone with you who can turn off the power or give artificial respiration. Several electric shocks can
cause breathing difficulties and can cause the heart to go into fibrillation.
1.18.Work with one hand when possible :
The worst kind of electric shock occurs when the current path is from one hand to other,
which permits the current to pass directly through the heart. A person can survive a severe shock between
the hand and foot that would cause death if the current path was from one hand to other.
1.19.Learn first aid :
Anyone working on electronic equipment, especially those working with voltages greater
than 50 volts should make an effort to learn first aid. A knowledge of first aid especially CPR
(Cardiopulmonary Resuscitation), may save your own or someone else’s life.
1.20.Avoid alcohol and drugs :
The use of alcohol and drugs has no place on a work site. Alcohol and drugs are not only
dangerous users and those who work around them; they also cost industry million of dollars a year. Alcohol
and drugs abusers kill thousand of people on the highway every year and are just as dangerous on a work
site as they are behind the wheel of a vehicle.
This part of manual is designed to increase student awareness of electricity and its uses
and dangers in their everyday, immediate environment – their homes.

12. Page No.12
2.HAND TOOL SAFETY INSTRUCTIONS
1. Select the right tool for the job, and use the tool correctly. The correct way is the way the teacher will
demonstrate to you.
2. Place tools and materials not being used in the center of the work table. Treat all tools and equipment
with respect. Do not shorten their useful life or yours through careless handling.
3. Never carry sharp tools in the pockets and when carrying tools in the hands, keep the cutting edge or
sharp point directed toward the floor.
4. Do not swing the hands excessively when carrying sharp tools and pass tools, handle first, to the person
who is to receive it.
5. Make sure tools are in good condition and sharp at all times when using them. Dull tools are dangerous.
6. Never use a hammer with a loose head or splintered handle. Do not tape any split handle
7. Always use screwdrivers of the correct size and shape for the job. Always use screwdrivers with the
blade in good condition.
8. Clamp small work on a bench or secure it in a vise when using a gouge, wood chisel, or screwdriver; but
never increase the vise jaw pressure by using a mallet or hammer.
9. Be sure your hands are free of dirt, grease, and oil when using tools
10. Control chisels, gouges and carving tools with one hand and allow the other hand to supply the power.
Always keep both hands behind the cutting edge and cut away from the body.
11. Wear goggles or face shield when chipping or cutting with any tool. Arrange the work so that
classmates are protected from flying chips.
12. Always keep punches and chisels ground off smoothly on the ends. Mushroomed ends may allow steel
to fly when hit. Report to the teacher any tools that are broken or appear to be defective.
13. Always use a screwdriver with an insulated handle when working around electricity, and never use a
screwdriver for a punch, chisel or other use it was not intended for.
14. Files should be checked for properly fitted handles in good condition.
15. Clear the work area before hand sawing. Make sure that the material to be sawed is well supported.
16. When using adjustable wrenches, pull toward the movable jaw. The pressure will then be against the
stationary jaw.
17. Saws will jump out of the cut when improperly engaged or forced. Start cut on the back stroke by
guiding the blade with your thumb. Move free hand away after cut is started.

13. Page No.13
3.METERS THEORY
As far as the measurement of electrical parameters such as Current, Voltage and Power is
concerned, the first thing that comes in mind is about measuring instruments. Electric measuring
instruments and meters are used to indicate directly the value of voltage, current, energy or power. There
are variety of measuring instruments available in market. The digital instruments use electronic circuitry,
where as analog instruments have an electromechanical arrangement (i.e. Input is an electrical signal
results mechanical force or torque as an output). This arrangement can be connected with suitable
components to act as an Ammeter or a Voltmeter. Our aim is to be familiar with the analog measuring
instruments and their principle of operation. The analog measuring instruments are classified, in following
types:
Permanent Magnet Moving Coil Instruments.
Moving Iron or Iron Vane Instruments.
Dynamometer type Instruments.
The instruments can be calibrated to measure quantities like current, voltage, power and
many more. The Voltmeter, Ammeter and Wattmeter are generally used to measure Voltage, Current and
Power of any electrical or electronics circuit respectively. Before we deal with the classifications of
measuring instruments, firstly take a view of the terminologies used in the description.
1.Deflecting torque/force:
The deflection of any instrument is determined by the combined effect of the deflecting
torque/force, control torque/force and damping torque/force. The value of deflecting torque must depend
on the electrical signal to be measured.
2.Controlling torque/force:
This torque/force must act in the opposite sense to the deflecting torque/force, and the
movement will take up an equilibrium or definite position when the deflecting and controlling torque are
equal in magnitude. Spiral springs or gravity usually provides the controlling torque.
3.Damping torque/force:
A damping force is required to act in a direction opposite to the movement of the moving
system. This brings the moving system to rest at the deflected position reasonably quickly without any
oscillation or very small oscillation. This is provided by i) air friction ii) fluid friction iii) eddy current. It
should be pointed out that any damping force shall not influence the steady state deflection produced by a
given deflecting force or torque.
4.Permanent Magnet Moving Coil (P.M.M.C.) Instruments:
A moving coil instrument consists basically a permanent magnet to provide a magnetic
field and a small lightweight coil is wound on a cylindrical soft iron core that is free to rotate around its
vertical axis. When a current is passed through the coil windings, a torque is developed on the coil by the
interaction of the magnetic field and the field set up by the current in the coil.
Aluminum pointer is attached to rotating coil and the pointer moves around the calibrated
scale indicates the deflection of the coil. To reduce parallax error a mirror is usually placed along with the
scale. A balance weight is also attached to the pointer to counteract its weight.
Hairspring is provided so as to return the coil to its original position in no current
conditions.

14. Page No.14
Hairspring not only supply a restoring torque but also provide an electric connection to
the rotating coil. With the use of hairsprings, the coil will return to its initial position when no current is
flowing though the coil. The springs will also resist the movement of coil when there is current through
coil. When the developing force between the magnetic fields (from permanent magnet and electro
magnet) is exactly equal to the force of the springs, the coil rotation will stop. The coil set up is supported
on jeweled bearings in order to achieve free movement.
Two other features are considered to increase the accuracy and efficiency of this meter
movement. First, an iron core is placed inside the coil to concentrate the magnetic fields. Second, the
curved pole faces ensure the turning force on the coil increases as the current increases.
The Permanent Magnet Moving Coil Instruments are used to measures only D.C. As it is
known that the average value of full wave rectifier current is 0.637 times actual current.
Principle of operation:
It has been mentioned that the interaction between the induced field and the field
produced by the permanent magnet causes a deflecting torque, which results in rotation of the coil.
Advantages:
The scale is uniformly divided (steady state θ = (G/C) Is).
The power consumption can be made very low (25μW to 200μW).
The torque-weight ratio can be made high with a view to achieve high accuracy.
A single instrument can be used for multi-range ammeters and voltmeters.
Error due to stray magnetic field is very small.
Figure 3.1
Limitations:
Suitable for direct current only High cost
Variation of magnet strength with time.
Errors:
Frictional error Magnetic decay
Thermo electric error
Temperature error.

15. Page No.15
Errors can be reduced by following the steps given below:
Proper pivoting and balancing weight may reduce the frictional error.
Use of resistance in series can nullify the effect of variation of resistance of the instrument circuit due
to temperature variation.
The stiffness of spring, permeability of magnetic core decreases with increases in temperature.
5.Moving Iron (M.I.) Instruments:
The deflecting torque in any moving-iron instrument is due to forces on a small piece of
magnetically ‘soft’ iron that is magnetized by a coil carrying the operating current.
Repulsion type M.I. instrument consists of two cylindrical soft iron vanes mounted within a fixed current-
carrying coil. One iron vane is held fixed to the coil frame and other is free to rotate, carrying with it the
pointer shaft. Two irons lie in the magnetic field produced by the coil that consists of only few turns if the
instrument is an ammeter or of many turns if the instrument is a voltmeter. Current in the coil induces
both vanes to become magnetized and repulsion between the similarly magnetized vanes produces a
proportional rotation.
Figure 3.2
The deflecting torque is proportional to the square of the current in the coil, making the
instrument reading true RMS quantity. Rotation is opposed by a hairspring that produces the restoring
torque. Only the fixed coil carries load current, and it is constructed so as to withstand high transient
current. Moving iron have non-linear scales and somewhat crowded in the lower range of calibration.
Attractive types of M.I. instrument this instrument consists of a few soft iron discs that are
fixed to the spindle, pivoted in jeweled bearings. The spindle also carries a pointer, a balance weight, a
controlling weight and a damping piston, which moves in a curved fixed cylinder. The special shape of the
moving-iron discs is for obtaining a scale of suitable form.
M.I. instruments may be used for DC current and voltage measurements and they are
subject to minor frequency errors only. The instruments may be effectively shielded from the influence of
external magnetic fields by enclosing the working parts, except the pointer, in a laminated iron cylinder
with laminated iron end covers.
Advantages:
Suitable for both A.C. & D.C. circuits.
Instruments are robust, owing to the simple construction of the moving parts. Low cost compared to
moving coil instrument.
Torque/weight ratio is high, thus less frictional error.

16. Page No.16
Errors:
Errors due to temperature variation.
Errors due to friction are quite small as torque-weight ratio is high in moving-iron instruments.
Stray fields cause relatively low values of magnetizing force produced by the coil. Efficient magnetic
screening is essential to reduce this effect.
Error due to variation of frequency causes change of reactance of the coil and also changes the eddy
currents induced in neighboring metal.
Deflecting torque is not exactly proportional to the square of the current due to non-linear
characteristics of iron material.
6.Dynamometer Type Instrument
Electrodynamic type instruments are similar to the P.M.M.C. instruments except the
magnet is replaced by two serially connected fixed coils that produce the magnetic field when energized.
The fixed coils are spaced far enough apart to allow passage of the shaft of the movable coil. The movable
coil carries a pointer, which is balanced by counter weights. Its rotation is controlled by springs.
Figure 3.3
The motor torque is proportional to the product of the currents in the moving and fixed
coils. If the current is reversed, the field polarity and the polarity of the moving coil reverse at the same
time, and the turning force continues in the original direction. Since reversing the current direction does
not reverse turning force, this type of instruments can be used to measure AC or DC current, voltage, or its
major application as a wattmeter (in our case) for power measurement.
For power measurement, one of the coils (usually the fixed coils) passes the load current
and other coil passes a current proportional to the load voltage. Air friction damping is employed for these
instruments and provided by a pair of aluminum-vanes attached to the spindle at the bottom. These vanes
move in a sector shaped chamber.
Cost and performance compared with the other types of instruments restrict the use of
this design to AC or DC power measurement. Electro-dynamic meters are typically expensive but have the
advantage of being more accurate than moving coil and moving iron instrument but its sensitivity is low.
Similar to moving iron vane instruments, the electro dynamic instruments are true RMS
responding meters. When electro dynamic instruments used for power measurement its scale is linear
because it predicts the average power delivered to the load and it is calibrated in average values for AC.
Voltage, current and power can all be measured if the fixed and moving.

17. Page No.17
Advantages:
Free from hysteresis and eddy current errors.
Applicable to both dc and ac circuits.
Precision grade accuracy for 40 Hz to 500 Hz.
Electrodynamic voltmeters give accurate r.m.s values of voltage irrespective of waveforms.
Limitations:
Low torque/weight ratio, hence more frictional errors.
More expensive than PMMC or MI instruments.
Power consumption higher than PMMC but less than MI instruments.
For these reasons, dynamometer ammeters and voltmeters are not in common use
(except for calibration purpose) especially in dc circuits. The most important application of the
dynamometer type instruments used as dynamometer wattmeter
Notes:
The moving iron type meters can be used to measure DC supply but the reading will have some errors.
The moving coil type meters do not give any deflection to the AC supply, rather buzz furiously.
The dynamometer type instruments can be subjected to both AC and DC supply, but generally used for
AC measurement.
7.Power Factor (PF) Meter Theory
The power factor of an AC electric circuit is defined as the ratio of the True power flowing
to the load to the apparent power and is a dimensionless number between 0 and 1 (frequently expressed
as a percentage, e.g. 0.5 pf = 50% pf).
Power Factor = True Power/Apperent Power
If Ø is the angle between voltage and current then
True power = (apparent power)cos(Ø)
Power Factor = COS(Ø)
7.a.Real Power:
Actual amount of power being used, or dissipated, in a circuit is called real power . It is
measured in watts (symbolized by the capital letter P, as always). As a rule, real power is a function of a
circuit's dissipative elements, usually resistances (R).
Real power = (apparent power)cos(Ø)
or V2/ R{only resistive element in circuit}
If we were to plot the voltage, current, and power waveforms for resistive circuit, it would look like this:
e voltage, I current , p power
Figure 3.4

18. Page No.18
Note that the waveform for power is always positive, never negative for this resistive
circuit. This means that power is always being dissipated by the resistive load
7.b.Reactive power:
Power merely absorbed and returned in load due to its reactive properties is referred to as
Reactive Power. Reactive Power is symbolized by the letter Q and is measured in the unit of Volt-Amps-
Reactive (VAR). Simply the power consumed in reactive load called reactive power.
Q = I2X
e voltage, I current , p power
Figure 3.5
7.c.Apperant Power(S) :
Total power in an AC circuit, both dissipated and absorbed/returned is referred to as
apparent power. Apparent power is symbolized by the letter s and is measured in the unit of Volt-Amps
(VA).
S = I2Z
S2 = P2 +Q2
Power Trangle:
These three types of power -- true, reactive ,apparent -- relate to one another in
trigonometric form. We call this the power triangle
Figure 3.6
Real power = (apparent power)cos(Ø)
Reactive power = (apparent power)sin(Ø)
Power Factor Calulations:
As was mentioned before, the angle of this "power triangle" graphically indicates the ratio
between the amount of dissipated (or consumed) power and the amount of absorbed/returned power. It
also happens to be the same angle as that of the circuit's impedance in polar form. When expressed as a
fraction, this ratio between true power and apparent power is called the power factor for this circuit.
Because true power and apparent power form the adjacent and hypotenuse sides of a right triangle,
respectively, the power factor ratio is also equal to the cosine of that phase angle.
Power Factor in purely resistive network:
In Purely Resistive circuit voltage & current are in phase with each other ,which means

19. Page No.19
that the peak voltage is reached at the same instant as peak current.
Figure 3.7
True Power(P) = I2R, apparent power(S) = i2z
Z = R + jX, Z = R + J 0,
Z = Impedance of Circuit, X = impedance of reactive element for the purely resistive circuit
X=0, SO Z = R
True Power = apparent power (from eq.1)
Power factor = 1
This can we also justify using power trangle relation (real
power) = (apparent power)cos(Ø)
Ø = 0, cos(Ø ) = 1 = power factor
For the purely resistive circuit, the power factor is 1 (perfect), because the reactive power equals zero.
Here, the power triangle would look like a horizontal line, because the opposite (reactive power) side would
have zero length.
Power Factor in purely inductive circuit& purely capacitive network:
An inductor is simply a coil of wire (often wrapped around a piece of ferromagnet). If we
now look at a circuit composed only of an inductor and an AC power source, we will again find that there is
a 90° phase difference between the voltage and the current in the inductor. This time, however, the
current lags the voltage by 90°, so it reaches its peak 1/4 cycle after the voltage peaks.
Figure 3.8
Impedance of inductor: XL =jωL, ω = 2πf,
f = frequency of applied voltage
Ø = 90, cos(Ø ) = 0 = power factor
Capacitive circuit: Consider now a circuit which has only a capacitor and an AC power source (such as a
wall outlet). A capacitor is a device for storing charge. It turns out that there is a 90° phase difference
between the current and voltage, with the current reaching its peak 90° (1/4 cycle) before the voltage
reaches its peak. Put another way, the current leads the voltage by 90° in a purely capacitive circuit.

20. Page No.20
Figure 3.9
Impedance of Capacitor: Xc =1/jωC , ω = 2πf ,
f = frequency of applied voltage
Ø = 90, cos(Ø ) = 0 = power factor
For the purely inductive circuit& purely capacitive, the power factor is zero, because true power equals
zero. Here, the power triangle would look like a vertical line, because the adjacent (true power) side would
have zero length.
Power Factor of RL circuit:
Figure 3.10
Impedance of inductor: XL =jωL, ω = 2πf
Z = R1 + j XL Z = R1 + jωL
Magnitude of impedance z = √R²+ XL²
Ø = tan-1(XL/R)
Power Factor = cos Ø,
Power Factor of RC circuit:
Figure 3.11
Impedance of Capacitor: Xc =1/jωc, ω = 2πf ,
Z = R1 + j XC Z = R1 + jωC
Magnitude of impedance z = √R²+ XC²
Ø = tan-1(XC/R)
Power Factor = cos Ø,
Power Factor of RLC circuit:
Figure 3.12

21. Page No.21
Impedance of Capacitor Xc =1/jωc, ω = 2πf,
Impedance of inductor: XL =jωL, ω = 2πf,
Impedance of RLC circuit: Z = R+J (XL –Xc) , XL. >XC
Z = R+J (XC –XL) , XC. >XL
Magnitude of impedance z = √R²+ (XC – XL)²
Ø = tan-1(XL-XC) / R , XL. >XC
Ø = tan-1(XC-XL )/ R , XC. >XL
Power Factor = cos Ø,
Importance of Power Factor:
Power factor can be an important aspect to consider in an AC circuit, because any power
factor less than 1 means that the circuit's wiring has to carry more current than what would be necessary
with zero reactance in the circuit to deliver the same amount of (true) power to the resistive load. The
poor power factor makes for an inefficient power delivery system. The significance of power factor lies in
the fact that utility companies supply customers with volt-amperes, but bill them for watts. Power factors
below 1.0 require a utility to generate more than the minimum volt-amperes necessary to supply the real
power (watts). This increases generation and transmission costs. For example, if the load power factor
were as low as 0.7, the apparent power would be 1.4 times the real power used by the load. Line current in
the circuit would also be 1.4 times the current required at 1.0 power factor, so the losses in the circuit
would be doubled (since they are proportional to the square of the current). Alternatively all components
of the system such as generators, conductors, transformers, and switchgear would be increased in size
(and cost) to carry the extra current.
Power Factore correction:
Poor power factor can be corrected, paradoxically, by adding another load to the circuit
drawing an equal and opposite amount of reactive power, to cancel out the effects of the load's inductive
reactance. Inductive reactance can only be canceled by capacitive reactance, so we have to add a capacitor
in parallel to circuit as the additional load. The effect of these two opposing reactance in parallel is to bring
the circuit's total impedance equal to its total resistance (to make the impedance phase angle equal, or at
least closer, to zero).
If we know that the (uncorrected) reactive power is Q VAR (inductive), we need to
calculate the correct capacitor size to produce the same quantity of (capacitive) reactive power. We need
to calculate the correct capacitor size to produce the same quantity of (capacitive) reactive power. Since
this capacitor will be directly in parallel with the source (of known voltage), we'll use the power formula
which starts from voltage and reactance:
Q = V2/XC, X C = V2/Q, Xc =1/jωC
C = 1/ jωXC
Capacitor is used parallel with load to improve power factor, If load impedance is
capacitive in nature then we have to put a inductor parallel with load impedance which nullify capacitive
effect and make power factor close to 1. To calculate value of inductor same above procedure apply.
Non linear Load:
A non-linear load on a power system is typically a rectifier (such as used in a power
supply), or some kind of arc discharge device such as a fluorescent lamp, electric welding machine, or arc
furnace. Because current in these systems is interrupted by a switching action, the current contains
frequency components that are multiples of the power system frequency. Distortion power factor is a
measure of how much the harmonic distortion of a load current decreases the average power
transferred to the load.

22. Page No.22
Non-linear loads change the shape of the current waveform from a sine wave to some
other form. Non-linear loads create harmonic currents in addition to the original (fundamental frequency)
AC current. Addition of linear components such as capacitors and inductors cannot cancel these harmonic
currents, so other methods such as filters or "power factor correction" (PFC) are required to smooth out
their current demand over each cycle of alternating current and so reduce the generated harmonic
currents.
Values of component in the Power Factor Demonstrator:
R1 = 500E ± 5%
R2 = 200E ± 5%
L1 =L2 = 200mH ± 5%
C1 = 12.5 F ± 5% , C2 = 20 F ± 5%
Note: 10E internally connected resistance should be considering in the calculation while performing the
experiment with the resistance which are given in “Network Component” Section.

23. Page No.23
4.PRINCIPLES OF THERMAL IMAGING
All materials, which are above 0 degrees Kelvin (-273 degrees C), emit infrared energy. The
infrared energy emitted from the measured object is converted into an electrical signal by the imaging
sensor (microbolometer) in the camera and displayed on a monitor as a color or monochrome thermal
image. The basic principle is explained as follows:
1.Infrared Radiation
The infrared ray is a form of electromagnetic radiation the same as radio waves, micro-
waves, ultraviolet rays, visible light, X-rays, and gamma rays. All these forms, which collectively make up
the electromagnetic spectrum, are similar in that they emit energy in the form of electromagnetic waves
traveling at the speed of light. The major differ-ence between each ‘band’ in the spectrum is in their
wavelength, which correlates to the amount of energy the waves carry. For example, while gamma rays
have wavelengths millions of times smaller than those of visible light, radio waves have wavelengths that
are billions of times longer than those of visible light.
Figure 4.1
The wavelength of the infrared radiation ‘band’ is 0.78 to 1000µm (micrometers). This is
longer than the wavelength of visible light yet shorter that radio waves. The wavelengths of infrared
radiation are classified from the near infrared to the far infrared.
2.Emissivity
Infrared radiation is energy radiated by the motion of atoms and molecules on the surface
of object, where the temperature of the object is more than absolute zero. The intensity of the emittance
is a function of the temperature of the material. In other words, the higher the temperature, the greater
the intensity of infrared energy that is emitted. As well as emitting infrared energy, materials also reflect
infrared, absorb infrared and, in some cases, transmit infrared. When the temperature of the material
equals that of its surroundings, the amount of thermal radiation absorbed by the object equals the amount
emitted by the object.

24. Page No.24
Figure 4.2
The figure above shows the three modes by which the radiant energy striking an object
may be dissipated. These modes of dissipation are:
a = absorption
t = transmission
r = reflection
The fractions of the total radiant energy, which are associated with each of the above
modes of dissipation, are referred to as the absorptivity (a) transmissivity (t) and the reflectivity (r) of the
body. According to the theory of conservation of energy, the extent to which materials reflect, absorb and
transmit IR energy is known as the emissivity of the material.
3.Blackbody Radiation
The emissivity of a body is defined formally by the equation below as the ratio of the
radiant energy emitted by the body to the radiation, which would be emitted by a black-body at the same
temperature.
Where,
Wo = total radiant energy emitted by a body at a given temperature T.
Wbb = total radiant energy emitted by a blackbody at the same temperature T.
If all energy falling on an object were absorbed (no transmission or reflection), the
absorptivity would equal to 1. At a steady temperature, all the energy absorbed could be re-radiated
(emitted) so that the emissivity of such a body would equal 1. Therefore in a blackbody,
absorptivity = emissivity = 1
Practical real life objects do not behave exactly as this ideal, but as described with trans-
missivity and reflectivity,
absorptivity + transmissivity + reflectivity = 1
Energy radiated from the blackbody is described as follows [“Planck’s Law”.]

25. Page No.25
(1)
In order to obtain total radiant emittance of the blackbody, integrate the equation (1) through all
wavelengths (0 to infinity). The result is as follows and is called “Stefan- Bolzmann equation.”
(2)
The temperature of blackbody can be obtained directly from the radiant energy of the blackbody by this
equation. In order to find out the wavelength on the maximum spec-tral radiant emittance, differentiate
Planck’s law and take the value to 0.
(3)
This equation is called “Wien’s displacement law”.
Where in 1) to 3),
Figure 4.3
In radiation of a normal object, as the emissivity is (<1) times of the blackbody, multiply
above equation by the emissivity. The following figures show the spectral radiant emittance of a
blackbody.
(a) is shown by logarithmic scale and (b) is shown by linear scale.
Figure 4.4

26. Page No.26
The graphs show that wavelength and spectral radiant emittance vary with the tempera-
ture. They also show that as the temperature rises, the peak of spectral radiant emittance is shifting to
shorter wavelengths. This phenomenon is observable in the visible light region as an object at a low
temperature appears red, and as the temperature increases, it changes to yellowish and then whitish
color—thus shifting to shorter & shorter wave-lengths as the temperature increases.
4.Blackbody Type Source and Emissivity
Although a blackbody is actually only a theoretical ideal, an object can be manufactured
which approximates it. A law closely related to the blackbody is Kirchhoff’s law that defines reflection,
transmission, absorption and radiation.
a = e = 1
Absorptivity equals emissivity, thus emissivity can be described by reflectivity and transmissivity.
a + t + r = 1
In order to obtain the true temperature of an object, it is necessary to obtain the
emissivity correctly. Therefore, the emissivity of the object has to be measured by using a blackbody-type
source which is closest to an ideal blackbody as possible. The black-body-type source can be designed to
meet the conditions pointed out by Kirchoff where “the radiation within an isothermal enclosure is
blackbody radiation.”
As a blackbody-type source for a measurement must radiate outside of the enclosed sur-
face, a small hole is cut through the wall of the enclosure small enough not to disturb the blackbody
condition. The radiation leaving this hole should closely approximate that of a blackbody. When the
diameter of the hole is as 2r and the depth is as L, if L/r is equal or more than 6, it is used as a blackbody-
type source for practical use. The following figure shows an example of a blackbody-type source based on
blackbody conditions.
Figure 4.5
5.Determining Emissivity
Emissivity is the ratio of energy radiated from an object to the exterior and energy radi-
ated from a blackbody. The emissivity varies with the surface condition of the object and also with
temperature and wavelength. If this value is not accurate, then the true temperature cannot be measured.
In other words, a variation or change in emissivity will cause a change in the indications on a thermal
imager.
To approach the true temperature therefore,
The emissivity must approximate 1.0 ( The measured object must be nearly a blackbody.)
The emissivity must be corrected. ( The emissivity of the measured
object must be internally corrected to 1 by the thermal imager.)

27. Page No.27
Therefore, in order to perform correct measurement for true temperature, the emissivity
is determined as follows:
1) By means of a printed table
Various books and literature carry physical constants tables, but if the measuring condition is
not identical, the constants may not usable. In such cases the literature should be used only
for reference.
2) Determination by ratio — Option 1
A contact-type thermometer is used to confirm that the measured object is in thermal
equilibrium and that the blackbody-type source is at the same temperature. The object and
the black-body-type source are then measured with the radiation ther-mometer and the
resulting energy ratio is then used to define the emissivity as follows:
EK : energy of blackbody-type source
ES: energy of measured object
X: emissivity of measured object
EK : ES = 1 : X
Where,
3) Determination by ratio — Option 2
An object, resembling a blackbody, is attached to a heat source to make the temperature of
the blackbody part and the measur-ing object the same. The ratio of infrared radiation
energies are then determined as in #2 above.
4) Comparison with blackbody surface — Option 1
A very small hole is made in the measured object to satisfy the aforementioned blackbody
conditions, and to make the temperature of the entire object uniform. Then, using the
emissivity correcting function of thermal imager, the emissivity is reduced until the
temperature of the point to be measured equals the temperature of the small hole measured
at an emissivity of 1. The emissivity setting should be the emissivity of the object. (This
applies only when the conditions are the same as at measurement.)
5) Comparison with blackbody surface — Option 2
If a small hole cannot be made in the object, then the emissivity can be obtained by applying
black paint to the object and reach-ing a thermal equilibrium through similar procedures. But
since the painted object will not provide a complete blackbody, the emissivity of the painted
object needs to be set first and then the temperature can be measured. The following figure
shows examples of blackbody paint.
6.Background Noise
When measuring the temperature of an object by a radiation thermometer, it is impor-tant

28. Page No.28
to take into consideration the above-mentioned emissivity correction as well as the environmental
conditions where the measurements will be performed.
Infrared rays enter the thermal imager from the measuring object as well as all other
objects nearby. Therefore, in order to avoid this influence, a function of environment reflection correction,
etc. is required. Also, when accurate data is required, it is necessary to minimize the influence by
shortening the transmission route of the infrared ray, for example.
The following methods may be useful to reduce background noise.
1. Shorten the distance between the measured object and of the thermal imager. Please keep a safe
distance to protect the operator as well as the instrument.
2. Have no high temperature object behind the measured object, such as the sun shining on the back
of the measured object.
3. Do not allow direct sunlight to strike thermal imager.
4. Do not allow obstacles such as dust or vapor (which attenuates the infrared signal) between the
measured object and the thermal imager.
7.Practical Measurement
There are a number of methods for correcting emissivity in order to obtain the true
temperature. The correction procedure with each method will be explained next.
1. Method of comparison or direct measurement with emissivity equal to approximately 1.0
a. Stabilize the temperature of the measured object or similar material.
b. Open a very small hole (hereafter called blackbody part) in the object which the thermal
imager must measure as to satisfy blackbody conditions.
c. Then set the emissivity correcting function of thermal imager so that the temperature of the
blackbody part and the measured surface will be the same. The obtained emissivity will be the
emissivity of the measured surface.
d. Thereafter when measuring the same type object, it is unnecessary to change the emissivity
setting.
2. Method of direct measurement of emissivity
If a hole cannot be made as in method 1, then apply black high emissivity paint and carry out the
same procedures to obtain the emissivity. Since the black paint will not provide a perfect
blackbody, first set the emissivity of the black paint and then measure the temperature.
3. Indirect measurement
Measure a sample similar to the measured object, and place it in a con-dition able to be heated by
a heater, etc. Then measure the object and the sample alternately with the camera and when the
indicated values are identical, measure the sample with a contact-type thermometer. Adjust the
emissivity of the thermal imager to cause the temperature readout to match that of the contact
measurement. The resulting emissivity is that of the sample.
4. Measuring by Wedge effect
With this method, the emissivity of the measured surface itself is enhanced through use of the

29. Page No.29
wedge or semi-wedge effect. But one must be careful about the number of reflections and/or the
measuring angle.
A small change in angle will reduce the emissivity enhancement.
Figure 4.6
8.Emissivity of Various Materials
From “Infrared Radiation, a Handbook for Applications “by Mikael A. Bramson

30. Page No.30

31. Page No.31

32. Page No.32

33. Page No.33
5.EARTHING FUNDAMENTALS
1.Soil Resistivity Testing
1.1. Introduction:
It is well known that the resistance of an earth electrode is heavily influenced by the
resistivity of the soil in which it is driven and as such, soil resistivity measurements are an important
parameter when designing earthing installations. A knowledge of the soil resistivity at the intended site,
and how this varies with parameters such as moisture content, temperature and depth, provides a
valuable insight into how the desired earth resistance value can be achieved and maintained over the life
of the installation with the minimum cost and effort.
One of the main objectives of earthing electrical systems is to establish a common
reference potential for the power supply system, building structure, plant steelwork, electrical conduits,
cable ladders & trays and the instrumentation system. To achieve this objective, a suitable low resistance
connection to earth is desirable. However, this is often difficult to achieve and depends on a number of
factors:
• Soil resistivity
• Stratification
• Size and type of electrode used
• Depth to which the electrode is buried
• Moisture and chemical content of the soil
Section 1.2 covers the first of these points.
1.2.Theory of Soil Resistivity:
Resistance is that property of a conductor which opposes electric current flow when a
voltage is applied across the two ends. Its unit of measure is the Ohm (Ω) and the commonly used symbol
is R. Resistance is the ratio of the applied voltage (V) to the resulting current flow (I) as defined by the well
known linear equation from Ohm’s Law:
V = I × R
where: V Potential Difference across the conductor (Volts)
I Current flowing through the conductor in (Amperes)
R Resistance of the conductor in (Ohms)
“Good conductors” are those with a low resistance. “Bad conductors” are those with a
high resistance. “Very bad conductors” are usually called insulators.
The Resistance of a conductor depends on the atomic structure of the material or its
Resistivity (measured in Ohm- m or Ω -m), which is that property of a material that measures its ability to
conduct electricity. A material with a low resistivity will behave as a “good conductor” and one with a high
resistivity will behave as a “bad conductor”. The commonly used symbol for resistivity is ρ (Greek symbol
rho).
The resistance (R) of a conductor, can be derived from the resistivity as:

34. Page No.34
Resistivity is also sometimes referred to as “Specific Resistance” because, from the above
formula, Resistivity (Ω-m) is the resistance between the opposite faces of a cube of material with a side
dimension of 1 metre.
Consequently, Soil Resistivity is the measure of the resistance between the opposite sides
of a cube of soil with a side dimension of 1 metre.
In the USA, a measurement of Ω-cm is used. (100 Ω-cm = 1 Ω-m)
1.3. Making A Measurements:
When designing an earthing system to meet safety and reliability criteria, an accurate
resistivity model of the soil is required. The following sections outline the major practical aspects of the
measurement procedure and result interpretation.
A) Principles:
Soil resistivity values in the Australian continent are widely varying depending on the type
of terrain, eg, silt on a river bank may have resistivity value in the order of 1.5Ωm, whereas dry sand or
granite in mountainous country areas may have values higher than 10,000Ωm. Factors that affect
resistivity may be summarised as:-
• Type of earth (eg, clay, loam, sandstone, granite).
• Stratification; layers of different types of soil (eg, loam backfill on a clay base).
• Moisture content; resistivity may fall rapidly as the moisture content is increased, however, after a
value of about 20% the rate of decrease is much less. Soil with content greater than 40% do not occur
very often.
• Temperature; above freezing point, the effect on earth resistivity is practically negligible.
• Chemical composition and concentration of dissolved salt.
• Presence of metal and concrete pipes, tanks, large slabs, cable ducts, rail tracks, metal pipes and
• Fences Topography; rugged topography has a similar effect on resistivity measurement as local surface
resistivity variation caused by weathering and moisture.
Figure 5.1
R =
ρ × L
A
where ρ Resistivity (Ω-m) of the conductor material
L Length of the conductor (m)
A Cross sectional Area (m2
)

35. Page No.35
Table 5-1 to Table 5-3 show how typical values alter with changes in soil, moisture and temperature.
Type of Soil or Water Typical Usual Limit
Resistivity Ωm
Ωm
Sea water 2 0.1 to 10
Clay 40 8 to 70
Ground well & spring water 50 10 to 150
Clay & sand mixtures 100 4 to 300
Shale, slates, sandstone etc 120 10 to 100
Peat, loam & mud 150 5 to 250
Lake & brook water 250 100 to 400
Sand 2000 200 to 3000
Moraine gravel 3000 40 to 10000
Ridge gravel 15000 3000 to 30000
Solid granite 25000 10000 to 50000
Ice 100000 10000 to 100000
Table 5-1 Resistivity values for several types of soils and water
Typical resistivity Ωm
Moisture Clay mixed Silica based
% by weight with sand sand
0 10 000 000 -
2.5 1 500 3 000 000
5 430 50 000
10 185 2 100
15 105 630
20 63 290
30 42 -
Table 5-2 - Variations in soil resistivity with moisture content
Temp. C Typical resistivity Ωm
20 72
10 99
0 (water) 138
0 (ice) 300
-5 790
-15 3300
Table 5-3 - Variations in resistivity with temperature for a mixture of sand and clay
with a moisture content of about 15% by weight

36. Page No.36
Typical resistivity Ωm
3500
3000
2500
2000
1500
1000
500
0
-15 -5 0 (ice) 0 (water) 10 20
Temperature °C
Figure 5-2 Variations in resistivity with temperature for a mixture of sand and
clay with a moisture content of about 15% by weight
When defining the electrical properties of a portion of the Earth, a distinction between
the geoelectric and geologic model is required. In the geoelectric model the boundaries between layers
are determined by changes in resistivity, being primarily dependent upon water and chemical content, as
well as texture. The geologic model, based upon such criteria as fossils and texture, may contain several
geoelectric sections. The converse is also common.
As earthing systems are installed near the surface of the Earth, the top soil layers being
subject to higher current densities are the most significant and require the most accurate modelling.
The Wenner and Schlumberger test methods are both recommended, with testing and
interpretation techniques summarised in the following sections.
B) Soil Resistivity Testing Procedure Guidelines:
The purpose of resistivity testing is to obtain a set of measurements which may be
interpreted to yield an equivalent model for the electrical performance of the earth, as seen by the
particular earthing system. However, the results may be incorrect or misleading if adequate investigation is
not made prior to the test, or the test is not correctly undertaken. To overcome these problems, the
following data gathering and testing guidelines are suggested:
An initial research phase is required to provide adequate background, upon which to
determine the testing program, and against which the results may be interpreted. Data related to nearby
metallic structures, as well as the geological, geographical and meteorological nature of the area is very
useful. For instance the geological data regarding strata types and thicknesses will give an indication of the
water retention properties of the upper layers and also the variation in resistivity to be expected due to
water content. By comparing recent rainfall data, against the seasonal average, maxima and minima for
the area it may be ascertained whether the results are realistic or not.
A number of guidelines associated with the preparation and implementation of a testing
program are summarised as follows:

37. Page No.37
(a) Test Method:
Factors such as maximum probe depths, lengths of cables required, efficiency of the
measuring technique, cost (determined by the time and the size of the survey crew) and ease of interpretation
of the data need to be considered, when selecting the test type. Three common test types are shown in Figure
5-2. The Schlumberger array is considered more accurate and economic than the Wenner or Driven Rod
methods, provided a current source of sufficient power is used.
Figure 5.3 Resistivity Test Probe Configurations
In the Wenner method, all four electrodes are moved for each test with the spacing
between each adjacent pair remaining the same. With the Schlumberger array the potential electrodes
remain stationary while the current electrodes are moved for a series of measurements. In each method
the depth penetration of the electrodes is less than 5% of the separation to ensure that the approximation
of point sources, required by the simplified formulae, remains valid.

38. Page No.38
(b) Selection of Test Method Type:
Wenner Array
The Wenner array is the least efficient from an operational perspective. It requires the
longest cable layout, largest electrode spreads and for large spacings one person per electrode is
necessary to complete the survey in a reasonable time. Also, because all four electrodes are moved after
each reading the Wenner Array is most susceptible to lateral variation effects.
However the Wenner array is the most efficient in terms of the ratio of received voltage
per unit of transmitted current.
Where unfavourable conditions such as very dry or frozen soil exist, considerable time
may be spent trying to improve the contact resistance between the electrode and the soil.
Schlumberger Array
Economy of manpower is gained with the Schlumberger array since the outer electrodes
are moved four or five times for each move of the inner electrodes. The reduction in the number of
electrode moves also reduces the effect of lateral variation on test results.
Considerable time saving can be achieved by using the reciprocity theorem with the
Schlumberger array when contact resistance is a problem. Since contact resistance normally affects the
current electrodes more than the potential electrodes, the inner fixed pair may be used as the current
electrodes, a configuration called the ‘Inverse Schlumberger Array’. Use of the inverse Schlumberger array
increases personal safety when a large current is injected. Heavier current cables may be needed if the
current is of large magnitude. The inverse Schlumberger reduces the heavier cable lengths and time spent
moving electrodes. The minimum spacing accessible is in the order of 10 m (for a 0.5m inner spacing),
thereby, necessitating the use of the Wenner configuration for smaller spacings.
Lower voltage readings are obtained when using Schlumberger arrays. This may be a
critical problem where the depth required to be tested is beyond the capability of the test equipment or
the voltage readings are too small to be considered.
Driven Rod Method
The driven rod method (or Three Pin or Fall-of-Potential Method) is normally suitable for
use in circumstances such as transmission line structure earths, or areas of difficult terrain, because of: the
shallow penetration that can be achieved in practical situations, the very localised measurement area, and
the inaccuracies encountered in two layer soil conditions.
(c) Traverse Locations.
Soil resistivity can vary significantly both with depth, and from one point to another at a
site, and as such, a single soil resistivity measurement is usually not sufficient. To obtain a better picture of
soil resistivity variations, it is advisable to conduct a detailed survey.
Figure 5.4 Performing a Line Traverse Survey

39. Page No.39
The Line Traverse technique is a commonly used method for performing soil resistivity
surveys. In this method, a series of imaginary parallel lines are drawn across the area to be surveyed, and a
number of soil resistivity measurements, at various stake separations, are performed along each of these
lines (see Figure 5.4). Larger earthing systems require a greater number of traverses ( >4).
Taking a number of measurements along each ‘line’, using different stake separations, will
provide an indication of how the soil resistivity varies with depth, whilst taking measurements along
different lines will indicate how the resistivity changes across the site.
In this way, a picture can be built up of the resistivity variation at the site and the areas of
lowest resistivity can be identified. By measuring the resistivity at different depths, it is possible to build up
information about the underlying soil and whether or not any advantage can be gained by installing the
earthing system to a greater depth.
A Line Traverse survey is a cheap and simple way of mapping variations in soil resistivity at
a site and could well provide significant cost savings, in terms of material and labour, when attempting to
achieve the required resistance figure.
It is also useful to include a ‘check’ traverse near to, yet beyond the influence of the grid.
Measurements are re-made on this traverse when undertaking an injection test on the installed grid, to
correlate the test results with the initial measured conditions at the time of design.
(d) Spacing Range.
The range of spacings recommended includes accurate close probe spacings ( >1m), which
are required to determine the upper layer resistivity, used in calculating the step and touch voltages, to
spacings larger than the radius or diagonal dimension of the proposed earth grid. The larger spacings are
used in the calculation of remote voltage gradients and grid impedance. Measurements at very large
spacings often present considerable problems (eg inductive coupling, insufficient resolution on test set,
physical barriers) they are important if the lower layer is of higher resistivity (ρ2 > ρ 1). In such cases
considerable error is introduced if a realistic value of ρ2 is not measured due to insufficient spacing.
(e) Practical Testing Recommendations.
It has been found that special care is required when testing to:
• Eliminate mutual coupling or interference due to leads parallel to power lines. Cable reels with
parallel axes for current injection and voltage measurements, and small cable separation for large
spacings (>100m) can result in errors;
• Ensure the instrumentation and set up is adequate (ie equipment selection criteria, power levels,
interference and filtering);
• Undertake operational checks for accuracy (ie, a field calibration check);
• Reduce contact resistance (use salt water, stakes and/or the reverse Schlumberger);
• Instruct staff to use finer test spacings in areas showing sharp changes (ie to identify the effect of
local inhomogeneities and give increased data for interpretation). Plot test results immediately
during testing to identify such problem areas.
2. Interpretation and Modelling of Result:
A) Apparent Resistivity Calculation:
Refer to section 1.2, Theory of Soil Resistivity, for basic calculations.
In homogeneous isotropic earth the resistivity will be constant. However, if the earth is
non homogeneous and the electrode spacing varied, a different value of resistivity (ρa) will be found for
each measurement. This measured value of resistivity is known as the apparent resistivity. The apparent

40. Page No.40
resistivity is a function of the array geometry, measured voltage (∆v), and injected current (I).
For the arrays described in the previous section the apparent resistivity is found from the
field measurements using the following formulae.
Wenner array
ρ
aw = 2πa ∆v
I
ρ
aw = 2πaR
Where Ρaw = apparent resistivity (Ω)
A = probe spacing (m)
∆v = voltage measured (volts)
I = injected current (Amps)
R = measured resistance (Ω)
Schlumberger array
ρ
as=
πL
2
R
2l
Where Ρas = apparent resistivity (Ωm)
L = distance from centre line to inner probes (m)
L = distance from centre line to outer probes (m)
R = measured resistance (Ω)
Driven Rod
ρ
2 πlR
a d =
ln
8 l
d
Where Ρad = Apparent resistivity (Ωm)
L = Length of driven rod in contact with earth (m)
D = Driven rod diameter (m)
R = Measured value of resistance (Ω)
B) Interpretation Of Resistivity Measurement:
The result of each resistivity test traverse is a value of apparent resistivity for each
spacing/configuration used. The interpretation task is the determination of the presence of layers of
material of common resistivity.
Both curve matching and analytical procedures may be used to identify the presence of
resistivity layering (eg. vertical, horizontal or dipping beds). Figure 2-1 shows several typical apparent
resistivity curves.
• Graphical curve matching is useful for field staff to detect anomalies and identify areas requiring
close examination and testing. However, the use of graphical curve matching is limited to soils of 3
layers or less.
• Computer based techniques are best used to identify two or more soil resistivity layers.

41. Page No.41
• Bad data is best eliminated or checked in the field, as statistical screening is only useful if a large
number of traverses are made and the resistivity layering over the area is uniform.
• The use of weighted averaging techniques to determine an equivalent homogeneous soil model or
average apparent resistivity values for each probe spacing is not mathematically sound. It is best to
first obtain a resistivity model for each traverse and then make a decision upon which information to
base the earthing system design.
It is recommended that a multi-layer model for apparent resistivity be generated. A two
layer model yields significant benefits in both economy, accuracy and safety, these should identify the
surface layer to about 1m and the average deep layer to the grid diagonal dimension. The multi-layer
model is useful in providing more accurate information regarding the presence of lower resistivity layers,
and hence optimising rod driving depths. However, the two layer model is considered sufficiently accurate
for modelling the behaviour of grids in the majority of cases. If more than two layers are identified, the
lower layers are usually combined to form a two layer equivalent model. This is done because the surface
potentials are closely related to the upper layer resistivity, whilst the grid resistance, which is primarily
effected by the deeper layers, is not usually adversely affected by this simplification.
Figure 5.5 Typical Resistivity Curves
Curve (A) - Homogenous resistivity
Curve (B) - Low resistivity layer overlaying higher resistivity layer
Curve (C) - High resistivity layer between two low resistivity layers
Curve (D) - High resistivity layer overlaying a lower resistivity layer
Curve (E) - Low resistivity layer over high resistivity layer with a vertical discontinuity
(typically a fault line).
3. How to Design a Lightning Earth System:
Once the soil resistivity is known, the design of the Earthing system can be made to
achieve the desired Earth resistance. Design parameters are given in the following sections.
A) Types Of Earth Electrodes:
Earth electrodes must ideally penetrate into the moisture level below the ground level.

42. Page No.42
They must also consist of a metal (or combination of metals) which do not corrode excessively for the
period of time they are expected to serve. Because of its high conductivity and resistance to corrosion,
copper is the most commonly used material for earth electrodes. Other popular materials are hot-
galvanised steel, stainless steel, aluminium and lead.
Earth electrodes may be rods, plates, strips, solid section wire or mats.
Three types of copper rods are commonly available.
• Solid Copper
• Copper clad steel rod ( copper shrunk onto the core)
• Copper Bonded steel core (coper is molecularly bonded to nickel plated steel rod)
Solid copper rods not prone to corrosion, but are expensive and difficult to drive into hard
ground without bending. A steel cored copper rod is used for this reason, however those rods that are
simply clad are prone to the cladding tearing away from the core when driven in rocky ground, or when
bent. This exposes the internal steel core to corrosion. The most cost effective solution is the
copperbonded electrode which is a molecularly bonded steel cored copper ground rod.
B) Common Earthing Systems:
The basic philosophy of any earth installation should be an attempt to maximise the
surface area contact with the surrounding soil. Not only does such this help to lower the earth resistance
of the grounding system, but it also greatly improves the surge impedance of the grounding system due to
the large capacitive coupling which is achieved.
The benefits of a small amount of capacitive coupling into the surrounding earth is greatly
enhanced when one considers that the fast rising edge associated with the lightning impulse has an
inherent high fundamental frequency.
The actual layout of the earthing system will vary with the usages (lightning protection
earth only, 50Hz power earth or combination) type of soil and space availability
Common lightning protection earths are shown in Figure 3-1- Methods of earthing.
A) Single earth rod, which is not generally acceptable other than in areas with a constantly high water
table.
B) Earth rod with Copper subterranean tape.This is an improvement upon A,but not as effective as D) & E).
C) Combination of rods and tapes are most commonly used. When two earth rods are used, spacing
should be at least twice the depth to which the rods are driven.
D) The use of radial tapes greatly improves the surge impedance of the earth system, by increasing the
capacitive coupling to the soil. This technique is more effective than a single long conductor, A), due to
the effective length of the conductor.
E) A crows foot earth mat further improves on D above when longer radials are required. The crows foot
fills in the area between the initial radials.
F) Earth enhancing compounds placed around the earth rod improve the rod-ground interface. Earth
enhancing compounds can also be used with horizontal conductors.

43. Page No.43
G) In areas where step potentials are of concern, an insulated PVC sleeve can be used to ensure the
lightning current is injected into the ground at a depth below the surrounding surface.
Figure 5.6 - Methods of earthing. A) Single earth rod, B) Earth rod with Cu subterranean tape, C) Multiple
rods, D) Radial tapes, E) Crows foot earth, F) Earth enhancing compounds placed around the earth rod,
G) Insulated PVC sleeve can be used to ensure the lightning current is injected into the ground at a depth
below the surrounding surface.
C) Earth Resistance of An Electrode – Calculation:
Since soil exhibits a resistance to the flow an electrical current and is not an “ideal”
conductor, there will always be some resistance (can never be zero) between the earth electrode and
“true Earth”. The resistance between the earth electrode and “true Earth” is known as the Earth
Resistance of an electrode and it will depend on the soil resistivity, the type and size of the electrode and
the depth to which it is buried.
If the soil resistivity is known or can be measured using the 4-point method, the Earth
Resistance of an electrode configuration may be calculated for the various types and sizes of the earth
electrode used. AS 1768-1991 pages 68 and 69 provides the formulae for calculating earth resistance for
various types and configurations of electrodes. The most common configurations will be covered below.
C.1} Rods Driven Vertically into the Ground
The Earth Resistance (Rg) of a single spike, of diameter (d) and driven length (L) driven

44. Page No.44
vertically into the soil of resistivity (ρ), can be calculated as follows:
R
ρ 8L
= ln −1
g
d
2πL
where: ρ Soil Resistivity in Ωm
L Buried Length of the electrode in m
d Diameter of the electrode in m
Examples
(a) 20mm rod of 3m length and Soil resistivity 50 Ω-m ..... R=16.1Ω
(b) 25mm rod of 2m length and Soil resistivity 30 Ω-m ..... R=13.0Ω
C.2} Rod Electrodes in Parallel
If the desired earth resistance cannot be achieved with one earth electrode, the overall
resistance can be reduced by connecting a number of electrodes in parallel. These are also sometimes
called “arrays of rod electrodes”.
The combined resistance of parallel electrodes is a complex function of several factors,
such as the number and configuration of electrodes, the separation between them, their dimensions and
soil resistivity. This does not take into account the effect of the horizontal conductors connecting the rods
in the array. The rule of thumb is that rods in parallel should be spaced at least twice their length to utilise
the full benefit of the additional rods.
If the separation of the electrodes is much larger than their lengths and only a few
electrodes are in parallel, then the resultant earth resistance can be calculated using the ordinary equation
for resistances in parallel. In practice, the effective earth resistance will usually be higher than this.
Typically, a 4 spike array may provide an improvement of about 2.5 to 3 times. An 8 spike array will
typically give an improvement of maybe 5 to 6 times.
The calculation of earth resistance (Rg) of rods spaced closer than the distance apart is
given by the following equation:
R
ρ 2 L
g
= ln −1
a'
πL
Where: ρ Soil Resistivity in Ωm
L Buried Length of the electrode in m
a’ Equivalent radius off the electrode at the surface in m
a ' =[( dh)0 .5
(ss')0.5
]0.5
a ' = [dhss'] 0.25
s ' = ( 4h2
+ s2
)0 .5
Where: d Diameter of the electrode in m
h Buried depth of the electrode in m
s Distance between two parallel electrodes in m
s’ Distance from one electrode to the image of the other, in m

45. Page No.45
C.3} Trench Electrodes - Horizontal Electrodes buried under the Surface
Trench Electrodes, conductors buried horizontally under the surface of the ground, also
make very good connections to earth. They are particularly effective when a down-conductor is connected
to a point in the middle of the trench electrode. With this configuration, the surge inductance is halved,
because there will be two parallel transmission paths. Trench electrodes are very effective when used in
combination with spike electrodes, located close to the junction of the down-conductor.
These horizontal electrodes should not have a very long length because, for a lightning
current rise time of 0.5µs and a propagation speed of 0.75 times the speed of light, the current will have
reached its peak at the connection point before the leading edge of the wave has travelled 100m along the
conductor.
Standard practice is to bury the conductor underground at a depth of >0.5m and a length
of >30m. For example, a 50m run of 10mm diameter solid copper conductor buried at a depth of 0.5m
below the surface yields a theoretical 4Ω earth resistance in a soil with resistivity of 90Ω-m.
ρ
ln
4L
R = −1
1
g
πL ( dh)2
where: ρ Soil Resistivity in Ω-m
L Buried Length of the electrode in m
d Diameter of the electrode in m
h Buried depth of the electrode in m
Practical earth connections can also be made from various “star” arrangements radiating
from a central point. This is sometimes called a “crow’s foot” earth electrode. This has an improved surge
impedance performance, with several parallel paths. But the multiple trench electrode will have a higher
low frequency (50Hz) resistance because of the interaction of the fields of each of the radial conductors.
C.4} Radial Conductors
Due to the interaction of the radial conductors doubling the number will not half the
resistance. The increase in resistance is approximated by:
For two wires at right angles, energised at the joint, the resistance is:
= R +
3R
100
For a 3 point star 4 point star 5 point star 6 point star
= R + 6R = R + 12R = R + 42R = R + 65R
100
100 100 100
where: R Resistance of a straight wire of same total length energised at one end
As discussed in the section titled “The Significance of Impedance”, due to the shorter
effective length of radials compared to a single run, the radial will be a much more effective earth under
transient conditions. It is better to run multiple radials, rather than a single long (or deep) conductor.
The calculation for buried radials is given by:

46. Page No.46
ρ 4L
Rg = − 1 + N (n)
ln
nπL 1
( dh)2
m = n −1
+ sin πm /
N (n) = ∑ ln 1n
sin πm / n
m=1
or
for n= 2 3 4 6 8 12
N(n)= 0.7 1.53 2.45 4.42 6.5 11
Where: Ρ Soil Resistivity in Ω-m
L Length of each radial in m
D Diameter of each radial in m
H Buried depth of the radials in m
n Number of radials
C.5} Ground-grid Mesh Electrodes
Another example of the use of conductors buried under the surface of the earth is the
ground-grid mesh. Grid meshes are often used to complement rods or can be used separately when deep
driven rods are impractical due to soil and terrain considerations.
Grid meshes are often used for the earthing in substations to create an equipotential
platform and also to handle the high fault currents returning to the transformer neutrals. They are
particularly useful when multiple injection points are required, at a substation for example. In this case a
number of items will be connected to the grid at various locations, the mesh provides a good earth
irrespective of the injection point of the fault current. Earthing resistance of buried grid meshes can be
considerably lower than those implemented using vertical earth spikes. Increasing the area of the grid
coverage can also significantly reduce the earth resistance.
D} Recommended Material Applications :
IEC 1024-1 (1990) gives guidance in types of metals used for the construction of facility
lightning protection systems.
MATERIAL
USE
IN OPEN AIR IN SOIL IN CONCRETE
COPPER SOLID, STRANDED SOLID, STRANDED DO NOT USE
OR AS A COATING OR AS A COATING
HOT SOLID OR SOLID SOLID
GALVANISED STRANDED
STEEL
STAINLESS SOLID OR SOLID DO NOT USE
STEEL STRANDED
ALUMINIUM SOLID OR DO NOT USE DO NOT USE
STRANDED
LEAD SOLID OR AS A SOLID OR AS A DO NOT USE
COATING COATING
Table 5.4 Recommended Usage of Materials from IEC 1024-1 1990

47. Page No.47
CORROSION
MATERIAL RESISTANCE INCREASED BY ELECTROLYTIC
WITH
COPPER AGAINST CONCENTRATED -
MANY CHLORIDES,
MATERIALS SULPHUR AND
ORGANIC
MATERIALS
HOT GALVANISED GOOD EVEN IN - COPPER
STEEL ACID SOILS
STAINLESS STEEL AGAINST WATER WITH -
MANY DISSOLVED
MATERIALS CHLORIDES
ALUMINIUM - BASIC AGENTS COPPER
LEAD HIGH ACID SOILS COPPER
CONCENTRATI
ON ON
SULPHATES
Table 5.5 The Corrosion properties of different materials
4. Testing an Earthing System
A} Earth Resistance Of An Electrode – Measurement :
When an electrode system has been designed and installed, it is usually necessary to
measure and confirm the earth resistance between the electrode and “true Earth”. The most commonly
used method of measuring the earth resistance of an earth electrode is the 3-point measuring technique
shown in Figure 5.8 This method is derived from the 4-point method, which is used for soil resistivity
measurements.
Figure 5.8: The 3-point Method of Earth Resistance Measurement
The 3-point method, called the “fall of potential” method, comprises the Earth Electrode
to be measured and two other electrically independent test electrodes, usually labelled P (Potential) and C
(Current). These test electrodes can be of lesser “quality” (higher earth resistance) but must be electrically
independent of the electrode to be measured. An alternating current (I) is passed through the outer
electrode C and the voltage is measured, by means of an inner electrode P, at some intermediary point
between them. The Earth Resistance is simply calculated using Ohm’s Law; Rg = V/I.
Other more complex methods, such as the Slope Method or the Four Pole Method, have
been developed to overcome specific problems associated with this simpler procedure, mainly for
measurements of the resistance of large earthing systems or at sites where space for locating the test

48. Page No.48
electrodes is restricted.
Regardless of the measurement method employed, it should be remembered that the
measurement of earth resistance is as much an art as it is a science, and resistance measurements can be
affected by many parameters, some of which may be difficult to quantify. As such, it is best to take a
number of separate readings and average them, rather than rely on the results of a single measurement.
When performing a measurement, the aim is to position the auxiliary test electrode C far
enough away from the earth electrode under test so that the auxiliary test electrode P will lie outside the
effective resistance areas of both the earth system and the other test electrode (see Figure 4-2). If the
current test electrode, C, is too close, the resistance areas will overlap and there will be a steep variation in
the measured resistance as the voltage test electrode is moved. If the current test electrode is correctly
positioned, there will be a ‘flat’ (or very nearly so) resistance area somewhere in between it and the earth
system, and variations in the position of the voltage test electrode should only produce very minor changes
in the resistance figure.
Figure 5.9: Resistance areas and the variation of the measured resistance with voltage electrode position
The instrument is connected to the earth system under test via a short length of test
cable, and a measurement is taken.
Measurement accuracy can be affected by the proximity of other buried metal objects to
the auxiliary test electrodes. Objects such as fences and building structures, buried metal pipes or even
other earthing systems can interfere with the measurement and introduce errors. Often it is difficult to
judge, merely from visual inspection of the site, a suitable location for the tests stakes and so it is always
advisable to perform more than one measurement to ensure the accuracy of the test.
a) Fall of Potential Method
This is one of the most common methods employed for the measurement of earth
resistance and is best suited to small systems that don’t cover a wide area. It is simple to carry out and
requires a minimal amount of calculation to obtain a result.
This method is generally not suited to large earthing installations, as the stake separations
needed to ensure an accurate measurement can be excessive, requiring the use of very long test leads
(refer to Table 5.2).
Normally, the outer test electrode, or current test stake, is driven into the ground 30 to 50
metres away from the earth system, (although this distance will depend on the size of the system being
tested - refer to Table 5.6) and the inner electrode, or voltage test stake, is then driven into the ground

49. Page No.49
mid-way between the earth electrode and the current test stake, and in a direct line between them.
Maximum Distance from Minimum distance from
dimension ‘electrical centre’ ‘electrical centre’ of
across earth of earth system to earth system to current
system voltage test stake test stake
1 15 30
2 20 40
5 30 60
10 43 85
20 60 120
50 100 200
100 140 280
Table 5.6 Variation of current and voltage electrode separation with maximum earth system dimensions, in metres.
The Fall of Potential method incorporates a check to ensure that the test electrodes are
indeed positioned far enough away for a correct reading to be obtained. It is advisable that this check be
carried, as it is really the only way of ensuring a correct result.
To perform a check on the resistance figure, two additional measurements should be
made; the first with the voltage test electrode (P) moved 10% of the original voltage electrode -to-earth
system separation away from its initial position, and the second with it moved a distance of 10% closer
than its original position, as shown in Figure 5.10.
Figure 5.10: Checking the validity of a resistance measurement
If these two additional measurements are in agreement with the original measurement,
within the required level of accuracy, then the test stakes have been correctly positioned and the DC
resistance figure can be obtained by averaging the three results. However, if there is substantial
disagreement amongst any of these results, then it is likely that the stakes have been incorrectly
positioned, either by being too close to the earth system being tested, too close to one another or too
close to other structures that are interfering with the results. The stakes should be repositioned at a larger
separation distance or in a different direction and the three measurements repeated. This process should
be repeated until a satisfactory result is achieved.
b) The 62% Method
The Fall of Potential method can be adapted slightly for use with medium sized earthing
systems. This adaptation is often referred to as the 62% Method, as it involves positioning the inner test
stake at 62% of the earth electrode-to-outer stake separation (recall that in the Fall-of-Potential method,
this figure was 50%).
All the other requirements of test stake location - that they be in a straight line and be
positioned away from other structures - remain valid. When using this method, it is also advisable to
repeat the measurements with the inner test stake moved ±10% of the earth electrode-inner test stake
separation distance, as before.
The main disadvantage with this method is that the theory on which it is based relies on
the assumption that the underlying soil is homogeneous, which in practice is rarely the case. Thus, care
should be taken in its use and a soil resistivity survey should always be carried out. Alternatively, one of the

50. Page No.50
other methods should be employed.
c) Other Test Methods
Many other methods exist for taking earth resistance measurements. Many of these
methods have been designed in an attempt to alleviate the necessity for excessive electrode separations,
when measuring large earth systems, or the requirement of having to know the electrical centre of the
system.
Three such methods are briefly described below. Specific details are not given here, but
instead the reader is referred to the relevant technical paper where these systems are described in detail.
c.1) The Slope Method
This method is suitable for use with large earthing systems, such as sub-station earths. It
involves taking a number of resistance measurements at various earth system to voltage electrode
separations and then plotting a curve of the resistance variation between the earth and the current. From
this graph, and from data obtained from tables, it is possible to calculate the theoretical optimum location
for the voltage electrode and thus, from the resistance curve, calculate the true resistance.
The additional measurement and calculation effort tends to relegate this system to use
with only very large or complex earthing systems.
For full details of this method, refer to paper 62975, written by Dr G.F. Tagg, taken from
the proceedings of IEE volume 117, No 11, Nov. 1970.
c.2) The Star-Delta Method
This technique is well suited to use with large systems in built up areas or on rocky terrain,
where it may be difficult to find suitable locations for the test electrodes, particularly over long distances in
a straight line.
Three test electrodes, set up at the corners of an equilateral triangle with the earth
system in the middle, are used and measurements are made of the total resistance between adjacent
electrodes, and also between each electrode and the earthing system.
Using these results, a number of calculations are performed and a result can be obtained
for the resistance of the earth system.
This method, developed by W. Hymers, is described in detail in Electrical Review, January
1975.
c.3) The Four Potential Method
This technique, helps overcome some of the problems associated with the requirement
for knowing the electrical centre of the earthing systems being tested.
This method is similar in set up to the standard Fall of Potential method, except that a
number of measurements are made with the voltage electrode at different positions and a set of equations
are used to calculate the theoretical resistance of the system.
The main draw back with the Four Potential method is that, like with the Fall of Potential
method, it can require excessive electrode separation distances if the earthing system being measured is
large.
5. The Significance of Impedance
A} Introduction
As mentioned before, the importance of ensuring that the grounding system affords a low
earth impedance and not simply a low earth resistance must be understood. A Fourier transform “spectral
study” of the typical waveform associated with the lightning impulse reveals both a high frequency and low
frequency component. The high frequency is associated with the extremely fast rising “front” (typically <
10 µs to peak current) of the lightning impulse while the lower frequency component resides in the long,
high energy, “tail” or follow on current, in the impulse. Figure 5.11 shows the percentage of less than a

51. Page No.51
specific value. For example, in an 8/20µs wave shape 10% of the energy is at frequencies less than 2.5kHz,
while 90% of the energy is less than 33kHz. For a 10/1000µs wave shape 90% of the energy is less than 1 or
2kHz.
Figure 5.11: Percentage of energy content below a given frequency
The grounding system appears to the lightning impulse as a transmission line where wave
propagation theory, with the normal rules of reflection and group velocity, apply. The soil can act as a
dielectric which under high potential stress at the electrode-soil junction can actually break-down,
decreasing the resistivity of the soil during the surge.
Measurement of earth resistance with conventional low frequency instruments may not
provide results which are indicative of the ground response to a lightning discharge. In complex
installations, many earths can be interconnected and the whole network is measured as one.
B} Theory
Lightning first strokes have 1 to 10 µs impulse current rise times while higher dI/dts occurs
with restrikes which occur in 75% of lightning discharges. These may have 0.2 µs rise time. In these
circumstances, the local earth is subject to the full discharge current before the wavefront has travelled
more than 60 metres. This assumes transmission at the speed of light (300 m/µs). The travel distance is
less if inductance and capacitance of conductors is considered.
The effect of distant earths is substantially negated with these values of dI/dt. They have
little effect in determining local voltage rise due to lead inductance and impedance effects. The following
describes a measurement technique which allows differentiation between impedance and resistance
effects.
B.1) Impulse Testing And The Transient Response
The transient performance of an earthing system is simply a measure of the systems
ability to discharge transient energy into the ground whilst minimising earth potential rise and ensuring
that equipment and personnel are safe from any danger. It is one of the most important factors in
determining the effectiveness of a lightning or surge protection system.
The two main elements of the earth system - the earth electrodes, which provide the
electrical connection to the ground, and the wiring which bonds equipment to the earth electrodes - both
impact on the transient performance. A poorly designed earth system, with a high impedance to ground
can cause excessive earth potential rise, increasing the risk of damage to equipment and personnel.