Earthing in electrical power systems

EARTHING IN ELECTRICAL
POWER SYSTEM
Faculty: Faculty of engineering Helwan
university
Department: Electrical power and
protection engineering
Prepared by: level 4 students
Supervised by: Prof. Mohiy Bahgat

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Content
chapter 1
Components
of Earthing
system
Chapter 2
LV Earthing
systems
Chapter 3
MV Earthing
Systems
Chapter 4
Influence of
Earthling
System in
Protection
System
Chapter 5
Selection of
Earthing
systems

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prepared by:
Chapter 1
•Mohamed Abd Elnasser
•Mahmoud Osama
•Bassel Yasser Abdallah
•Ibrahiem Shrief
•Hossam Hassan Abd Allah
Chapter 2
•Ahmed Ayman Emam
•Hadeel Khalid
•Reham Al-Husseiny
•Mohamed Ragab
•Ahmed Yahya Zakaria
Chapter 3
•Ahmed Magdy Mohamed
•Mohamed Tarek Abo el Safa
•Mohamed Essam shawky
•Oma Esmail Saad
•Mohamed Magdy Abdallah
chapter 4
•Rahaf Waheb
•Hossam El Den khaled
•Abdallah Abd Elazez
•Omar Sherief
•Mohamed Amin Mohamed
Chapter 5
•Mohamed Essam Elden
•Marwa Ghareb

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Chapter 1
Components of Earthing
system

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What Are Some Different Types of Grounding Electrodes?
Grounding is the process of electrically connecting any metallic object to
the earth by the way of an earth electrode system. The National Electric
Code requires that the grounding electrodes be tested to ensure that they
are under 25-ohms resistance-to-ground (Earth). It is important to know
that aluminum electrodes are not allowed for use in grounding.
Driven Rod
The standard driven rod or copper-clad rod consists of an 8 to 10 foot
length of steel with a 5 to 10-mil coating of copper. This is by far the
most common grounding device used in the field today. The driven rod
has been in use since the earliest days of electricity with a history dating
as far back as Benjamin Franklin.
Driven rods are relatively inexpensive to purchase, however ease of
installation is dependent upon the type of soil and terrain where the rod is
to be installed. The steel used in the manufacture of a standard driven rod
tends to be relatively soft. Mushrooming can occur on both the tip of the
rod, as it encounters rocks on its way down, and the end where force is
being applied to drive the rod through the earth. Driving these rods can be
extremely labor-intensive when rocky terrain creates problems as the tips
of the rods continue to mushroom. Often, these rods will hit a rock and
actually turn back around on themselves and pop back up a few feet away
from the installation point.

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Because driven rods range in length from 8 to 10 feet, a ladder is often
required to reach the top of the rod, which can become a safety issue.
Many falls have resulted from personnel trying to literally ‘whack’ these
rods into the earth, while hanging from a ladder, many feet in the air.
The National Electric Code (NEC) requires that driven rods be a
minimum of 8 feet in length and that 8 feet of length must be in direct
contact with the soil. Typically, a shovel is used to dig down into the
ground 18 inches before a driven rod is installed. The most common rods
used by commercial and industrial contractors are 10 ft in length. Many
industrial specifications require this length as a minimum.
A common misconception is that the copper coating on a standard driven
rod has been applied for electrical reasons. While copper is certainly a
conductive material, its real purpose on the rod is to provide corrosion
protection for the steel underneath. Many corrosion problems can occur
because copper is not always the best choice in corrosion protection. It
should be noted that galvanized driven rods have been developed to
address the corrosion concerns that copper presents, and in many cases
are a better choice for prolonging the life of the grounding rod and
grounding systems. Generally speaking, galvanized rods are a better
choice in all but high salt environments.
An additional drawback of the copper-clad driven rod is that copper and
steel are two dissimilar metals. When an electrical current is imposed,
electrolysis will occur. Additionally, the act of driving the rod into the
soil can damage the copper cladding, allowing corrosive elements in the
soil to attack the bared steel and further decrease the life expectancy of
the rod. Environment, aging, temperature and moisture also easily affect
driven rods, giving them a typical life expectancy of five to 15 years in
good soil conditions. Driven rods also have a very small surface area,
which is not always conducive to good contact with the soil. This is
especially true in rocky soils, in which the rod will only make contact on
the edges of the surrounding rock.
A good example of this is to imagine a driven rod surrounded by large
marbles. Actual contact between the marbles and the driven rod will be
very small. Because of this small surface contact with the surrounding
soil, the rod will increase in resistance-to-ground, lowering the
conductance, and limiting its ability to handle high-current faults.

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Grounding Plates
Grounding plates are typically thin copper plates buried in direct contact
with the earth. The National Electric Code requires that ground plates
have at least 2 ft2 of surface area exposed to the surrounding soil. Ferrous
materials must be at least .20 inches thick, while non-ferrous materials
(copper) need only be .060 inches thick. Grounding plates are typically
placed under poles or supplementing counterpoises.
As shown, grounding plates should be buried at least 30 inches below
grade level. While the surface area of grounding plates is greatly
increased over that of a driven rod, the zone of influence is relatively
small as shown in “B”. The zone of influence of a grounding plate can be
as small as 17 inches. This ultra-small zone of influence typically causes
grounding plates to have a higher resistance reading than other electrodes
of similar mass. Similar environmental conditions that lead to the failure
of the driven rod also plague the grounding plate, such as corrosion,
aging, temperature, and moisture.
Ufer Ground or Concrete Encased Electrodes
Originally, Ufer grounds were copper electrodes encased in the concrete
surrounding ammunition bunkers. In today’s terminology, Ufer grounds
consist of any concrete-encased electrode, such as the rebar in a building
foundation, when used for grounding, or a wire or wire mesh in concrete.

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Concrete Encased Electrode
The National Electric Code requires that Concrete Encased Electrodes
use a minimum No. 4 AWG copper wire at least 20 feet in length and
encased in at least 2 inches of concrete. The advantages of concrete
encased electrodes are that they dramatically increase the surface area and
degree of contact with the surrounding soil. However, the zone of
influence is not increased; therefore the resistance to ground is typically
only slightly lower than the wire would be without the concrete.
Concrete encased electrodes also have some significant disadvantages.
When an electrical fault occurs, the electric current must flow through the
concrete into the earth. Concrete, by nature retains a lot of water, which
rises in temperature as the electricity flows through the concrete. If the
extent of the electrode is not sufficiently great for the total current
flowing, the boiling point of the water may be reached, resulting in an
explosive conversion of water into steam. Many concrete encased
electrodes have been destroyed after receiving relatively small electrical
faults. Once the concrete cracks apart and falls away from the conductor,
the concrete pieces act as a shield preventing the copper wire from
contacting the surrounding soil, resulting in a dramatic increase in the
resistance-to-ground of the electrode.
There are many new products available on the market designed to
improve concrete encased electrodes. The most common are modified
concrete products that incorporate conductive materials into the cement
mix, usually carbon. The advantage of these products is that they are
fairly effective in reducing the resistivity of the concrete, thus lowering
the resistance-to-ground of the electrode encased. The most significant

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improvement of these new products is in reducing heat buildup in the
concrete during fault conditions, which can lower the chances that steam
will destroy the concrete encased electrode. However some disadvantages
are still evident. Again, these products do not increase the zone-of-
influence and as such the resistance-to-ground of the concrete encased
electrode is only slightly better than what a bare copper wire or driven
rod would be in the ground. Also a primary concern regarding enhanced
grounding concretes is the use of carbon in the mix. Carbon and copper
are of different nobilities and will sacrificially corrode each other over
time. Many of these products claim to have buffer materials designed to
reduce the accelerated corrosion of the copper caused by the addition of
carbon into the mix. However, few independent long-term studies are
being conducted to test these claims.
Ufer Ground or Building Foundations
Ufer Grounds or building foundations may be used provided that the
concrete is in direct contact with the earth (no plastic moisture barriers),
that rebar is at least 0.500 inches in diameter and that there is a direct
metallic connection from the service ground to the rebar buried inside the
concrete.
This concept is based on the conductivity of the concrete and the large
surface area, which will usually provide a grounding system that, can
handle very high current loads. The primary drawback occurs during fault
conditions, if the fault current is too great compared with the area of the
rebar system, when moisture in the concrete superheats and rapidly
expands, cracking the surrounding concrete and the threatening the

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integrity of the building foundation. Another drawback to the Ufer
ground is they are not testable under normal circumstances as isolating
the concrete slab in order to properly perform resistance-to-ground testing
is nearly impossible. The metal frame of a building may also be used as a
grounding point, provided that the building foundation meets the above
requirements, and is commonly used in high-rise buildings. It should be
noted that many owners of these high-rise buildings are banning this
practice and insisting that tenants run ground wires all the way back to
the secondary service locations on each floor. The owners will have
already run ground wires from the secondary services back to the primary
service locations and installed dedicated grounding systems at these
service locations. The goal is to avoid the flow of stray currents, which
can interfere with the operation of sensitive electronic equipment.
Water Pipes
Water pipes have been used extensively over time as a grounding
electrode. Water pipe connections are not testable and are unreliable due
to the use of tar coatings and plastic fittings. City water departments have
begun to specifically install plastic insulators in the pipelines to prevent
the flow of current and reduce the corrosive effects of electrolysis. The
National Electric Code requires that at least one additional electrode be
installed when using water pipes as an electrode. There are several
additional requirements including:
• 10 feet of the water pipe is in direct contact with the earth
• Joints must be electrically continuous
• Water meters may not be relied upon for the grounding path
• Bonding jumpers must be used around any insulating joints, pipe or
meters
• Primary connection to the water pipe must be on the street side of the
water meter
• Primary connection to the water pipe shall be within five feet of the
point of entrance to the building
• The National Electric Code requires that water pipes be bonded to
ground, even if water pipes are not used as part of the grounding
system.

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Electrolytic Electrode
The electrolytic electrode was specifically engineered to eliminate the
drawbacks of other grounding electrodes. This active grounding electrode
consists of a hollow copper shaft filled with natural earth salts and
desiccants whose hygroscopic nature draws moisture from the air. The
moisture mixes with the salts to form an electrolytic solution that
continuously seeps into the surrounding backfill material, keeping it
moist and high in ionic content.
The electrolytic electrode is installed into an augured hole and backfilled
with a special highly conductive product. This specialty product should
protect the electrode from corrosion and improve its conductivity. The
electrolytic solution and the special backfill material work together to
provide a solid connection between the electrode and the surrounding soil
that is free from the effects of temperature, environment, and corrosion.
This active electrode is the only grounding electrode that improves with
age. All other electrode types will have a rapidly increasing resistance-to-
ground as the season’s change and the years pass. The drawbacks to these
electrodes are the cost of installation and the cost of the electrode itself.

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Earth-Electrode Comparison Chart
The following chart compares the various types of electrodes versus some
important characteristics that may prove helpful in selecting proper
electrode usage.
Driven
Rod
Grounding
Plate
Concrete
Encased
Electrode
Building
FoundationWater Pipe
Electrolytic
Electrode
Resistance-to-Ground
(RTG) Poor Poor Average
Above
Average
Poor to
Excellent** Excellent
Corrosion Resistance Poor Poor Good * Good * Varies High
Increase in RTG in
Cold Weather
Highly
Affected
Highly
Affected
Slightly
Affected
Slightly
Affected
Minimally
Affected
Minimally
Affected
Increase in RTG over
Time
RTG
Worsens
RTG
Increases
RTG
typically
unaffected
RTG
typically
unaffected
RTG
typically
unaffected
RTG
Improves
Electrode Ampacity Poor Average Average *
Above
Average *
Poor to
Excellent** Excellent
Installation Cost Average
Below
Average
Below
Average Average Average Poor
Life Expectancy
Poor 5–
10 years
Poor 5-10
years
Average
*15-20
years
Above
Average
*20-30
years
Below
Average*10
-15 years
Excellent30-
50 years

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MAIN EARTHING TERMINAL OR BAR
Main earthing bar is to be provided at point of service entrance or main
distribution room, and as described in the Specification. Connect all
earthing conductors, protective conductors and bonding conductors to the
main earthing bar. Provide 2 insulated main earthing conductors, I at each
end of the bar, connected via testing joints to the earth electrode at 2
separate earth pits. Conductor is to be sized to carry maximum earth fault
current of system at point of application with final conductor temperature
not exceeding 160 deg. C (320 deg. F) for at least 5 seconds. Main
earthing conductors are to be minimum 120 mm2 or as otherwise
required by the particular Section of the Specification. Main earthing bar
shall be positioned at an accessible location within the electrical room
and clearly labeled.
The main earth bar shall be in the form of a ring or rings of bare
conductors surrounding or within an area in which items to be earthed are
located. Where 2 or more rings are installed they shall be interconnected
by at least two conductors, which shall be widely separated.
Testing joints (test links) are to be provided, in an accessible position, on
each main earthing conductor, between earthing terminal or bar and earth
electrode.

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Earth bar design
1) Component of Earth bar
1: Outgoing Way
Outgoing ways enable the earth connection to be isolated from the test
link.
This enables the test link to operate without the need to loosen the earth
connection to the Earth Bar.
2: Standard Link
Standard test links are supplied on all Earth Bars unless outgoing ways
are requested.
3- Parallel Fixings
Parallel fixings are typically used when there are space restrictions
impacting on the installation of the Earth Bar.
By using parallel fixings, the length of an Earth Bar can be significantly
reduced. Also, parallel fixings can be used to accommodate cable lugs
with two fixing holes.
4-Staggered Fixings
Like parallel fixings, staggered fixings are typically utilized to lower the
overall length of an Earth Bar in the event of space restrictions.
Parallel fixings can also be spaced to allow cable lug connections to be
made from both sides of the Earth Bar.
5- No Fixings
Earth Bars can also be supplied without fixings. Options include punched
holes, tapped holes or plain bar.

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6- Fixings
The Earth Bars are supplied with M10 brass fixings as standard.
We can also supply fixings of different size and material. These include
M4 – M16 sizes and stainless steel, phosphor bronze and BZP materials.
7- Bases
Supply all Earth Bars on metallic bases. All standard stock Earth Bars up
to 12 ways (standard, single and twin link) are supplied on powder coated
bases. Can also supply galvanized and
Hot-dipped galvanized bases for use in external or more humid
environments e.g. coastal.
Fixing Centers
The Earth Bars are supplied as standard with a minimum 35mm distance
between fixing centers. Typically, this allows for up to a 185mm² cable
lugs to be installed on adjacent fixings. Larger cable lugs may require
larger fixing center’s to accommodate wider cable lug palms.
2) Standard and tinned Earth Bars
Standard Earth Bars are manufactured from bare copper bar. In most
applications this does not cause any problems since the Earth Bars are
located on the inside of a building, usually inside a dry, warm
substation/communication room.
When this is not possible or practical, the Earth bar has to be located
externally in a location that has higher moisture or humidity. In these
cases, we recommend the use of a Tinned Earth Bar.
Tin is a soft white metal. It can easily be polished, scratch brushed or
flow melted to give a bright finish. It is non-toxic and it is not greatly
affected by organic acids. Sulphur compounds do not readily tarnish tin.
Neither is it impaired by air or water. Tin is one of the least susceptible
metals to corrosion.

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Benefits of a Tinned Earth Bar
Tinning a copper bar protects against atmospheric corrosion and hence
provides a longer life when exposed to corrosive atmospheres.
A layer of tin protects the copper in the Earth Bar from the formation of
copper oxide, thus preventing oxidation.
Tinned Earth Bars may be used in external applications or where
atmospheric conditions are more severe and aggressive than normal ie
high moisture content areas, high humidity etc.
A tinned Earth Bar resists corrosion from water.
Types of Earth bars
Standard earth bar
Standard and Tinned Earth Bars are an efficient and convenient way of
providing a common earth point.
The Standard and Tinned Earth Bars are supplied with a powder coated
base as standard. The standard connections are M10.
1. Standard Earth Bars
2. Standard Tinned Earth Bars
Single link earth bar
Earth Bars with Single Disconnecting Link are supplied with a powder
coated base and M10 connections as standard.
The Single Disconnecting Link is mainly used to offer a temporary break
in the connection to the earth allowing the inspection and testing of
multiple Earth Rods/systems while disconnecting it from the Lightning
and Earthing system.
1. Earth Bars with Single Disconnecting Link
2. Tinned Earth Bars with Single Disconnecting Link

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Twin link earth bar
Twin Disconnecting Links give greater flexibility to offer a temporary
break in the connection to the earth allowing the inspection and testing of
multiple Earth Rods/systems while disconnecting it from the Lightning
and Earthing system.
1. Earth Bars with Twin Disconnecting Link
2. Tinned Earth Bars with Twin Disconnecting Links

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Equipotential bonding:
• Is the connection of all metal elements together, which are not
designed to transmit electricity, in a room or building, to protect
against electric shocks.
• In case of a failure in the electrical insulation, all the metal objects
in the room will have the same electrical voltage, so that the person
in the room cannot touch two different components in the voltage.
• Even if the grounding system is lost, the person will be protected
from the difference of the potential difference as the high voltage
difference means a danger to the human.
Two figures show how we connect metal elements in
Equipotential bonding method.

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What requires protective equipotential bonding?
• Other installation pipework and ducting
• Central heating and air conditioning systems
• Exposed metallic structural parts of the building.
• Gas installation pipes
• Water installation pipes
Supplementary bonding conductors
These conductors connect together extraneous conductive parts - that is,
metalwork which is not associated with the electrical installation but
which may provide a conducting path giving rise to shock. The object is
to ensure that potential differences in excess of 50 V between accessible
metalwork cannot occur; this means that the resistance of the bonding
conductors must be low this table shows some of the extraneous
metalwork in a bathroom which must be bonded.

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The cross-sectional areas required for supplementary bonding conductors
are shown in the Table Where connections are between extraneous parts
only, the conductors may be 2.5 mm² if mechanically protected or 4 mm²
if not protected. If the circuit protective conductor is larger than 10 mm²,
the supplementary bonding conductor must have at least half this cross-
sectional area. Supplementary bonding conductors of less than 16 mm²
cross sectional area must not be aluminium.
The application of a supplementary bonding conductor to prevent the
severe shock which could otherwise occur between the live case of a
faulty electric kettle and an adjacent water tap.
There will sometimes be doubt if a particular piece of metalwork should
be bonded. The answer must always be that bonding will be necessary if
there is a danger of severe shock when contact is made between a live
system and the metal work in question. Thus if the resistance between the
metalwork and the general mass of earth is low enough to permit the
passage of a dangerous shock current, then the metalwork must be
bonded.
The question can be resolved by measuring the resistance (Rx) from the
metalwork concerned to the main earthing terminal. Using this value in
the formula:
Ib= Uo
Rp + Rx
Will allow calculation of the maximum current likely to pass through the
human body where:
Ib - is the shock current through the body (A)
Uo - Is the voltage of the supply (V)
RP -is the resistance of the human body (Ohms)
and
Rx - is the measured resistance from the
metalwork concerned
to the main earthing terminal (Ohms)
The resistance of the human body, RP can in most cases be taken as 1000
Ohms although 200 Ohms would be a safer value if the metalwork in
question can be touched by a person in a bath. Although no hard and fast
rules are possible for the value of a safe shock current, IB, it is probable
that 10 mA is seldom likely to prove fatal. Using this value with 240 V
for the supply voltage, up, and 1000 Ohms as the human body resistance,

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RP, the minimum safe value of RP calculates to 23 kOhms. If the safer
values of 5 mA for IB and 200 Ohms for RP are used, the value of Rx
would be 47.8 kOhms for a 240 V supply.
What is the difference between supplementary and equipotential
bonding?
Protective equipotential bonding is different from supplementary
bonding. Supplementary bonding is the practice of connecting two
conductive simultaneously accessible parts together to reduce the
potential difference between the parts
Exposed conductive part:
• Can readily be touched and which is not normally alive, but which
may become alive under fault conditions.

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Equipotential bonding inside and outside buildings
Stray currents are inevitably propagated in an earthing network. It is
impossible to eliminate all the sources of disturbances for a site. Earth
loops are also inevitable. When a magnetic field affects a site, e.g. the
field created by lightning, differences in potential are created in the loops
formed by the various conductors and the currents flowing in the earthing
system. Consequently, the earthing network is directly affected by any
counter-measures taken outside the building.
As long as the currents flow in the earthing system and not in the
electronic circuits, they do no damage. However, when earthing networks
are not equipotential, e.g. when they are star connected to the earth
electrode, the HF stray currents will flow wherever they can, including in
control wires. Equipment can be disturbed, damaged or even destroyed.
The only inexpensive means to divide the currents in an earthing system
and maintain satisfactory equipotential characteristics is to interconnect
the earthing networks. This contributes to better equipotential bonding
within the earthing system, but does not remove the need for protective
conductors. To meet legal requirements in terms of the safety of persons,
sufficiently sized and identified protective conductors must remain in
place between each piece of equipment and the earthing terminal. What is
more, with the possible exception of a building with a steel structure, a
large number of conductors for the lightning rods or the lightning-
protection network must be directly connected to the earth electrode.
The fundamental difference between a protective conductor (PE) and a
lightning rod down-conductor is that the first conducts internal currents to
the neutral of the MV/LV transformer whereas the second carries external
current (from outside the installation) to the earth electrode.
In a building, it is advised to connect an earthing network to all accessible
conducting structures, namely metal beams and door frames, pipes, etc. It
is generally sufficient to connect metal trunking, cable trays and lintels,
pipes, ventilation ducts, etc. at as many points as possible. In places
where there is a large amount of equipment and the size of the mesh in
the bonding network is greater than four metres, an equipotential
conductor should be added. The size and type of conductor are not of
critical importance.
It is imperative to interconnect the earthing networks of buildings that
have shared cable connections. Interconnection of the earthing networks
must take place via a number of conductors and all the internal metal

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structures of the buildings or linking the buildings (on the condition that
they are not interrupted).
In a given building, the various earthing networks (electronics,
computing, telecom, etc.) must be interconnected to form a single
equipotential bonding network.
This earthing-network must be as meshed as possible. If the earthing
network is equipotential, the differences in potential between
communicating devices will be low and a large number of EMC problems
disappear. Differences in potential are also reduced in the event of
insulation faults or lightning strikes.
If equipotential conditions between buildings cannot be achieved or if the
distance between buildings is greater than ten metres, it is highly
recommended to use optical fibre for communication links and galvanic
insulators for measurement and communication systems.
These measures are mandatory if the electrical supply system uses the IT
or TN-C system.
Earthing or grounding conductors.
The earthing conductor is commonly called the earthing lead. It joins the
installation earthing terminal to the earth electrode or to the earth terminal
provided by the Electricity Supply Company.
It is a vital link in the protective system, so care must be taken to see that
its integrity will be preserved at all times.
Aluminum conductors and cables may now be used for earthing and
bonding, but great care must be taken when doing so to ensure that there
will be no problems with corrosion or with electrolytic action where they
come into contact with other metals.
Where the final connection to the earth electrode or earthing terminal is
made there must be a clear and permanent label Safety Electrical
Connection - Do not remove

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Where a buried earthing conductor is not protected against mechanical
damage but is protected against corrosion by a sheath, its minimum size
must be 16 mm² whether made of copper or coated steel.
If it has no corrosion protection, minimum sizes for mechanically
unprotected earthing conductors are 25 mm² for copper and 50 mm² for
coated steel.
If not protected against corrosion the latter sizes again apply, whether
protected from mechanical damage or not.
Earthing conductors, as well as protective and bonding conductors, must
be protected against corrosion Probably the most common type of
corrosion is electrolytic, which is an electro-chemical effect between two
different metals when a current passes between them whilst they are in
contact with each other and with a weak acid.
The acid is likely to be any moisture which has become contaminated
with chemicals carried in the air or in the ground.
The effect is small on ac supplies because any metal removed whilst
current flows in one direction is replaced as it reverses in the next half
cycle. For dc Systems, however, it will be necessary to ensure that the
system remains perfectly dry (a very difficult task) or to use the
'sacrificial anode' principle.

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A main earth terminal or bar must be provided for each installation to
collect and connect together all protective and bonding conductors. It
must be possible to disconnect the earthing conductor from this terminal
for test purposes, but only by the use of a tool. This requirement is
intended to prevent unauthorized or unknowing removal of protection.

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Protective conductors types
The circuit protective conductor (increasingly called the 'c.p.c.') is a
system of conductors joining together all exposed conductive parts and
connecting them to the main earthing terminal. Strictly speaking, the term
includes the earthing conductor as well as the equipotential bonding
conductors.
The circuit protective conductor can take many forms, such as:
1- A separate conductor which must be green/yellow insulated if equal to
or less than 10 mm2
cross-sectional area.
2- A conductor included in a sheathed cable with other conductors.
3- The metal sheath and/or armoring of a cable.
4- Conducting cable enclosures such as conduit or trucking.
5- Exposed conductive parts, such as the conducting cases of equipment.
This list is by no means exhaustive and there may be many other items
forming parts of the circuit protective conductor as indicated in {Fig
5.10}.
Note that gas or oil pipes must not be used for the purpose, because of the
possible future change to plastic (non-conducting) pipes.
Fig 5.10 some types of circuit protective conductor

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Of course, very important that the protective conductor remains effective
throughout the life of the installation.
Thus, great care is needed to ensure that steel conduit used for the
purpose is tightly jointed and unlikely to corrode.
The difficulty of ensuring this point is leading to the increasing use of a
c.p.c. run inside the conduit with the phase conductors. Such a c.p.c. will,
of course, always be necessary where plastic conduits are used. Where an
accessory is connected to a system (for example, by means of a socket
outlet) which uses conduit as its c.p.c., the appliance (or socket outlet)
earthing terminal must be connected by a separate conductor to the earth
terminal of the conduit box (see {Fig 5.11}).
This connection will ensure that the accessory remains properly earthed
even if the screws holding it into the box become loose damaged or
corroded
Fig 5.11 Protective connection for socket outlet in conduit system

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Chapter 2
Earthing in LV systems

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1- Introduction to LV Earthing:
Electricity: is very important in our life, but it also has dangerous
problems that have bad effects on the electric power systems and human
beings as well.
For the electric power systems: In case of fault conditions, the fault
may lead to damage or failure in equipment of the system.
For human being: During these fault conditions, a high potential
difference between the metal parts of the system that should not be
carrying current in the normal operation and earth will be produced. If a
man touches these parts, he will be exposed to an electric shock and it can
be lethal. All of these hazards urged the engineers to do something to
limit these problems and protect man life, so earthing was introduced.
2- What is the difference between Neutral, Earth AND
Ground?
Neutral: The neutral is the common point of three star-connected
windings.
Earthing: The term Earthing means that the circuit is physically
connected to the ground which is zero-volt potential (Earth).
Earth is the conductive mass of earth, whose electric potential at any
point is conventionally taken as zero
Grounding: The term Grounding means that the circuit is not physically
connected to ground, but its potential is zero with respect to other points.
System earthing
System Earthing is a connection of the conductors of a distribution the
system to the earth
Safety earthing / Equipment earthing
Safety Earthing is a connection of one or more of the non-currents
carrying metal parts (frames or enclosures) to the earth.

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3- What Is Step and Touch Potential?
Step Potential
Step potential is the step voltage between the feet of a person standing
near an energized grounded object. It is equal to the difference in
voltage, given by the voltage distribution curve, between two points at
different distances from the electrode. A person could be at risk of
injury during a fault simply by standing near the grounding point.
Touch Potential

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Touch potential is the touch voltage between the energized object and
the feet of a person in contact with the object. It is equal to the
difference in voltage between the object and a point some distance
away. The touch potential or touch voltage could be nearly the full
voltage across the grounded object if that object is grounded at a point
remote from the place where the person is in contact with it. For
example, a crane that was grounded to the system neutral and that
contacted an energized line would expose any person in contact with
the crane or its uninsulated load line to a touch potential nearly equal
to the full fault voltage.
But what’s the difference between their effects:-
Step Potential effect:
When a fault occurs at a tower or substation, the current will enter the
earth. Based on the distribution of varying resistivity in the soil
(typically, a horizontally layered soil is assumed) a corresponding
voltage distribution will occur. The voltage drop in the soil
surrounding the grounding system can present hazards for personnel
standing in the vicinity of the grounding system. Personnel “stepping”
in the direction of the voltage gradient could be subjected to hazardous
voltages.

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In the case of Step Potentials or step voltage, electricity will flow if a
difference in potential exists between the two legs of a person.
Calculations must be performed that determine how great the tolerable
step potentials are and then compare those results to the step voltages
expected to occur at the site.
Hazardous Step Potentials or step voltage can occur a significant
distance away from any given site. The more current that is pumped
into the ground, the greater the hazard. Soil resistivity and layering
plays a major role in how hazardous a fault occurring on a specific site
may be. High soil resistivities tend to increase Step Potentials. A high
resistivity top layer and low resistivity bottom layer tends to result in
the highest step voltages close to the ground electrode: the low
resistivity bottom layer draws more current out of the electrode
through the high resistivity layer, resulting in large voltage drops near
the electrode. Further from the ground electrode, the worst case
scenario occurs when the soil has conductive top layers and resistive
bottom layers: in this case, the fault current remains in the conductive
top layer for much greater distances away from the electrode.
Fault clearing time is an important factor to consider as well. The
more time it takes the electric utility company to clear the fault, the
more likely it is for a given level of current to cause the human heart
to fibrillate.
Touch Potential effect:
When a fault occurs at a tower or substation, the current will pass
through any metallic object and enter the earth. Those personnel
“touching” an object in the vicinity of the GPR will be subjected to
these touch voltages which may be hazardous.

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For example if a person happens to be touching a high-voltage tower
leg when a fault occurs, the electricity would travel down the tower
leg into the person’s hand and through vital organs of the body. It
would then continue on its path and exit out through the feet and into
the earth. Careful analysis is required to determine the acceptable
Fibrillation Currents that can be withstood by the body if a fault were
to occur.
Engineering standards use a one-meter (3.28 ft) reach distance for
calculating Touch Potentials. A two-meter (6.54 ft) reach distance is
used when two or more objects are inside the GPR event area. For
example, a person could be outstretching both arms and touching two
objects at once such as a tower leg and a metal cabinet. Occasionally,
engineers will use a three-meter distance to be particularly cautious, as
they assume someone may be using a power tool with a power cord 3
meters in length.
The selection of where to place the reference points used in the Touch
Potential or touch voltage calculations are critical in getting an
accurate understanding of the level of hazard at a given site.
The actual calculation of Touch Potentials uses a specified object
(such as a tower leg) as the first reference point.
This means that the further away from the tower the other reference
point is located, the greater the difference in potential.
If you can imagine a person with incredibly long arms touching the
tower leg and yet standing many dozens of feet away, you would have
a huge difference in potential between their feet and the tower.

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Obviously, this example is not possible: this is why setting where and
how far away the reference points used in the touch calculation is so
important and why the one-meter rule has been established.
Mitigating Step and Touch Potential hazards is usually accomplished
through one or more of the following three (3) main techniques:
• Reduction in the Resistance to Ground of the grounding system
• Proper placement of ground conductors
• The addition of resistive surface layers
4- Critical Currents Thresholds
Standard IEC Standard IEC 60479 60479--1 1

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5- Different types of neutral point connection to earth.
The neutral may or may not be not be earthed.
The different types of neutral point connection to earth are:
➢ Solidly (or directly) earthed neutral
➢ Unearthed or Isolated Neutral, or high impedance-earthed
neutral
➢ Resistance Earthed Neutral
➢ Reactance earthed neutral
➢ Petersen coil earthing.
The neutral may be connected to earth either directly or via a resistor or
reactor.
When there is no connection between the neutral point and earth, we say
that the neutral is isolated or unearthed.
Importance of Neutral Grounding:
There are many neutral grounding options available for both Low and
Medium voltage power systems. The neutral points of transformers,
generators and rotating machinery to the earth ground network provides a
reference point of zero volts. This protective measure offers many
advantages over an ungrounded system, like:-
➢ Reduced magnitude of transient over voltages
➢ Simplified ground fault location
➢ Improved system and equipment fault protection
➢ Reduced maintenance time and expense
➢ Greater safety for personnel
➢ Improved lightning protection
➢ Reduction in frequency of faults.

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Let’s have some notes about each type of neutral point connection to
earth:
➢ Unearthed neutral/ High impedance earthing
There is no electrical connection between the neutral point and earth,
except for measuring and protective devices.
▪ In ungrounded system there is no internal connection between the
conductors and earth. However, as system, a capacitive coupling exists
between the system conductors and the adjacent grounded surfaces.
Consequently, the “ungrounded system” is, in reality, a “capacitive
grounded system” by virtue of the distributed capacitance.
▪ Under normal operating conditions, this distributed capacitance causes
no problems. In fact, it is beneficial because it establishes, in effect, a
neutral point for the system; As a result, the phase conductors are
stressed at only line-to-neutral voltage above ground.
▪ But problems can rise in ground fault conditions. A ground fault on
one-line results in full line-to-line voltage appearing throughout the
system. Thus, a voltage 1.73 times the normal voltage is present on all
insulation in the system. This situation can often cause failures in older
motors and transformers, due to insulation breakdown.
Advantage:
1. After the first ground fault, assuming it remains as a single fault, the
circuit may continue in operation, permitting continued production
until a convenient shut down for maintenance can be scheduled.
Disadvantages:
1. The interaction between the faulted system and its distributed
capacitance may cause transient over-voltages (several times normal)
to appear from line to ground during normal switching of a circuit
having a line-to ground fault (short). These over voltages may cause
insulation failures at points other than the original fault.
2. A second fault on another phase may occur before the first fault can be
cleared. This can result in very high line-to-line fault currents,
equipment damage and disruption of both circuits.
3. The cost of equipment damage.

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4. Complicate for locating fault(s), involving a tedious process of trial
and error: first isolating the correct feeder, then the branch, and finally,
the equipment at fault. The result is unnecessarily lengthy and
expensive down downtime.
High impedance is inserted between the neutral point and earth.
➢ Solidly earthed neutral
An electrical connection is intentionally made between the
neutral point and earth.
▪ Solidly grounded systems are usually used in low voltage applications
at 600 volts or less.
▪ In solidly grounded system, the neutral point is connected to earth.
▪ Solidly Neutral Grounding slightly reduces the problem of transient
over voltages found on the ungrounded system and provided path for
the ground fault current is in the range of 25 to 100% of the system

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three phase fault current. However, if the reactance of the generator or
transformer is too great, the problem of transient over voltages will not
be solved.
▪ While solidly grounded systems are an improvement over ungrounded
systems, and speed up the location of faults, they lack the current
limiting ability of resistance grounding and the extra protection this
provides.
▪ To maintain systems health and safe, Transformer neutral is grounded
and grounding conductor must be extend from the source to the
furthest point of the system within the same raceway or conduit. Its
purpose is to maintain very low impedance to ground faults so that a
relatively high fault current will flow thus insuring that circuit breakers
or fuses will clear the fault quickly and therefore minimize damage. It
also greatly reduces the shock hazard to personnel
▪ If the system is not solidly grounded, the neutral point of the system
would “float” with respect to ground as a function of load subjecting
the line-to-neutral loads to voltage unbalances and instability.
▪ The single-phase earth fault current in a solidly earthed system may
exceed the three phase fault current. The magnitude of the current
depends on the fault location and the fault resistance. One way to
reduce the earth fault current is to leave some of the transformer
neutrals unearthed.
Advantage:
1. The main advantage of solidly earthed systems is low over voltages,
which makes the earthing design common at high voltage levels (HV).
Disadvantage:
1. This system involves all the drawbacks and hazards of high earth fault
current: maximum damage and disturbances.
2. There is no service continuity on the faulty feeder.
3. The danger for personnel is high during the fault since the touch
voltages created are high.
Applications:
1. Distributed neutral conductor.
2. 3-phase + neutral distribution.
3. Use of the neutral conductor as a protective conductor with systematic
earthing at each transmission pole.
4. Used when the short-circuit power of the source is low.

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➢ Resistance earthing
A resistor is inserted between the neutral point and earth.
▪ Resistance grounding has been used in three-phase industrial
applications for many years and it resolves many of the problems
associated with solidly grounded and ungrounded systems.
▪ Resistance Grounding Systems limits the phase-to-ground fault
currents. The reasons for limiting the Phase to ground Fault current by
resistance grounding are:
1. To reduce burning and melting effects in faulted electrical equipment
like switchgear, transformers, cables, and rotating machines.
2. To reduce mechanical stresses in circuits/Equipment’s carrying fault
currents.
3. To reduce electrical-shock hazards to personnel caused by stray ground
fault.
4. To reduce the arc blast or flash hazard.
5. To reduce the momentary line-voltage dip.
6. To secure control of the transient over-voltages while at the same time.
7. To improve the detection of the earth fault in a power system.
▪ Grounding Resistors are generally connected between ground and
neutral of transformers, generators and grounding transformers to limit
maximum fault current as per Ohms Law to a value which will not
damage the equipment in the power system and allow sufficient flow
of fault current to detect and operate Earth protective relays to clear the
fault. Although it is possible to limit fault currents with high resistance
Neutral grounding Resistors, earth short circuit currents can be
extremely reduced. As a result of this fact, protection devices may not
sense the fault.

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▪ Therefore, it is the most common application to limit single phase fault
currents with low resistance Neutral Grounding Resistors to
approximately rated current of transformer and / or generator.
▪ In addition, limiting fault currents to predetermined maximum values
permits the designer to selectively coordinate the operation of
protective devices, which minimizes system disruption and allows for
quick location of the fault.
▪ There are two categories of resistance grounding:
(1) Low resistance Grounding.
(2) High resistance Grounding.
▪ Ground fault current flowing through either type of resistor when a
single phase faults to ground will increase the phase-to-ground voltage
of the remaining two phases. As a result, conductor insulation and
surge arrestor ratings must be based on line-to-line voltage. This
temporary increase in phase-to-ground voltage should also be
considered when selecting two and three pole breakers installed on
resistance grounded low voltage systems.
▪ The increase in phase-to-ground voltage associated with ground fault
currents also precludes the connection of line-to-neutral loads directly
to the system. If line-to neutral loads (such as 277V lighting) are
present, they must be served by a solidly grounded system. This can be
achieved with an isolation transformer that has a three-phase delta
primary and a three-phase, four-wire, wye secondary
▪ Neither of these grounding systems (low or high resistance) reduces
arc-flash hazards associated with phase-to-phase faults, but both
systems significantly reduce or essentially eliminate the arc-flash
hazards associated with phase-to-ground faults. Both types of
grounding systems limit mechanical stresses and reduce thermal
damage to electrical equipment, circuits, and apparatus carrying
faulted current.
▪ The difference between Low Resistance Grounding and High
Resistance Grounding is a matter of perception and, therefore, is not
well defined. Generally speaking high-resistance grounding refers to a
system in which the NGR let-through current is less than 50 to 100 A.
Low resistance grounding indicates that NGR current would be above
100 A.

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▪ A better distinction between the two levels might be alarm only and
tripping. An alarm-only system continues to operate with a single
ground fault on the system for an unspecified amount of time. In a
tripping system a ground fault is automatically removed by protective
relaying and circuit interrupting devices. Alarm-only systems usually
limit NGR current to 10 A or less.
Rating of The Neutral grounding resistor:
1. 1. Voltage: Line-to-neutral voltage of the system to which it is
connected.
2. 2. Initial Current: The initial current which will flow through the
resistor with rated voltage applied.
3. 3. Time: The “on time” for which the resistor can operate without
exceeding the allowable temperature rise.
(A). Low Resistance Grounded:
▪ Low Resistance Grounding is used for large electrical systems where
there is a high investment in capital equipment or prolonged loss of
service of equipment has a significant economic impact and it is not
commonly used in low voltage systems because the limited ground
fault current is too low to reliably operate breaker trip units or fuses.
This makes system selectivity hard to achieve. Moreover, low
resistance grounded systems are not suitable for 4-wire loads and
hence have not been used in commercial market applications
▪ A resistor is connected from the system neutral point to ground and
generally sized to permit only 200A to 1200 amps of ground fault
current to flow. Enough current must flow such that protective devices
can detect the faulted circuit and trip it off-line but not so much current
as to create major damage at the fault point.
▪ Since the grounding impedance is in the form of resistance, any
transient over voltages are quickly damped out and the whole transient
overvoltage phenomena is no longer applicable. Although theoretically

43| P a g e
possible to be applied in low voltage systems (e.g. 480V),significant
amount of the system voltage dropped across the grounding resistor,
there is not enough voltage across the arc forcing current to flow, for
the fault to be reliably detected. For this reason, low resistance
grounding is not used for low voltage systems (under 1000 volts line
to-line).
Advantages:
1. Limits phase-to-ground currents to 200-400A.
2. Reduces arcing current and, to some extent, limits arc-flash hazards
associated with phase-to-ground arcing current conditions only.
3. May limit the mechanical damage and thermal damage to shorted
transformer and rotating machinery windings.
Disadvantages:
1. Does not prevent operation of over current devices.
2. Does not require a ground fault detection system.
3. May be utilized on medium or high voltage systems.
4. Conductor insulation and surge arrestors must be rated based on the
line to-line voltage. Phase-to-neutral loads must be served through an
isolation transformer.
Used: Up to 400 amps for 10 sec are commonly found on medium
voltage systems.
(B).High Resistance Grounded:
▪ High resistance grounding is almost identical to low resistance
grounding except that the ground fault current magnitude is typically
limited to 10 amperes or less. High resistance grounding accomplishes
two things.
▪ The first is that the ground fault current magnitude is sufficiently low
enough such that no appreciable damage is done at the fault point. This
means that the faulted circuit need not be tripped off-line when the
fault first occurs. Means that once a fault does occur, we do not know
where the fault is located. In this respect, it performs just like an
ungrounded system.
▪ The second point is it can control the transient overvoltage
phenomenon present on ungrounded systems if engineered properly.
▪ Under earth fault conditions, the resistance must dominate over the
system charging capacitance but not to the point of permitting
excessive current to flow and thereby excluding continuous operation

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▪ High Resistance Grounding (HRG) systems limit the fault current
when one phase of the system shorts or arcs to ground, but at lower
levels than low resistance systems.
▪ In the event that a ground fault condition exists, the HRG typically
limits the current to 5-10A.
▪ HRG’s are continuous current rated, so the description of a particular
unit does not include a time rating. Unlike NGR’s, ground fault current
flowing through a HRG is usually not of significant magnitude to
result in the operation of an over current device. Since the ground fault
current is not interrupted, a ground fault detection system must be
installed.
▪ These systems include a bypass contactor tapped across a portion of
the resistor that pulses (periodically opens and closes). When the
contactor is open, ground fault current flows through the entire
resistor. When the contactor is closed a portion of the resistor is
bypassed resulting in slightly lower resistance and slightly higher
ground fault current.
▪ To avoid transient over-voltages, an HRG resistor must be sized so that
the amount of ground fault current the unit will allow to flow exceeds
the electrical system’s charging current. As a rule of thumb, charging
current is estimated at 1A per 2000KVA of system capacity for low
voltage systems and 2A per 2000KVA of system capacity at 4.16kV.
▪ These estimated charging currents increase if surge suppressors are
present. Each set of suppressors installed on a low voltage system
results in approximately 0.5A of additional charging current and each
set of suppressors installed on a 4.16kV system adds 1.5A of additional
charging current.
▪ A system with 3000KVA of capacity at 480 volts would have an
estimated charging current of 1.5A.Add one set of surge suppressors
and the total charging current increases by 0.5A to 2.0A. A standard
5A resistor could be used on this system. Most resistor manufacturers
publish detailed estimation tables that can be used to more closely
estimate an electrical system’s charging current.

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Advantages:
1. Enables high impedance fault detection in systems with weak
capacitive connection to earth
2. Some phase-to-earth faults are self-cleared.
3. The neutral point resistance can be chosen to limit the possible over
voltage transients to 2.5 times the fundamental frequency maximum
voltage.
4. Limits phase-to-ground currents to 5-10A.
5. Reduces arcing current and essentially eliminates arc-flash hazards
associated with phase-to-ground arcing current conditions only.
6. Will eliminate the mechanical damage and may limit thermal damage
to shorted transformer and rotating machinery windings.
7. Prevents operation of over current devices until the fault can be located
(when only one phase faults to ground).
8. May be utilized on low voltage systems or medium voltage systems up
to 5kV. IEEE Standard 141-1993 states that “high resistance grounding
should be restricted to 5kV class or lower systems with charging
currents of about 5.5A or less and should not be attempted on 15kV
systems, unless proper grounding relaying is employed”.
9. Conductor insulation and surge arrestors must be rated based on the
line to-line voltage. Phase-to-neutral loads must be served through an
isolation transformer.
Disadvantages:
1. Generates extensive earth fault currents when combined with strong or
moderate capacitive connection to earth Cost involved.
2. Requires a ground fault detection system to notify the facility engineer
that a ground fault condition has occurred.

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➢ Reactance earthing
A reactor is inserted between the neutral point and earth.
▪ Adding inductive reactance from the system neutral point to ground is
an easy method of limiting the available ground fault from something
near the maximum 3 phase short circuit capacity (thousands of
amperes) to a relatively low value (200 to 800 amperes).
▪ To limit the reactive part of the earth fault current in a power system a
neutral point reactor can be connected between the transformer neutral
and the station earthing system.
▪ A system in which at least one of the neutrals is connected to earth
through an Inductive reactance.
▪ The current generated by the reactance during an earth fault
approximately compensates the capacitive component of the single
phase earth fault current, is called a resonant earthed system.
▪ The system is hardly ever exactly tuned, i.e. the reactive current does
not exactly equal the capacitive earth fault current of the system.
▪ A system in which the inductive current is slightly larger than the
capacitive earth fault current is over compensated. A system in which
the induced earth fault current is slightly smaller than the capacitive
earth fault current is under compensated

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➢ Petersen coil earthing
A reactor tuned to the network capacitances is inserted between
the neutral point and earth so that if an earth fault occurs, the
fault current is zero.
➢ 𝑰 𝒇
⃗⃗⃗ = 𝑰 𝑳
⃗⃗⃗ + 𝑰 𝑪
⃗⃗⃗ = 𝟎⃗⃗
➢ 𝑰 𝒇 = fault current
➢ 𝑰 𝑳 = current in the neutral earthing reactor
➢ 𝑰 𝑪 = current in phase to earth capacitances
▪ A Petersen Coil is connected between the neutral point of the system
and earth, and is rated so that the capacitive current in the earth fault is
compensated by an inductive current passed by the Petersen Coil. A
small residual current will remain, but this is so small that any arc
between the faulted phase and earth will not be maintained and the
fault will extinguish. Minor earth faults such as a broken pin insulator,
could be held on the system without the supply being interrupted.
Transient faults would not result in supply interruptions.
▪ Although the standard ‘Peterson coil’ does not compensate the entire
earth fault current in a network due to the presence of resistive losses
in the lines and coil, it is now possible to apply ‘residual current
compensation’ by injecting an additional 180° out of phase current into
the neutral via the Peterson coil. The fault current is thereby reduced to
practically zero. Such systems are known as ‘Resonant earthing with
residual compensation’, and can be considered as a special case of
reactive earthing.

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▪ Resonant earthing can reduce EPR to a safe level. This is because the
Petersen coil can often effectively act as a high impedance NER,
which will substantially reduce any earth fault currents, and hence also
any corresponding EPR hazards (e.g. touch voltages, step voltages and
transferred voltages, including any EPR hazards impressed onto nearby
telecommunication networks).
Advantages:
1. Small reactive earth fault current independent of the phase to earth
capacitance of the system.
2. Enables high impedance fault detection.
Disadvantages:
1. Risk of extensive active earth fault losses.
2. High costs associated.
Comparison of Neutral Earthing System:
Condition
Un
grounded
Solid
Grounded
Low
Resistance
Grounded
High
Resistance
Grounded
Reactance
Grounding
Immunity to
Transient
Over
voltages
Worse Good Good Best Best
73%
Increase in
Voltage
Stress Under
Line-to-
Ground
Fault
Condition
Poor Best Good Poor
Equipment
Protected
Worse Poor Better Best Best

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Safety to
Personnel
Worse Better Good Best Best
Service
Reliability
Worse Good Better Best Best
Maintenance
Cost
Ease of
Locating
First Ground
Fault
Permits
Designer to
Coordinate
Protective
Devices
Not
Possible
Good Better Best Best
Reduction in
Frequency
of Faults
Worse Better Good Best Best
Lighting
Arrestor
Ungrounded
neutral type
Grounded-
neutral
type
Ungrounded
neutral type
Ungrounded
neutral type
Ungrounded
neutral type
Current for
phase-to
ground fault
in percent of
three-phase
fault current
Less than
1%
Varies,
may be
100% or
greater
5 to 20%
Less than
1%
5 to 25%

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6- Types of Earthing systems
The BS 7671 lists five different types of earthing systems as
follows:
1. TN-S system.
2. TN-C-S system.
3. TT system.
4. TN-C system.
5. IT system.
Where:
➢ T = Earth
➢ N = Neutral
➢ S = Separate
➢ C = Combined
➢ I = Isolated
6.1 TT Earthing system technique
The neutral point of the LV transformer is directly connected to an earth
electrode.
The exposed conductive parts of the installation are connected to an
electrically separate earth electrode.

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6.2 IT earthing system technique

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6.3 TT System:
➢ One point at the supply source is connected directly to earth. All
exposed and extraneous conductive parts are connected to a
separate earth electrode at the installation.
➢ Earth electrode must have a low resistance to be able trip the
circuit breaker. But sometime it is difficult to achieve low

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resistance. Therefore, RCD device must be used to protect for
leakage current in the circuit.
➢ PE and N must never be connected together.
Characteristic of TT:

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6.4 TN-C System:
➢ Combined PE and N conductor all the way from source to the
device. The neutral conductor is used as a protective conductor.
➢ It is not permitted for conductors of less than 10 or for portable
equipment.
➢ PEN conductor must be connected to a number of earth electrodes
in the installation since the TN-C system requires an effective
equipotential environment within the installation.
➢ Caution: In the TN-C system, the protective conductor function
has priority over the neutral function. Thus, a PEN conductor must
always be connected to the earthing terminal of a load and a
jumper is used to connect this terminal to the neutral terminal
➢ Example

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✓ To be able to apply TN-C system sizes of conductors must be at
least 10 .
✓ PEN conductor must be connected to earthing terminal of a device
because protection has priority with respect to return current. And a
jumper is used to connect to neutral of the device.
6.5 TN-S System:
✓ Protective earth (PE) and neutral (N) conductors are separated from
source to device.
✓ This system is obligatory for circuits with cross-sectional areas less
than 10 devices.
✓ PE and N conductors are never connected together in TN-S system.
6.6 TN-C-S Systems:

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✓ Combined PEN conductor from source to building distribution point,
but separate PE and N conductors in fixed indoor wiring and flexible
power cords.
✓ TN-C and TN-S systems can be used in the same installation.
✓ In the TN-C-S system, TN-C (4 wires) system must never be used
downstream of the TN-S (5 wires) system, since any accidental
interruption in the neutral on the upstream part would lead to an
interruption in the protective conductor in the downstream part and
therefore create a danger.
✓ TN-C-S System (Example):
✓ 5 wire system or TN-S can be used for any size conductors but
mandatory for conductors with sizes less than 10 .
✓ TN-C and TN-S systems can be used in the same installation.
In the TN-C-S system, TN-C (4 wires) system must never be used
downstream of the TN-S (5 wires) system, since any accidental
interruption in the neutral on the upstream part would lead to an
interruption in the protective conductor in the downstream part and
therefore create a danger.
Characteristic of TN:

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6.7 IT System:
✓ No intentional connection is made between the neutral point of the
supply source and earth.
✓ Exposed and extraneous conductive parts of the installation are
connected to an earth electrode.
✓ IT System: Leakage impedances

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In practice, all circuits have leakage impedances to earth. For
example, in a LV 3-phase 3-wire system, 1km of cable will have
leakage impedance which is equivalent to 3-4 kΩ.
✓ IT System: Impedance-earthed neutral
➢ Impedance in order of 1-2 kΩ is connected permanently between
the neutral point of the transformer LV winding and earth.
➢ The reason of connecting impedance to neutral is to fix the
potential of a small network with respect to earth and to reduce the
level of overvoltage, such as transmitted surges from the MV
windings, static charges, etc. with respect of earth.
➢ Impedance slightly increases the first-fault current level.
Characteristic of IT:

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Measurements and calculations of earthing resistance :
The purpose of earthing
System EARTHING involves the provision of a connection to the general
mass of earth. This connection should have a resistance not greater than
that required to operate safety mechanisms to isolate the electricity supply
from a fault situation.
Second important characteristic of EARTHING connection is that it
MUST be capable of carrying the maximum expected fault current.
The value of resistance required might not always be amenable to an
automatically set value. Therefore, the various factors which affect the
resistance to earth and fault current capacity of the buried conductor
designated the earth electrode should be considered.
This should include the size and shape of the earth conductor, the
resistivity of the soil in which it is buried and the connection of the
system to it. It is also essential to consider the current density at the
surface of the earth electrode and the ground potentials in its vicinity.
Why Testing the Soil Resistivity Important?
The resistance of an earth electrode is related to the resistivity of the soil
in which it is placed and driven, and thus soil resistivity calculations and
measurements is a crucial aspect when designing EARTHING
installations.
The property of resistivity can be defined for any material and is done so
by the American Society for Testing and Materials (ASTM), which
publishes standards for testing and measurement. When applied to soil,
resistivity is an indication of a given soil’s ability to carry electric current.
The flow of electricity in the soil is largely electrolytic, determined by the
transport of ions dissolved in moisture. An awareness of soil resistivity at
the determined location and how it varies with various factors such as
temperature, depth, moisture content etc. gives us an understanding of
how the wanted earth resistance value should be obtained and retained
over the lifespan of the installation with least cost and trouble.

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A major aim of grounding system is to set up a shared reference potential
for the building structure, power supply system, electrical conduits, plant
steel work, 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
• Deepness to which the electrode is covered
• Dampness and chemical composition of the soil
The purposes of soil resistivity testing are:
• 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
• Geophysical surveys are performed using these values as assistance
in finding depth to bedrock, core locations and other geological
phenomena.
• The degree of corrosion in underground pipelines is determined. A
drop in resistivity is proportional to an indent in corrosion in
subversive pipelines.
Soil resistivity influences the plan of an EARTHING system absolutely
and is the major factor that decides the resistance to earth of a grounding
system. Thus before designing and installing a new grounding system, the
determined location should be tested to find out the soil’s resistivity.

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What Is Done During The Testing Of Soil Resistivity?
Soil Resistivity varies widely with following factors:
• Type of earth
• Stratification
• Moisture content; resistivity may fall rapidly as the moisture
content is increased
• Temperature
• Chemical composition and concentration of dissolved salt.
• Presence of metal and concrete pipes, tanks, large slabs.
• Topography

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Types of earth electrodes and their resistance calculation:
Earth Rod Type
➢ Stainless Steel Earth Rods
➢ Galvanized Steel Earth Rods
➢ Copper bond Threaded Earth Rods
➢ Solid Copper Earth Rods
Copper is the optimal choice of earth electrode material and underground
conductor – solid copper is recommended for high fault current
installations whereas copper bonded rods are usually installed for smaller
sections.
Copper bonded steel core earth rods are the most specified due to
electrical and mechanical strength, resistance to corrosion as
comparatively lower cost compared to solid copper or stainless-steel
types – the lowest cost galvanized rods for usually installed non-critical,
short-term or temporary earthing requirements.

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An earthing system should be of the highest integrity and of robust
construction to ensure that it remains safe and will not endanger
the health and safety of persons or their surroundings. The majority of the
formulae presented in this section relate to low frequency currents and
high frequency examples are not included.
It is therefore important to recognize this issue if a long horizontal tape or
bare cable is being considered for producing a low earth resistance, even
though the impedance will ultimately be limited to a final value (see
Figure 4).
Earthing systems should consist of copper conductors, copper clad or
austenitic steel rods of appropriate dimensions, cast iron plates, or steel
piles used individually or connected together in combination to form a
single local earth electrode system.

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The effect of shape on an electrode resistance is related to the current
density around the particular electrode considered. To obtain a low
overall resistance the current density should be as low as possible in the
medium surrounding the electrode.
This may be achieved by making the dimensions in one direction large by
comparison to the other two. Thus a pipe rod or strip has a much lower
resistance than a plate of equal surface area.
a) Plates
The approximate resistance to earth of a plate R in ohms (Ω) may be
calculated from:
Plates, if used, should be installed as small units of not greater than 1.2 m
× 1.2 m connected in parallel vertically and at least 2 m apart. The
minimum ground cover should not be less than 600 mm and ideally the
surrounding soil should be damp.
Where the plate is placed in a cut out slot, e.g. in a chalk bed near the
surface, the slot should be big enough to allow at least 300 mm thickness
of soil or other conducting low resistivity medium cover around the
whole plate. This requires careful assembly during installation to ensure
that the bottom of the plate is resting in the medium used and not on the
chalk or high resistivity substrata.
NOTE! For conventional sizes, the resistance is approximately inversely
proportional to the linear dimensions, not to the surface area, i.e. a 0.9 m
× 0.9 m plate has a resistance approximately 25% higher than a 1.2 m ×
1.2 m plate.

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b) Rod electrode
The resistance of a rod RR in ohms (Ω) may be calculated from:
NOTE! – Change of diameter has little effect on the overall value of
resistance, and the size is more governed by the mechanical strength of
the rod to withstand being mechanically driven when deep earth rods are
required e.g. to depths of 20 m or more.
c) Parallel connection of aligned rods
The resistance RT in ohms (Ω) of n vertically driven rods set (s) meters
apart may be calculated from:
This model is more closely aligned to the basic theory behind the
electrostatic behavior of an earthing system component, in effect stating
more clearly the interactive effect of the value of s which for practical
purposes has long been set as being not less than twice the depth of the
rod.
This is related to the hemispherical radius of the rod and that has avoided
the effects of using less than The Two-Times constraint in design

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thinking. This affects the interference characteristics of multiple rod/tape,
etc. systems when the spacing is reduced below The Two-Times value.
d) Strip or round conductor electrodes
This section deals only with a straight run of conductor. Other shapes are
not covered here. The resistance RTA in ohms (Ω) of a strip or round
conductor may be calculated from:
When two or more strips in straight lengths, each of length L in meters
(m) and a separation distance s meters are laid parallel to each other and
connected together at one end only the combined resistance may be
calculated from the following equation:
RN = F R1
WHERE:
• RN is the resistance of n conductors in parallel, in ohms (Ω)
• R1 is the resistance of a single strip of length L, calculated from the
preceding RTA equation, in ohms (Ω).
• F has the following value:
• For two lengths: F = 0.5 + [0.078(s/L)] − 0.307
• For three lengths: F = 0.33 + [0.071(s/L)] − 0.408
• For four lengths: F = 0.25 + [0.067(s/L)] − 0.451

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• Provided that 0.02 < (s/L) < 0.3
F) Resistance of an electrode encased in low resistivity material, e.g.
conducting concrete
The resistance of a backfilled electrode RB in ohms (Ω) may be calculated
from:
g) Miscellaneous electrodes
There are many configurations that can be set out under this heading, but
a few of those which one is most likely to try first in order to achieve the
required value are included especially when dealing with deep reinforced
piles, etc.
• Three rods at the vertices of an equilateral triangle
• Two strips set at right angles to each other meeting at one corner
• Three strips set at 120° meeting at the star point all of equal length
• Four strips set in a cruciform
• Structural steelwork

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Three rods at the vertices of an equilateral triangle
The resistance Re in ohms (Ω) of three interconnected rods set out at the
vertices of an equilateral triangle (see Figure 5) of side s meters length
may be calculated from:
Two strips set at right angles to each other meeting at one corner
The resistance RL in ohms (Ω) of two strips of equal length set at 90°
with one corner touching (see Figure 6) may be calculated from:

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Three strips set at 120° meeting at the star point all of equal length
The resistance RS in ohms (Ω) of a star arranged strip (see Figure 7) may
be calculated from:
Four strips set in a cruciform
The resistance RCR in ohms (Ω) of four strips set out in a cruciform (see
Figure 8) may be calculated from:

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Structural steelwork
Foundation metalwork in concrete may be used as ready and effective
earth electrode. The total electrode area formed by the underground
metalwork of large structure may often be used to provide an earth
resistance lower than that obtainable by other methods .Overall values
well below 1 Ω are obtainable.

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Improve Earth Electrode Resistance
3 good ways to improve earth electrode resistance
1. Lengthen the earth electrode in the earth
2. Use multiple rods
3. Treat the soil
1. Effect of Rod Size
As you might suspect, driving a longer rod deeper into the earth,
materially decreases its resistance. In general, doubling the rod length
reduces resistance by about 40 percent. The curve of Figure 1 shows this
effect. For example, note that a rod driven 2 FT down has a resistance
of 88 Ω.
The same rod driven 4 FT down has a resistance of about 50 Ω. using the
40 percent reduction rule, 88 × 0.4 = 35 Ω reduction. By this calculation,
a 4-ft deep rod would have a resistance of 88 – 35 or 53 Ω — comparing
closely with the curve values.

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2. Use of Multiple Rods
Two well-spaced rods driven into the earth provide parallel paths. They
are, in effect, two resistances in parallel. The rule for two resistances in
parallel does not apply exactly. That is, the resultant resistance is not one-
half the individual rod resistances (assuming they are of the same size and
depth).
Actually, the reduction for two equal resistance rods is about 40 percent.
If three rods are used, the reduction is 60 percent, if four, 66 percent (see
Figure 3).
When you use multiple rods, they must be spaced apart further than the
length of their immersion. There are theoretical reasons for this, but you
need only refer to curves such as Figure 4 above.

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3. Treatment of the Soil
Chemical treatment of soil is a good way to improve earth electrode
resistance when you cannot drive deeper ground rods because of hard
underlying rock, for example. It is beyond the scope of this manual to
recommend the best treatment chemicals for all situations. You have to
consider the possible corrosive effect on the electrode as well as EPA and
local environmental regulations. Magnesium sulfate, copper sulfate, and
ordinary rock salt are suitable non-corrosive materials. Magnesium
sulfate is the least corrosive, but rock salt is cheaper and does the job if
applied in a trench dug around the electrode (see Figure 5).
It should be noted that soluble sulfates attack concrete, and should be
kept away from building foundations. Another popular approach is to
backfill around the electrode with a specialized conductive concrete.
NOTE! Chemical treatment is not a permanent way to improve your earth
electrode resistance. The chemicals are gradually washed away by rainfall
and natural drainage through the soil. Depending upon the porosity of the
soil and the amount of rainfall, the period for replacement varies. It may
be several years before another treatment is required.

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Methods of measuring the resistance of an earth electrode
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 1).

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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.

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Chapter 3
MV Earthing Systems

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Contents
➢ Earthing systems overview
➢ Types of earthing systems used in MV
installations.
➢ MV earthing connection techniques
➢ Residual current devices
➢ Earthing transformer.

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1. Earthing systems overview.
1.1 Introduction.
In any medium or low voltage three-phase system there are three single-
phase voltages which are measured between each phase and a common
point called the "neutral point". In balanced operating conditions these
three voltages are phase shifted by 120° and have the value:
U /
U being the phase-to-phase voltage measured between phases (see fig.
1).
From a physical point of view, the neutral is the common point of three
star-connected windings. It may or may not be accessible, may or may
not be distributed and may or may not be earthed, which is why we
refer to the earthing system.
The neutral may be connected to earth either directly or via a resistor or
reactor. In the first case, we say that the neutral is solidly (or directly)
earthed and, in the second case, we say that the neutral is impedance-
earthed.
When there is no intentional connection between the neutral point and
earth, we say that the neutral is isolated or unearthed.
The earthing system plays a very important role in a network. On
occurrence of an insulation fault or a phase being accidentally earthed,
the values taken by the fault currents, touch voltages and overvoltages
are closely related to the type of neutral earthing connection.
A solidly earthed neutral helps to limit overvoltages; however, it
generates very high fault currents. On the other hand, an isolated or
unearthed neutral limits fault currents to very low values but
encourages the occurrence of high overvoltages.
3

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In any installation, service continuity in the presence of an insulation
fault also depends on the earthing system. An unearthed neutral allows
continuity of service in medium voltage, as long as the security of
persons is respected. On the other hand, a solidly earthed neutral, or
low impedance-earthed neutral, requires tripping to take place on
occurrence of the first insulation fault.
The extent of the damage to some equipment, such as motors and
generators having an internal insulation fault, also depends on the
earthing system.
In a network with a solidly earthed neutral, a machine affected by an
insulation fault suffers extensive damage due to the high fault currents.
On the other hand, in an unearthed network or high impedance-earthed
network, the damage is reduced, but the equipment must have an
insulation level compatible with the level of overvoltages able to
develop in this type of network.
The earthing system also has a considerable amount of influence on the
nature and level of electromagnetic disturbances generated in an
electrical installation.
Earthing systems which encourage high fault currents and their
circulation in the metallic structures of buildings are highly disturbing.
On the other hand, earthing systems which tend to reduce these
currents and which guarantee good equipotential bonding of exposed
conductive parts and metallic structures are not very disturbing.
The choice of earthing system, as much in low voltage as in medium
voltage, depends both on the type of installation and network. It is also
influenced by the type of loads, the service continuity required and the
limitation of the level of disturbance applied to sensitive equipment.

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Ph3
Ph1
Ph2
Figure 1: three-phase system
Vi : phase-to-neutral voltage
UiJ : phase-to-phase voltage
The fault of high current flow to substation grounding may be the
phenomenon of lightning or short-circuit grounding system.
It causes potential difference and the result of potential difference
causes electrical current paths through human body (between the two
feet or between foot and hand).
If the current is higher than tolerable human, the result may shock him
to death.

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Good design of substation grounding system should have low grounding
system resistance with considerable touch and step voltages in tolerable
human.
Step and touch voltages play an important role when designing high
voltage substation.
Step and touch potentials near high voltage substation due to severe
ground faults present a hazard to anyone in proximity to substation
when a fault occurs.
Figure 2

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1.2 Human body impedence.
Touch voltage thresholds are related to touch current thresholds by the
body’s impedance according to Ohm’s law. Also human body impedance
is a function of several factors, such as:
❖ the type of power source (DC or AC ).
❖ the magnitude of the touch voltage.
❖ the pathway of the current through the body (hand-to-hand or
both- hands-to-feet or hand to- seat).
❖ the area of contact with the skin.
❖ the condition of the skin contact area (saltwater-wet, water-wet,
dry).
❖ duration of the current flow through human body.
Internal impedance of the human body as a percentage indicated is
shown in Fig. 3.
Thus, one can calculate the percentage for the current path of the hand -
the hand is :
26,4% +10,9% + 6,9% + 6,1% + 6,9% + 10,9% + 26,4% =94,5% (1)
Applying the same reasoning, as indicated in Fig. 3, it will be calculated
the percentage for the current route of the hand –feet:

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Table 1
26,4% +10,9% + 9,9% +1,3% +
(5,1% +14,1% + 32,3%)
= 74,25%
2
(2)
and for current path foot - foot:
32,3% + 14,1% + 8.7% + 14,1% + 32.3% = 101,5% (3)
Table 1 shows the total human body impedance values for a current
path hand – hand, AC, 50/60 Hz, for important contact surfaces,
representing the most complete knowledge of the total impedance for
adult subjects [4].
Touch voltage
(V)
Values for the total body impedances (Ω) that are not
exceeded for:
5% of the
population
50% of the
population
95% of the
population
25 1 750 3 250 6 100
50 1 375 2 500 4 600
75 1 125 2 000 3 600
100 990 1 725 3 125
125 900 1 550 2 675
150 850 1 400 2 350
175 825 1 325 2 175
200 800 1 275 2 050
225 775 1 225 1 900
400 700 950 1 275
500 625 850 1 150
700 575 775 1 050
1 000 575 775 1 050
Asymptotic
value
575 775 1 050

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Total body impedances for a current path hand to hand a.c. 50/60 Hz,
for large surface areas of contact in dry conditions. [4]
Taking as a reference the value for the percentage of the hand - hand
current path [3] (equation 1) and total body impedance values human
(Table 1) we will determine the impedance values for different current
path through the human body.
Thus we can calculate the internal resistance of the human body for
hand - feet path (which will be used to determine the touch voltage) and
foot - foot path (which will be used to determine the step voltage) from
the value of internal resistance hand - hand path, namely:
Ri_hand-feet = Ri_hand-hand •0,786 (4)

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Figure 3: Percentage of internal resistance of the human body for the
part of the body concerned. [4]
respectively:
Ri_foot-foot = Ri_hand-hand •1,074 (5)

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1.3 Touch voltage.
Touch voltage - non-dangerous limit voltage any person entering into
contact with a live part is subjected to a difference in potential: the
person therefore risks being electrified (i.e. receiving a non-lethal
electric shock). There are two types of contact: direct contact and
indirect contact.
❖ Direct contact:
This is the contact of a person with a live part of a piece of
equipment that is energized. Contact may occur with a phase or
with the neutral (see fig. 4).
Figure 4
❖ Indirect contact:
This is the contact of a person with the exposed conductive part of
a load which is accidentally live following an insulation fault (see
fig. 5).
Figure 5

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We have UT = rP If as the impedance of the human body is very high
compared with rP .
UT : touch voltage
I f : fault current
rP : earth electrode resistance
Contrary to what is generally believed, the risk for persons is not only
related to the value of the voltage applied to the human body, but also
to that of the current likely to go through it and the contact time. The
current and voltage are related by Ohm's law: I = U / R where R is the
impedance of the human body.
This impedance varies in relation to the touch voltage, the state and
dampness of the skin, as well as the path that the current takes inside
the human body.
IEC publication 479 gives the human body impedance values in relation
to the touch voltage to which it is subject (see fig. 6).
Figure 6

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This publication also gives the effects of electrical current on persons in
relation to its value and duration. These effects are shown in figure 7for
alternating current at 50 or 60 Hz.
Figure 7
Curve C1 defines the time-current limit of exposition to an electric
shock, which must not be exceeded.
Tables 2 and 3 are based on these data and fix the maximum supply
disconnection times in relation to the prospective touch voltage to
which a person is subject. They have been drawn up using graphs 6 and
7 and taking into account an additional resistance created by shoes
being worn and contact with the ground. They allow conventional limit
voltages U L , which can be held without this being dangerous for
persons, to be defined in relation to the type of premises. In other
words, a touch voltage below U L does not require disconnection. On the
other hand, any touch voltage above U L requires the fault to be cleared

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in a time at the most equal to the time stipulated in tables 2 and 3. The
conventional limit voltages have been set at 50 V for dry premises and
25 V for damp premises.
The disconnection times to be used in practice and the protections to be
implemented for disconnecting the power supply depend on the
earthing systems (TT, TN , IT ) .
Table 2
Table 3

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❖ Touch voltage on occurrence of an insulation fault:
Let us assume that owing to an insulation fault in a network a
phase accidentally comes into contact with the exposed
conductive part of a load (see fig. 8). A fault current If is then
established between the load and earth and any person entering
in contact with the exposed conductive part is subject to a
difference of potential UT referred to as the touch voltage:
UT = rP I f
As for the fault current, the touch voltage is closely linked to the
value of the impedance ZN , and thus to the earthing system. If
the neutral is unearthed, the values of the fault current If and
touch voltage UT are very low. It is therefore not necessary to
disconnect the power supply. They are, on the other hand, high if
the neutral is solidly earthed and, in this case, the power supply
must be disconnected.
Figure 8

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1.4 Step voltage.
Is the voltage between the feet of a person standing near an energized
grounded object. It is equal to the difference in voltage, given by the
voltage distribution curve, between two points at different distances
from the "electrode". A person could be at risk of injury during a fault
simply by standing near the grounding point Step potential is the voltage
between the feet of a person standing near an energized grounded
object.
In step potential, electricity spreads like ripples or rings over the surface
of the ground away from the point of the contact. Each ring carries a
different voltage as it travels into the ground.
If they step on one ring while their foot is on the other ring electricity
will make up to difference in voltage through the body.
Eliminate Step Voltage Threats by providing additional ground
conductors (electrodes) in the area of the step voltage hazard such as
the perimeter of the Substation Grid.
Options include:
❖ Additional Grid Conductor.
❖ Additional Ground Rods.
❖ Use of Deep Driven Ground Rods.
❖ Prefabricated Wire Mesh.
Figure 9: step voltage

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Eliminate Touch Voltage Threats by providing additional ground
conductors in the area of the Touch Voltage hazard such as a switch
handle. Options include:
❖ Personnel Safety Mats.
❖ Counterpoise Wires.
Figure 10: touch voltage
1.5 Damage caused to equipment.
The damage that may be caused to the equipment of an electrical
network having an insulation fault depends on the values of currents
and overvoltages which are developed in the network the moment the
fault occurs. These are thus limited as far as possible when there are
sensitive loads.
It is advisable to find a compromise as the means which reduce fault
currents tend to encourage the occurrence of overvoltages and vice
versa.

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❖ MV rotating machines:
The most frequent fault affecting a medium voltage generator or
motor is flashover between a phase and the magnetic circuit. This
type of fault is called a "stator frame" fault.
When a stator frame fault occurs in a machine, the entire phase-
earth fault current circulates in the faulty phase and in the magnetic
circuit earthed via the frame. An electric arc develops at the place
where the faulty phase touches the magnetic circuit and a lot of
energy is dissipated leading to deterioration of the magnetic circuit
and insulation. The extent of the damage depends on the fault
current value. Experience shows that not very high currents, present
for a very short time, do not lead to deterioration deep in the iron
core. It is generally admitted that a fault current below 20 or 30 A
does not result in extensive damage and does not require the
magnetic circuit to be remade. Tests have shown that a 7.5 A fault for
10 minutes does not cause extensive damage whereas a 200 A fault
for 0.3 s does.
For fault times below one second, the empirical law:
I 4 t = cte
relates the fault current value to the time during which it can be
applied without causing considerable damage.
To reduce the risks in a medium voltage network comprising motors
and generators, the phase-earth fault current is as far as possible
limited to a value of 20 A maximum by choosing the limiting resistor
earthing system: Ir  20 A

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However, in order to limit the amplitude of transient overvoltages
which are created when a phase to earth fault is cleared by a circuit-
breaker, the relation Ir  2 IC should be respected as far as possible.
Ir : current circulating in the neutral point earthing resistor
IC : network capacitive current
For a very long network (high capacitive current), these two relations
may be incompatible and a compromise must therefore be made.
❖ effects of fault currents on MV cable screens:
Medium voltage cables, whether they are the individually screened or
collectively screened type all have earthed metal screens.
When the insulating material of one phase breaks down, the conductor
and screen are practically joined: all the fault current then flows via the
screen which must be able to withstand it without being damaged. With
the fault current being directly dependent on the earthing system, we
can see how important the earthing system is in the choice and sizing of
cable screens.
The screens can be made of copper or aluminium and take various
forms, the most notable being:
❖ one or several copper or aluminium bands wrapped around
the insulating material.
❖ a thin copper or aluminium band installed lengthways
❖ a flat arrangement of copper or aluminium wires
❖ a braid of copper wires.

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Lead is also used to make screens for very high currents, or when it is
necessary to endow the insulating material with a particularly effective
protection against damp or corrosive products.
The permissible current in a screen depends on the material it is made
of, its cross-sectional area, the type of insulating material with which it is
in contact and the time during which the fault current will flow through
it.
In all cases, it is important to check that the screen is suited to the
operating conditions.
1.6 Practical Solution.
To reduce step and touch voltages to manageable levels, there are some
traditional methods:
❖ Install a grading ring at 1m from the equipment to provide an
equipotential zone.
❖ Use Neutral/Earthing impedances at zone substations to reduce
the earth fault currents for ”close to substation‟ faults.
❖ Overhead earth wires to split/redirect the earth fault current for
”close to substation‟ faults.
❖ Reduce the earth fault clearance time.
❖ Introduce impedance between the person and the source voltage
with the introduction of concrete poles; there are new hazards in
the community.
❖ Traditional timber poles provided a degree of insulation, and little
danger arose from step or touch voltages arising from an insulator
fault on a pole

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2. Types of earthing systems used in MV installations.
2.1 types of MV earthing systems.
This paragraph deals with the type of earthing for the HV/MV or MV/MV
substation exposed conductive parts, the MV neutral and the MV
installation exposed conductive parts.
Principles and earthing systems used in medium voltage:
If we consider public distribution networks and industrial or tertiary
private networks without making any distinction between them, we can
see that the same earthing system principles are used in each. In other
words:
❖ solidly earthed neutral.
❖ Unearthed neutral.
❖ resistance earthing.
❖ reactance earthing.
❖ partially or totally tuned Petersen coil earthing.
These principles are summarized in the table . The advantages and
drawbacks of each system are described below.

100| P a g e
Solidly earthed neutral
An electrical connection is
intentionally made between the
neutral point and earth.
Unearthed neutral
There is no electrical connection
between the neutral point and
earth, except for measuring and
protective devices.
High impedance earthing
A high impedance is inserted
between the neutral point and
earth
Resistance earthing
A resistor is inserted between the
neutral point and earth
Reactance earthing
A reactor is inserted between the
neutral point and earth

101| P a g e
Petersen coil earthing
A reactor L tuned to the network
capacitances is inserted between
the neutral point and earth so that
if an earth fault occurs, the fault
current is zero.
2.2 Comparison of different MV earthing systems.
a) unearthed neutral
Operating technique
No supply disconnection on occurrence of first insulation fault; it is thus
compulsory to:
❖ carry out permanent insulation monitoring
❖ signal the first insulation fault
❖ locate and clear the first insulation fault
❖ carry out disconnection on occurrence of the second insulation
fault.

Earthing in electrical power systems

Earthing in electrical power systems

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