This document discusses earthing systems and the hazards of a broken neutral connection for a power transformer. It defines system earthing and equipment earthing, and explains that a broken neutral connection can cause overvoltage issues for the transformer and prevent protective relays from operating during a fault. The document also discusses the objectives and importance of proper earthing, including providing an alternative path for fault currents, ensuring safety from electric shocks, and maintaining system voltages. It provides examples of what can occur when a transformer's neutral connection to earth is broken.

1. EARTHING SYSTEM,HAZARDS OF
BROKEN EARTH OF POWER
TRANSFORMER
BY
Prof . J .C . Bhola

3. Types of Earthing
i)system earthing
ii) Equipment earthing

4. SYSTEM EARTHING
• It includes physical metallic
connection of Transformer neutral
with earth pit or /and with metallic
earth mesh.

5. EQUIPMENT EARTHING
• It includes physical metallic
connection of all non-current
carrying metallic parts with earth
pit /earth mesh.

6. Definition
• Ground or earth in a mains (AC power)
electrical wiring system is a conductor that
provides a low impedance path to the earth to
prevent hazardous voltages from appearing on
equipment (the terms "ground") and "earth"
are used synonymously here). Normally a
grounding conductor does not carry current.

7. • In electricity supply systems,
an earthing system defines the electrical
potential of the conductors relative to that of
the Earth's conductive surface. The choice
of earthing system has implications for
the safety and electromagnetic
compatibility of the power supply.

8. • A protective earth (PE) connection ensures that all
exposed conductive surfaces are at the same
electrical potential as the surface of the Earth, to
avoid the risk of electrical shock if a person
touches a device in which an insulation fault has
occurred. It ensures that in the case of an
insulation fault (a "short circuit"), a very high
current flows, which will trigger an overcurrent
protection device (fuse, circuit breaker) that
disconnects the power supply.

9. • A functional earth connection serves a purpose
other than providing protection against electrical
shock. In contrast to a protective earth connection,
a functional earth connection may carry a current
during the normal operation of a device. Functional
earth connections may be required by devices such
as surge suppression and electromagnetic
interference filters, some types of antennas and
various measurement instruments.

10. GROUNDING IN SUB-
STATION

13. EARTHING OF
EQUIPMENT BODY

16. The main objectives of the earthing
• to Provide an alternative path for the fault
current to flow so that it will not endanger the
user
• to Ensure that all exposed conductive parts do
not reach a dangerous potential
• to Maintain the voltage at any part of an
electrical system at a known value so as to
prevent over current or excessive voltage on the
appliances or equipment.

17. Objects of Earthing
• The object of an earthing system in a
substation is to provide safety and reliability
of equipment protection .
• Earthing system shall ensure that no human
being in the sub station is subject to shock or
injury on the occurrence of a short circuit to
EARTH i.e. earth fault or lightning on the
equipment installed in the substations yard or
on the transmission lines radiating from sub-
station..

18. Effect of Current Flow Through Human Body
Current Level + Shock Hazard
100 mA Threshold of perception
1–5 mA Sensation of pain
5–10 mA Increased pain
10–20 mA Intense pain; unable to release grip
30 mA Breathing affected
40–60 mA Feeling of asphyxiation
75 mA Ventricular fibrillation, irregular heartbeat

19. Permissible Body current Limits
• The magnitude and duration of the current
conducted through a human body should be
less than those that cause “Ventricular
fibrillation”.
• For a 50 kg body weight the allowable body
current is 116 mA for 1 sec &367 mA for 0.1
sec.
• Therefore good earthling system should not
allow more than allowable body current even
during faults in the s/s or lines.

20. Resistance of the Human body
• The resistance of the internal body tissues, not
including skin, from one hand to both feet, is
approximately 300 ohms.
• Whereas the body resistance including skin
ranging from 500-3000 ohms.
• For voltages above 1 KV and currents above 5A,
the human resistance is decreased by damage/
puncture of the skin at the point of contact.
• Therefore with solid earth system , values of
voltages and currents affecting human body
shall be within safe limits.

21. Duties of Earthing System
• It stabilizes circuit potential with respect to
ground
• It should protect the life of personnel moving
in the s/s and property
• It should provide low impedance path to fault
current

22. Ground Mat is connected to
• The natural point of each system through its
own independent earth.
• Equipment framework and other non-current
carrying parts.
• The earth point of lightning arresters,voltage
transformers in the substation through their
permanent independent earth electrode.
• Substation fence.

23. What, if neutral of transformer breaks
• It prevents flow of earth-fault current ,from
the location of fault via earth strata to the
neutral of transformer through earth-mesh
and earth pit.

24. What, if neutral of transformer break’s
• With the result ,due to non completion of
fault current -loop, replica of fault current is
not seen by the over-current and earth fault
relay . Thereby the O/C &E/F relay does not
operates, even if the earth fault exists on the
line..

25. What, if neutral of transformer break’s
• Fault current will try level best to enter all
other neutral earth pits , i.e. neutrals of D.C
system , earthing of battery charger , trip coil
closing coil , C.T. secondary, over current relay
& earth fault relay ,earthing points of control
panels &those of isolator handles and all types
of sub-station structures.

26. A CASE STUDY OF A 33/11 KV TRANSFORMER WITH
BROKEN NEUTRAL.
• Earth resistance values at a 33/11 KV sub-
station were 20 ohms, neutral conductor of
the transformer was traceable up to some
distance and earth pit was found filled with
water.
• Further it was not sure that continuity of earth
conductor up to real earth is there or not.

27. What does it indicate
• It indicates that even though the neutral
grounding conductor is visible up to some
distance on the ground and the high values of
earth resistance indicates that the neutral
grounding is not properly connected between
transformer neutral and the earth strata, to
complete the fault current path loop.

28. EFFECT AT A SUB-STATION
• During a fault on 11kv line at 33/11 kv sub-
station, the faulty feeder did not trip.
• D.C. fuses at charger blows off.
• Relay covers fell down due to vibration of
panel.
• Trip-coils of other 33kv VCB located at sub-
station got burnt.

29. Step and Touch Potential
• Step Potential:- A voltage between two feet
separated by 1 meter along the earth with
ground current flowing during fault condition.
• Touch Potential :-A voltage between the
object touched and the ground point just
below the person touching the object when
ground currents are flowing.

30. Solutions suggested to obtain ground
resistance of 1 ohm or less
i)Making a combination of horizontal grid(by
using M.S.flat o size 75x4 mm square,burried in
the ground at depth of 0.5 meter),and a number
of long vertical electrodes penetrating the lower
strata of soil of substation,upto moist layer of
soil.

31. Solutions suggested to obtain ground
resistance of 1 ohm or less
ii) Connecting an existing adjacent grounding
grid to the newly designed grid i.e. M.S. flat
mesh grid & vertical pipe electrodes penetrating
up to moist layer of soil.
iii) By penetrating long vertical pipe electrodes
penetrating the lower strata of soil , upto moist
layer, around the s/s fence.

33. The qualities of a good earthing
system are:
• Must be of low electrical resistance
• Must be of good corrosion resistance
• Must be able to dissipate high fault current
repeatedly

34. EARTHING
• The earthing material that is electrically
conductive and a fault current will flow to
'earth' through the live conductor, provided.
This is to prevent a potentially live conductor
from rising above the safe level. All exposed
metal parts of an electrical installation or
electrical appliance must be earthed.

35. The main objectives of the earthing
• Provide an alternative path for the fault
current to flow so that it will not endanger the
user
• Ensure that all exposed conductive parts do
not reach a dangerous potential
• Maintain the voltage at any part of an
electrical system at a known value so as to
prevent over current or excessive voltage on
the appliances or equipment.

36. The qualities of a good earthing system
• Must be of low electrical resistance
• Must be of good corrosion resistance
• Must be able to dissipate high fault current
repeatedly

37. Neutral Treatment
All neutral wires of the same electrical system should
have the same electrical potential, because they are
all connected together through the system ground.
Neutral conductors are usually insulated for the same
voltage as the live conductors, with interesting
exceptions.
The values of over-voltages and fault currents
expected in the network are crucial, since they
affect Power Quality parameters, namely
interruptions and voltage dips.

38. System substations in general tend to have:
• A high fault level
• A large earth grid that is walked over by
substation staff
• A number of items of electrical equipment with
electrical protection of various clearing times
• A number of aerial and/or cable feeders
• Fences with associated prospective touch
voltages for people external to the substation
• Other services (eg water, communications)
connected

39. Types of System Substations
Traction Substations
These locations are the supply points for the
overhead wiring. relevant.
dc protection and voltage regulation. All the
sections in this document All the sections in this
document may be may be relevant.
AC Switching Stations
These locations are ac switching stations that have
high voltage circuit breakers.

40. Earth grid ,Electrode
Standard Electrode
The standard electrode is a 3.6 m length of copper
tube (14.29 mm outside diameter, 11.03 mm inside
diameter). Longer electrodes may be used if there is
some difficulty obtaining the required resistance as
the soil resistivity is usually found to be lower at a
greater depth.
The current rating of the earthing electrode is 5 kA for
1 second when tested in free air in an ambient of
15°C to 25°C without exceeding a temperature rise of
350°C

41. Electrode Spacing
The earthing system at a System Substation consists
of a minimum of 4 earth electrodes installed around
the inside perimeter of the substation and connected
together with the earth mesh.
The exact spacing of the electrodes will be
determined by the final design, which will be based
on local conditions, resistivity of the area and space
available for electrodes. The spacing between
electrodes should be greater than the electrodes ’
length.

42. • Although the earth mesh will often result in a
low enough resistance without the use of
electrodes, a minimum of 4 electrodes are still
necessary to ensure the fault level capability
i.e. 4 x 5 kA. Electrodes are also required in
case of the drying out of the soil at the depth
of the earth mesh in long dry spells.

43. Surge Arresters
The connection between the earth side of the
high voltage arrester and the earth side of the
equipment being protected must be as short
as possible (the same applies to the live side of
the surge arrester). The resistance connection
to remote earth is not critical to the surge
arrester operation but it is important to
consider the touch potentials during the surge
arrester operation.

44. Earth grid
• Standard Electrode :The standard electrode is
a 3.6 m length of M.S. tube (14.29 mm outside
diameter, 11.03 mm inside diameter). Longer
electrodes may be used if there is some
difficulty obtaining the required resistance as
the soil resistivity is usually found to be lower
at a greater depth. The current rating of the
earthing electrode is 5 kA for 1 second when
tested in freeair in an ambient of 15°C to 25°C
without exceeding a temperature rise of
350°C.

45. Neutral Grounding Practice Adopted in
MP System
• Solidly / Effectively grounded/
earthed system

46. WHAT IF BREAKAGE IN NEUTRAL GROUNDING
OF POWER TRANSFORMERS
• Neutral of Transformer is solidly
connected to make electrical
connection with moist layer of
under-ground soil.

47. • Breakage in neutral grounding may be either
due to mechanical breakage of neutral
grounding conductor or due to breakage in
electrical path of neutral ground with the
conducting moist layer of earth i.e. due to
high earth resistance value of s/s .

48. Effects of breakage of neutral grounding of
Power Transformer
• Transformer experiences double the overvoltage
during faults.
• Due to neutral floating , transformer will have
premature failures
• Due to neutral floating, over current & earth fault
protection of transformer will not operate, due to
non-completion of fault current loop, through
transformer neutral & winding.
• Earth resistance value of s/s will rise to high values.

49. TYPES OF EARTHING
• GROUND.— Grounding of living components of
system are termed as GROUND ,such as
grounding of Generator, motor, transformer and
battery charger and inverter or converter.
• NEUTRAL ---Equipment/ structure earthling is
termed as NEUTRAL, such as connecting of non
current carrying parts of equipment i.e. body of
transformer ,CT ,PT , Circuit Breaker ,motor ,
generator etc.

50. Neutral
• Neutral is a circuit conductor (that carries current
in normal operation), which is connected to earth
(or ground) generally at the service panel with
the main disconnecting switch or breaker.
• In a poly phase or three-wire (single-phase) AC
system, the neutral conductor is intended to have
similar voltages to each of the other circuit
conductors. By this definition, a circuit must have
at least three wires for one to serve as a neutral.

51. Importance of good Earthing:
• 1) Computers perform better, if they are provided
near to zero voltage reference and it is only possible
by good earth.
• 2) For your very Important PLC based machines it is
recommended that you need to maintain neutral to
earth voltage below 2 volts. Many times it is
observed by electrical people that their machine
failed and the reason was neglected proper earth or
maintaining the existing earth.
• Sometimes shocks are observed in home appliances
i.e. Frieze , cooler, washing m/c etc.

52. • 3) It is fact that currents of 3 phases doesn't
maintained same but we need to maintain voltages
of 3 phases on same level. Proper neutral earthing
maintains voltages between 3 phases on balance
position. No maintenance of Neutral Earthings has
bad effect on motor.
• i) With 5% Unbalanced Input Voltage between
phase to phase - the LOAD will be unbalanced and
the capacity of the motor reduces by 40%.

53. • ii)Consumption of the three phase motor will
increase by 20 - 25%.
• To avoid frequent tripping of the overload
relays and to continue production, setting of
motor relay is kept higher than actual
required. The motors (particularly smaller
motors upto 7.5 H.P.) cannot withstand this
high current for long time and in most cases
the motor burns out and this high current
flows through the relays, contactors, cables
etc. resulting in failure of the same.

54. • 4) Electrical Shock prevention is one of the biggest
requirement.
• 5) To ground fault current, good earthing is
required.
• 6) To ground lightning energy, earthing must be
proper.

55. • . Increased volume makes tight contact from
Electrode to Earth Stimulating Material and
Material to soil. It never wash away in water. It
never dries out, as it is "hydrophilic" and
absorbs moisture from the soil to remain
hydrated and will maintain its density. In
direct sunlight the top few inches will become
hard and seal itself off, but the rest of the
material stays moist and conductive. It is non
corrosive, non-toxic and harmless to the soil
or surrounding vegetation.

56. Maintenance Free Earthing System Details
Earth Electrode:
Earth Electrodes are made from GI
pipes & Strips of Hot Dipped GI &
Copper. The Strip is inserted in Pipe
and space around this strip is filled
by a special highly conductive and
100% non-corrosive Material.
.

57. Earth Stimulating Material :
It is ideal earth fill material to get lower earth
resistance. It is used as protective and contact
material to fill between earth electrode and
surrounding soil. It enriches the electrode
surrounding with charge carriers for high
conductivity. When water is added to it, it
absorbs plenty of water about four to six times
its dry volume, it becomes a dense paste. This
consistency holds its own shape and sticks on to
any surface it touches. This consistence helps
hold the Earth Electrode in place

58. Limits of earth resistance
i)Generating Power Station:
765KV S/S and 400KV S/S - Less than
0.5Ώ
ii)EHV grid s/s up to 220 KV - up to 1.0Ώ
iii)33 kv s/s -- 1.0 to 3.0 Ώ
iv)Line / tower footing resistance-up to8Ώ

59. Earthing system needs to satisfy three demands
In general any earthing system needs to
satisfy three demands:
i) Lightning and short circuit: the earth
system must protect the occupants,
prevent direct damage such as fire,
flashover or explosions due to a direct
lightning strike and overheating due to a
short-circuit current.

60. ii) Safety: the earthing system must
conduct lightning and short-circuit
currents without introducing
intolerable step-voltage and touch-
voltages.

61. • iii) Equipment protection and
functionality: the earthing system must
protect electronics by providing a low
impedance path to interconnect
equipment. Proper cable routing, zoning
and shielding are important aspects and
serve the purpose of preventing sources
of disturbance from interfering with the
operation of electrical equipment.

63. Typical Earth Resistance Values.
1. Power Stations – 0.5 to 1.0 ohms.
2. EHV Sub-Stations –around 1.0 ohm.
3. 33/11 KV S/S – 2 to 3 ohms.
4. Other Installations – 5 to 8 ohms.

65. The ground
electrode :The
design of the ground
electrode - its size,
shape and
disposition - are
critically important,
not only for
producing a
sufficiently low
impedance but also
for controlling the
shape of the electric
field on the surface
of the ground..

66. Ground Potential Rise
The resistance of the earth electrode system and
the current into the soil determine the voltage
difference between the system and the ground. For
large fault currents this voltage will be very high at
the electrode and will decrease with distance from
the earth conductor as the volume of earth through
which the current is flowing increases. This ground
potential rise (GPR) can result in dangerous
situations.

67. • Before examining the issue further it is necessary to
define some terms (see Figure). ‘Touch potential’ is
the voltage difference between the earthed
structure and a person standing on the ground
within touching distance of the structure. ‘Step
potential’ is the voltage between the feet of a
person (assumed to be 1 meter apart) standing on
the ground. The maximum touch and step
potentials are limited by various Standards.

68. It is quite possible to achieve a low
impedance - under the right soil
conditions – with a single earth rod. Note
that the ground potential slope is very
steep – i.e. the step and touch voltages
would be high – so this is not a suitable
choice of electrode.

69. • Figure 1 shows (right hand side) the effect of
adding a guard ring 1 metre outside the
perimeter, buried 0.5 metres deep, to the
system. Not only does this reduce the
impedance and therefore the GPR (because a
greater volume of earth is carrying the
current), but the shape of the field is also
controlled within the guard ring reducing step
and touch voltages.

70. As can be seen, the step and touch voltages around
the building are much smaller when an earthing
ring is used. The top of the ‘potential mountain’ is
made wider and flatter by the field shaping
characteristics of the ring in the soil. When no ring
is used the ‘mountain’ is steeper and higher,
especially near a grounding rod or concrete pile,
thus possibly leading to dangerous situations.

71. The buried earth electrode ring around the
building should be located at a distance of least
1 metre from the exterior wall. It should be
deep enough to ensure that it will not be
affected by freezing in winter and will not dry
out in summer. Where there is no applicable
local guidance the depth should be at least 0.5
metres. The earth ring should be made of
copper with a cross-sectional area of at least 50
mm2 .

72. The earth electrode ring
must be connected to a
meshed network under
the structure and to the
meshed network around
the structure if applicable.
Connections between the
ring and the rest of the
earthing system of the
building or site must be
made at several points.

73. Site-wide earthing system
A low impedance path to earth is required to
conduct lightning and short circuit currents into
the soil. This main earthing system has to be a
network that provides a low impedance
connection between all objects and a good
distributed contact with the soil. It must be able
to conduct all possibly occurring currents while
avoiding dangerous touch voltages and large
currents on cables connecting remote objects.

74. Electromagnetic compatibility
Every piece of electrical and electronic equipment
produces some electromagnetic radiation. Similarly
, every piece of equipment is also sensitive, to a
greater or lesser extent, to electromagnetic
radiation. If everything is going to work, the
cumulative level of radiation in an environment
must be rather less than the level that will disrupt
the operation of the equipment working in that
environment. To achieve this goal , equipment is
designed, built and tested to standards to reduce
the amount of radiation that is emitted and increase
the amount that can be tolerated.

75. EMC is defined in the IEC 61000 series
as:
‘The ability of an equipment or
system to function satisfactorily
in its electromagnetic
environment without introducing
intolerable electromagnetic
disturbances to anything in that
environment.’

76. • Maintaining this compatibility in
practice requires great care in the
design and implementation of the
installation and the earthing
system.

77. Improvement in earth resistance values after taking
corrective steps
• Initial earth resistance value….20 ohms.
• After completing M.S.flat mesh work buried
below 0.5 meter ,making the connections of all
its raisers with body earth of all equipments &
making its interconnection with 2 no. earth pit for
Transformer neutral, results of earth resistance
were reduced to 4 ohms.
• After making interconnection with long driven
pipe electrode of 150 mm G.I.pipe, results of
earth resistance were further reduced to 0.4
ohms.

78. Conclusion
• By adopting M S flat mesh and earth pits &
long driven GI pipe electrodes together have
been found useful to reduce the earth
resistance of s/s from 20 ohms to 0.4 ohms.
• It rectified the problem of breakage of neutral
grounding of Power Transformer.
• Mal-operations carried at s/s due to floating
neutral were overcome and protection started
working in normal manner.
• Floating neutral condition of Xmer was also
rectified.

80. THANK YOU