The document discusses different methods of earthing or grounding electrical systems. It defines earthing as connecting electrical equipment to earth through a low resistance wire to provide an alternative path for fault currents. The key methods discussed are: - Solid or effective grounding, which directly connects the neutral to earth. This allows large fault currents but limits equipment costs. - Resistance grounding, which connects the neutral to earth through a resistor. This limits fault currents but increases equipment costs. - Reactance grounding uses an inductor instead of resistor to limit fault currents. - Peterson coil grounding cancels out capacitive fault currents through resonant tuning of an inductor.

1. Neutral Earthing
by, vaza hasmukh(rockey)

2. 2
What Is Earthing?

The process of connecting metallic bodies of all the electrical apparatus
and equipment to huge mass of earth by a wire having negligible
resistance is called Earthing.

The term earthing means connecting the neutral point of supply system
or the non current carrying parts of the electrical apparatus to the
general mass of earth in such a manner that all times an immediate
discharge of electrical energy takes place without danger.

3. 3
Objectives of the earthing
Provide an alternative path for the fault current to flow so that it will
not endanger the user.
It makes all conductive parts to have a normal voltage (potential)
value to prevent over current or excessive voltage on the
appliances or equipment to protect them.

4. Isolated Neutral
•
The system neutral is not connected to earth. Thus the
neutral is isolated from earth.
•
This method is typically used for LV networks.

5. •
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.

6. Neutral Grounding
•
The process of connecting neutral point of 3-phase
system to earth either directly or through some circuit
element (e.g. resistance, reactance etc.) is called
neutral grounding.
•
Neutral grounding provides protection to personal
and equipment.
•
It is because during earth fault, the current path is
completed through the earthed neutral and the
protective devices (e.g. a fuse etc.) operate to isolate
the faulty conductor from the rest of the system.

8. •
3-phase, star-connected system with neutral
earthed. Suppose a single line to ground fault
occurs in line R at point F.
•
This will cause the current to flow through
ground path. Note that current flows from R
phase to earth, then to neutral point N and
back to R-phase.
•
Since the impedance of the current path is low,
a large current flows through this path. This
large current will blow the fuse in R-phase and
isolate the faulty line R. This will protect the

9. Methods of Neutral Grounding
•
The methods commonly used for grounding
the neutral point of a 3-phase system are :
(i) Solid or effective grounding
(ii) Resistance grounding
(iii)Reactance grounding
(iv) Peterson-coil grounding
•
The choice of the method of grounding
depends upon many factors including the size
of the system, system voltage and the scheme

10. Solid Grounding
•
When the neutral point of a 3-phase system
(e.g. 3- phase generator, 3-phase transformer
etc.) is directly connected to earth through a
wire of negligible resistance and reactance, it
is called solid grounding or effective
grounding.
•
Fig. shows the solid grounding of the neutral
point. Since the neutral point is directly
connected to earth through a wire, the neutral
point is held at earth potential under all

12. •
Advantages:- The solid grounding of
neutral point has the following advantages :
–
The neutral is effectively held at earth potential.
–
When there is an earth fault on any phase of the
system, the phase to earth voltage of the faulty phase
becomes zero. However, the phase to earth voltages
of the remaining two healthy phases remain at
normal phase voltage because the potential of the
neutral is fixed at earth potential.
–
This permits to insulate the equipment for phase
voltage. Therefore, there is a saving in the cost of
equipment.

13. –
It becomes easier to protect the system from earth faults
which frequently occur on the system. When there is an
earth fault on any phase of the system, a large fault current
flows between the fault point and the grounded neutral. This
permits the easy operation of earth fault relay.
•
Disadvantages:-
–
Since most of the faults on an overhead system are
phase to earth faults, the system has to bear a large number
of severe shocks. This causes the system to become unstable.

14. –
The solid grounding results in heavy earth fault
currents. Since the fault has to be cleared by the circuit
breakers, the heavy earth fault currents may cause the burning
of circuit breaker contacts.
–
The increased earth fault current results in greater
interference in the neighboring communication lines.
•
Applications:- This system of grounding is used for voltages
upto 33 kV.

15. Resistance Grounding
•
In order to limit the magnitude of earth fault current, it is a common practice to
connect the neutral point of a 3-phase system to earth through a resistor. This is
called resistance grounding.
•
When the neutral point of a 3-phase system (e.g. 3-phase generator, 3-phase
transformer etc.) is connected to earth (i.e. soil) through a resistor, it is called
resistance grounding.
•
Fig. shows the grounding of neutral point through a resistor R. The value of R
should neither be very low nor very high. If the value of earthing resistance R is
very low, the earth fault current will be large and the system becomes similar to
the solid grounding system.
•
On the other hand, if the earthing resistance R is very high, the system
conditions become similar to ungrounded neutral system.

16. •
The value of R is so chosen such that the earth fault current is
limited to safe value but still sufficient to permit the operation
of earth fault protection system.
•
In practice, that value of R is selected that limits the earth fault
current to 2 times the normal full load current of the earthed
generator or transformer.

18. •
Advantages:-
–
It increases the stability of electrical system.
–
Reduces electrical & mechanical stresses in
electrical system through which fault currents are
flowing.
–
Limits electric shocks to working personnel &
interference in communication by limiting current
values.
–
Reduces heating effects in faulted
electric/electronic devices.

19. •
Disadvantages:-
–
Electrical equipments are designed for higher
voltages requiring larger insulation.
–
Cost is more.
–
A large amount of energy is produced in the
earthing resistance during earth faults. Sometimes it
becomes difficult to dissipate this energy to
atmosphere.
•
Applications:- It is used on a system operating
at voltages between 2.2 kV and 33 kV.

20. Reactance Grounding
•
In this system, a reactance is inserted between
the neutral and ground.
•
The purpose of reactance is to limit the earth
fault current. By changing the earthing
reactance, the earth fault current can be
changed to obtain the conditions similar to that
of solid grounding.
•
The ground fault current should be at least 55
% of 3 phase fault current to prevent transient
over voltages.

22. •
This method is not used these days because of
the following disadvantages :
(i) In this system, the fault current required to
operate the protective device is higher than that of
resistance grounding for the same fault conditions.
(ii) Possibility of high transient overvoltages.

23. Arc Suppression Coil Grounding
(or Resonant Grounding)
•
We have seen that capacitive currents are responsible for
producing arcing grounds. These capacitive currents flow
because capacitance exists between each line and earth.
•
If inductance L of appropriate value is connected in parallel
with the capacitance of the system, the fault current IF flowing
through L will be in phase opposition to the capacitive current
IC of the system. If L is so adjusted that IL = IC, then resultant
current in the fault will be zero. This condition is known as
resonant grounding.

24. •
When the value of L of arc suppression coil is
such that the fault current IF exactly balances
the capacitive current IC, it is called resonant
grounding.
•
An arc suppression coil (also called
Peterson coil) is an iron-cored coil
connected between the neutral and earth.
•
The reactor is provided with tappings to
change the inductance of the coil. By adjusting
the tappings on the coil, the coil can be tuned

26. •
Operation. Fig. shows the 3-phase system
employing Peterson coil grounding. Suppose
line to ground fault occurs in the line B at
point F. The fault current IF and capacitive
currents IR and IY will flow as shown in Fig.
Note that IF flows through the Peterson coil
(or Arc suppression coil) to neutral and back
through the fault.
•
The total capacitive current IC is the phasor

28. The Peterson coil grounding has the following
advantages:
•
(i) The Peterson coil is completely effective in
preventing any damage by an arcing ground.
•
(ii) The Peterson coil has the advantages of
ungrounded neutral system.
Disadvantages. The Peterson coil grounding has
the following disadvantages :
•

29. Voltage Transformer Earthing
•
In this method of neutral earthing, the primary of a single-
phase voltage transformer is connected between the neutral
and the earth.
•
A low resistor in series with a relay is connected across the
secondary of the voltage transformer.
•
The voltage transformer provides a high reactance in the
neutral earthing circuit and operates virtually as an
ungrounded neutral system.
•
Earth fault on any phase produces a voltage across the relay.
This causes the operation of the protective device.

31. Advantages. The following are the advantages
of voltage transformer earthing :
(i) The transient over voltages on the system due
to switching and arcing grounds are reduced.
It is because voltage transformer provides high
reactance to the earth path.
(ii) Arcing grounds are eliminated.

32. Disadvantages. The following are the disadvantages of
voltage transformer earthing :
(i) When earth fault occurs on any phase, the line voltage
appears across line to earth capacitances. The system
insulation will be overstressed.
(ii) The earthed neutral acts as a reflection point for the
travelling waves through the machine winding. This may
result in high voltage build up.
Applications:- The use of this system of neutral earthing is
normally confined to generator equipments which are
directly connected to step-up power transformers.

33. Earthing Transformer
•
The neutral or star point is usually available at every voltage
level from generator or transformer neutral.
•
However, if no such point is available due to delta
connection or if neutral point is desired on bus-bars, a zig-
zag transformer is most commonly used.
•
To convert 3 phase 3 wire system into 4 wire system,
earthing transformer is used.

34. •
Such transformer has no secondary. It is a core type transformer
having 3 limbs.
•
It is seen from the fig. , that currents in the two halves of winding on
each limb are in opposite directions.

35. •
As a result there will be not be any undesirable harmonic
prevailing in circuit & so stresses on the insulation of
transformer are considerably reduced.
•
The fault current is quite high as the impedance of earthing
transformer is quite low.
•
The resistance is inserted either in neutral circuit or in
windings of earthing transformer, in order to limit the
magnitude of fault current.