EARTHING
INTRODUCTION
Earthing
means a connection to the general mass of earth. The use of earthing is so widespread in an
electric system that at practically every point in the system, from the
generating system to the consumers’ equipment, earth connections are made.
Earthing
is divided into two main categories:
- Neutral Earthing
- General Earthing
OBJECTIVE OF EARTHING
Neutral Earthing
This
is the earthing of the star or neutral point of power system lines and
apparatus.
The objective of neutral earthing are:
a) To
reduce the voltage stress due to switching and lightning surges and to
discharge safely into the ground over voltages occurring in the system.
b) To
permit the use of graded insulation in H.V. and E.H.V systems with consequent reduction
in weight, size and cost.
c) To
control the fault currents to satisfactory values.
d) To
ensure the operation of ground or earth fault relays.
General Earthing
This is a term applied to all
earthing of metal parts of lines and apparatus used in electrical systems and
equipment used in the utilisation of electrical energy other than neutral
earthing.
The objects of general earthing are:
a) To
provide protection to plant and personnel due to accidental grounding of
equipment.
b) To
cordon off the zone of dead line working to make it safe during working to
prevent electrostatic and electromagnetic induction and also accidental contact
from other energized lines and apparatus.
Examples
of general earthing are the earthing of the frames of generators, rotors,
motors, tanks of transformers, circuit breakers, body of domestic apparatus,
lines, electric stoves, electric irons etc.
3.0 NEUTRAL EARTHING
The
various methods of neutral earthing are:
a) Solid
Earthing or Effectively Grounded Earthing
b) Resistance
Earthing
c) Reactance
Earthing
d) Arc
suppression coil earthing.
However
before discussing the effects, the merits and demerits of the above methods, an
isolated Neutral system is considered.
3.1 ISOLATED NEUTRAL SYSTEM

But the voltage across the other two phases
rises to phase to phase voltage, as shown.
The
fault phase B supplied the currents ICGR and ICGY. These being capacitive
Currents,
no current flows when the line capacitance is charged. Hence, an arcing takes place at the faulted
point. During this period, the line
capacitance discharges and capacitive current once again flows. This repetitive cycle of charging and
discharging causes intermittent arcing at the point of fault and also gives
rise to abnormal voltages across the healthy phases due to the capacitance
effect. In practice, voltages of 3 to 4
times the system phase voltage may occur thereby causing damage to the system
insulation. Hence isolated neutral
system is not being practiced.
Solid
Earthing
In
solid earthing a direct metallic connection is made between the system neutral
and the ground. The ground electrode
resistance will be very small usually less than one ohm.
The Main Advantages are:
a) There
is no abnormal voltage rise on the other healthy phases.
b) Permits
the use of discriminative protective gear.
c) No
voltage stress on the system insulation.
d) Efficient
and correct operation of Earth fault Relays is ensured.
e) Additional
savings are possible in power transformers of 132KV and above with the use of
graded insulation.
f) No
arcing grounds.
3.22 Disadvantages are:
a) On
overhead transmission lines, a majority of the faults are to the ground. Thus, the number of severe shocks to the
system is relatively much greater than with resistance or reactance grounding.
b) The
ground fault current is generally lower than the three-phase current. But near generating stations, it may be
relatively higher and may exceed the three phase short circuit currents. In such cases circuit breakers with higher
rupturing capacity are required.
c) The
increased ground fault currents affect neighboring telecommunication circuits.
Most
of the adverse effects have been overcome nowadays by the use of high rupturing
capacity, high speed circuit breaker and fast acting protective relays. Hence in the world over, it is the practice
to adopt solid earthing for the neutrals of power systems.
3.3 Resistance Earthing
This
is one form of impedance earthing and introduced when it becomes necessary to
limit the earth fault current. The
resistance used may be a solid metallic resistor or a liquid resistor or a
metallic resistor immersed in a liquid like transformer oil.
The
main advantages are:
1) Permits
the use of discriminative gear.
2) Effects
of arcing grounds are avoided with suitable low ohmic resistance.
3) Ground
fault currents are reduced, thus obviating the harmful effects of the large
currents associated with solid earthing.
4) Interference
with adjoining communication circuits is avoided.
The disadvantages are:
1) System
neutral will almost invariably be fully displaced in the case of a ground
fault, thereby necessitating the use of 100% lightning Arresters at an increase
in cost.
2) Cost
of transformers will increase because graded insulation cannot be used.
Resistance
earthing, if at all used, is limited to system voltages of 33KV and below and
when the total system capacity does not exceed 5000 KVA.
3.4 Reactance Earthing
This
is another form of impedance earthing also called `Peferson Coil Earthing'
after the name of the inventor.
This
is a logical development of reactance earthing and is based on a value of
reactance in the system neutral such that the reactance current due to the coil
exactly neutralises the network capacitance current at the fault. The resultant capacity current is theoretically
nil and in any case inadequate to maintain the arc. Hence the name `arc suppression coil'.