Answer / pankaj rajput

Explanations given by Mahmud are very well in line with
your query. I am just adding something more...

Due to line capacitance (between phases & earth),
intermittent arcing may take place during single phase to
ground fault in an Isolated Neutral system. This phenomenon
is called Arcing Grounds and should be avoided to have
better equipment safety & power quality.

SQRT 3 times of normal capacitance currents of healthy
phases flows between the fault location & ground. This
current is oscillatory in nature and creates arc. Also the
voltage of healthy phases will become line voltage because
the faulty phase is connected to ground.

Hence system star point should be grounded.

- Henereunder is one more clarification to Mahmud's
explanation...

Impedance grounding is applicable only to MV systems. LV &
HV system star points are generally solidly grounded i.e.
grounding without impedance.

The reason of solid grounding in LV system is that the
amount of earth fault current is very less e.g. for a
single phase to earth fault in a 415V 3 phase system, the
earth fault current is only 240A (considering earthing
resistance 1 Ohm). Whereas in 11kV system, the amount of
fault current will be 6351A. Hence resistance/impedance is
added to reduce the current to a safe value e.g. a 1250 Ohm
resistance in 11kV system limits the fault current to 5A.

The reason of solid grounding in HV/EHV systems can be
explained as below:

- Usually these are medium & long transmission lines. Hence
the line capacitance is considerably high.
- The capacitance current of the healthy phases flows in
reverse direction to earth fault current (predominantly
inductive current) fed by the faulty phase. Therefore it
nullifies/reduces the flow of earth fault current. Hence no
impedance is required to limit the earth fault current.

Answer / sanjay

Neutral grounding is the method of connecting the neutral
point of system to the earth.

Like : Resistance earthing
Petersen coil earthingi.e. arc suppression coil
earthing.
Reactance earthing

It is required because if the neutral point is isolated at
that time during L-G fault the voltage of the remaining two
phase will be increased up to twice the normal voltage
which makes system unbalanced so to avoid this the neutral
grounding is required .

Neutral grounding resister is the resistor through which
the neutral of the system is connected to the ground.

Answer / mahmud

Impedance earthing - The purpose of this method is to limit
the current for greater safety.
There are 3 type-
1.Earthing through reactance.
2.Earthing through ARC - Suppression coil.
3.Earthing through resistor.

These parameters are needed to specify the earthing
resistor:
U : Rated voltage line to line voltage or system voltage
(kV)
If : Rated fault current (A)
T s : Rated time (Sec) - Duration of the fault
Resistance value :

R=U/(I.Sqare root3)

Answer / santhosh(adityabirla group)

tranformer neutral is connected to earth is called neutral
grounding
if neutral is not earthed in case of one phase is ground
faulty then other two healthy phases voltage become
dangerously high which result insulation failure
generator neutral is connected to earth through resistance
is called NGR
R(NGR) = vph / FAULT CURRENT
EX VPH = 3.8KV , FAULT =100
R= 38OHM

Answer / jagdish ghediya

when two generator running in paralal at that time we
ground only one nuetral if we ground both genrator nuetral
then what hepan

Answer / sophia

neutral grounding or earthing is connecting the neutral point to ground.
Advantages of Neutral Earthing:-
 Connecting the neutral of an electrical system to earth has following
advantages.
 Arcing grounds are eliminated.
 Voltage of healthy phases with respect to earth does not rise √3 times to
normal value.
 Insulation is prevented from stress of high surge voltage and hence
provides long life to insulation & equipment.
 Stable neutral point.
 Earth fault relay function becomes simple & reliable.
 Greater safety to personnel & equip
neutral grounding resistor is the resistor through which neutral is grounded.

Answer / minu

@ Pankaj:
"The reason of solid grounding in LV system is that the
amount of earth fault current is very less e.g. for a
single phase to earth fault in a 415V 3 phase system, the
earth fault current is only 240A (considering earthing
resistance 1 Ohm). Whereas in 11kV system, the amount of
fault current will be 6351A. Hence resistance/impedance is
added to reduce the current to a safe value e.g. a 1250 Ohm
resistance in 11kV system limits the fault current to 5A"

i would like to differ in this point of yours. the LG fault
current in a system is never very less. it usually amounts
almost equal to the phase fault current or even more. I can
say this coz i have conducted short circuit studies for many
industrial plants where the LV system is solidly grounded.

Answer / yaswanth

the earthing is nothing but the connection of neutral point of the supply system to general mass of earth in such a way that immediate discharge of electricity can takes place with out danger.
when grounding is provided it ensures the safety of personal against electric shocks andf avoids accidents. the equipment is also provided with aprotection against lightning and voltage surges.

Answer / biswajit sen gupta

Neutral grounding is the method of connecting the neutral
of the system to the ground.

it is neccessary so that during an unbalanced
condition,neutral current is created which is not required,
so to remove it neutral grounding is required

Answer / jivan r rajput

Neutral is a circuit conductor that may carry current in normal operation, which is connected to ground (or earth) only at the main electrical panel. In a polyphase or three-wire (single-phase) AC system, the neutral conductor is intended to have similar voltages to each of the other circuit conductors

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