1.
Earthing
Earthing of the system is connecting any one point of
electrical system (for e.g. neutral for AC systems) to ground
Earthing is necessary for
• For discharging the accumulated charges to ground to
prevent hazard to man & machine.
• Stabilizing system voltage with respect to ground.
• Fast & efficient operation of protective gear in case of an
earth fault.
• Efficient return of fault / leakage current to it’s source
with out causing any disturbance to the running system.

2.
Types of Earthing
TN - S System
• Neutral is solidly earthed at source.
• Earth & Neutral are shorted only at a source.
• Two separate conductors (earth & neutral) run
throughout the system.
• It is necessary to have a separate protective earth
electrode in consumer’s premises.
L
O
A
D
R
Y
B
N
E

3.
TN - C System
• Neutral is solidly grounded at source.
• Single conductor does the function of Earth & Neutral.
• Local earth electrode is required at consumer’s
premises.
Types of Earthing
L
O
A
D
R
Y
B
N & E

4.
TT System
• Neutral is solidly grounded at source.
• Neutral conductor is not used for the function of
protective earthing.
• Separate local earth electrode is used for protective
earthing.
• Local earth point is not connected to the neutral.
• This system is not used in India.
Types of Earthing
L
O
A
D
R
Y
B
N
E

5.
IT System
• Neutral is either earthed through resistance / impedance
at source or is kept isolated.
• Separate local earth electrode is used for protective
earthing.
• Local earth point is not connected to the neutral.
Types of Earthing
L
O
A
D
R
Y
B
N
E

6.
• Capacitive charging between conductors & ground.
• Transient overvoltage between system & ground
causing excessive stress on insulation.
R
L
O
A
D
Y
B
N
415 V
239 V
?? V ?? V
Effects of using unearthed system

7.
• No operation of earth fault protection in case of
single phase to earth fault.
L
O
A
D
R
Y
B
N
• √3 times higher voltage between other two phases
& earth if one phase has earth fault developed.
415 V
• Earth fault developed in second phase causes √3
times higher fault current for same fault resistance.
Effects of using unearthed system

8.
Types of Neutral Earthing for Alternator
 Capability of alternator of carrying the fault current.
 Based on the permissible current & type of earth fault
protection used, following are the types.
• Solid Grounding
• Medium Resistance Grounding
• High Resistance Grounding
• Grounding through PT (inductance)
 It is also necessary to study other NGR’s used in same
system (Grid / paralleling transformer).
Basis of calculation –

9.
Types of Neutral Earthing for Alternator
G G
High
Resistance
G G
Medium / Low
Resistance

10.
• Proper operation of NGR’s, during parallel operation of
DG sets.
• Neutral & protective grounding should not be mixed with
each other.
• Interconnection of all earth pits including neutral earth
pits.
• Earthing the Armour of power cable at one end.
• Taking the earthing lead back through CBCT (if used),
to avoid false operation of earth fault relay.
Precautions

11.
• Grounding of shield only at one end.
• Connecting all shields to a separate bus & earthing the
bus in a dedicated pit.
• Grounding of Alternator shaft at DE & insulating it on
NDE.
• Insulating all pipelines for bearing lubrication.
• Providing return path while welding & not using earth
as a return path.
• Strictly avoiding looping of earth & neutral at any point
other than source.
Precautions

12.
• Checking of earth resistance at least once a year on a
dry day.
• Following is the guideline specifying the values.
• Large Power Stations 0.5 Ω
• Major Substations 1.0 Ω
• Small Substations 2.0 Ω
• In all other cases 8.0 Ω
• Earth continuity 1.0 Ω
• (From earth plate to any other point)
• Avoid using earth as a return path for any charge.
• Earthing is done for the efficient return of electrical
energy to it’s source & earth will not soak up all
electrical noise.
Precautions