Earthing, or grounding, is connecting the electrical system to earth to discharge accumulated charges and prevent hazards. It stabilizes voltages and allows protective devices to operate during faults. There are different earthing systems like TN-S, TN-C, and TT that use separate earth and neutral conductors or combine them. Proper earthing is important for safety and performance of the electrical network.

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