OBJECT OF EARTHING
The object of earthing system is, to provide as nearly as possible a surface under and
around a station, which shall be at a uniform potential and as nearly to zero or absolute earth
potential as possible. This is in order to ensure that, in general, all parts of apparatus other than
live parts shall be at earth potential, as well as, to ensure that operators and attendants shall be at
earth potential at all times, so that there exists no potential difference, to cause shock or injure a
person, when short circuit or any type of earth fault takes place in the system.
CLASSIFICATION OF EARTHING
Earthing can be broadly classified as follows
• System earthing.
• Equipment earthing.
• Static and lightning protection earthing.
System earthing is designed primarily to preserve the security of the system by ensuring that the
potential on each conductor is restricted to such a value as is consistent with the level of insulation
applied. From the point of view of safety, it is equally important that earthing should ensure efficient and
fast operation of protective gear in the case of earth faults. The system earth resistance should be such
that, when any fault occurs against which earthing is designed to give protection, the protective gear
will operate to make the faulty main or plant harmless. In most cases such operation involves
isolation of the faulty main or plant by circuit breakers or fuses.
In case of underground systems, there is no difficulty whatever, but, in case of overhead line
systems protected by fuses or circuit breakers, fitted with over protection only, there may be difficulty in
arranging the value of the system earth resistance in such a way that a conductor falling and making
good contact with the ground results in operation of the protection systems. A low system earth
resistance is required even in the cases where an arc suppression coil is installed, as its operation may be
frustrated by too high an earth electrode resistance.
Earthing may not give protection against faults that are not essentially earth faults. For
example, if a phase conductor on an overhead spur line breaks and the part remote from the supply falls
to the ground, it is unlikely that any protective gear relying on earthing, other than current balance
protection at the sub-station, will operate since the earth fault current circuit includes the impedance of
the load that would be high relative to the rest of the circuit.
When the insulation of the motor windings breaks down, current starts leaking into the metal
body of the apparatus which would get charged with the supply voltage and become alive and
dangerous, unless the charge is drained to earth by an earth connection. When the body is
earthed, a fault in the winding would cause a heavy current to flow into the earth, which would
then blow out the fuse, thereby disconnecting the supply to the motor.
The permissible value of the earth resistance is directly related to the system voltage, maximum
fault current and the sensitivity of the protective equipment and the reasonable potential rise. If the
resistance to earth is high, the protective system will not function under a fault, creating a dangerous
condition.