Introduction
Earthing
practices adopted at Generating Stations, Substations, Distribution structures
and lines are of great importance. It is however observed that this item is
most often neglected. The codes of practice, Technical Reference books,
Handbooks contain a chapter on this subject but they are often skipped
considering them as too elementary or even as unimportant. Many reference books
on this subject are referred to and such of those points which are most
important are compiled in the following paragraphs. These are of importance of
every practicing Engineer in charge of Substations.
OBJECTIVE
OF EARTHING
Prime
Objective of Earthing is to provide a Zero potential surface in and around and
under the area where the electrical equipment is installed or erected.
To
achieve this objective the non-current carrying parts of the electrical
equipment is connected to the general mass of the earth which prevents the
appearance of dangerous voltage on the enclosures and helps to provide safety
to working staff and public.
Importance of Earthing & Practices
·
The
earthing is provided for
a)
Safety
of Personnel
b)
Prevent
or atleast minimise damage to equipment as a result of flow of heavy fault
currents.
c)
Improve
reliability of Power supply
·
The
earthing is broadly divided as
a)
System
earthing (Connection between part of plant in an operating system like LV
neutral of a Power Transformer winding and earth).
b)
Equipment
earthing (Safety grouding)
Connecting frames of equipment (like
motor body, Transformer tank, Switch gear box, Operating rods of Air break
switches, etc) to earth.
·
The
system earthing and safety earthing are interconnected and therefore fault
current flowing through system ground raises the potential of the safety ground
and also causes steep potential gradient in and around the Substation. But
separating the two earthing systems have disadvantages like higher short
circuit current, low current flows through relays and long distance to be
covered to separate the two earths. After weighing the merits and demerits in
each case, the common practice of common and solid (direct) grounding system
designed for effective earthing and safe potential gradients is being adopted.
·
Factors
that change the requirement of earth electrode
a)
If
an electrical facility can expand in system, it creates different routes in the
electrode. What was formerly a suitable
low earth resistance can become obsolete standard.
b)
More
number of metallic pipes, which were buried underground become less and less
dependable as effective low resistance ground connection.
c)
Most
of the location, the water table gradually falling. In a year or two, area end up with dry earth
of high resistance.
d)
These
factors emphasize the importance of a continuous, periodic program of earth
resistance testing.
·
The
earth resistance shall be as low as possible and shall not exceed the following
limits:
Power
Stations - 0.5 Ohms
EHT Substations - 1.0
Ohms
33KV Stations - 2.0
Ohms
D/t Structures - 5.0
Ohms
Tower foot resistance - 10.0
Ohms
File Size: 46 Kb
1. As per ANSI/IEEE std 80-1986 code
2. Earthing Conductor
3. DESIGN OF EARTHING MAT
4. No of Earthing Rod used in Earthing Mat
5. Total Length of Earthing Mat Conductor
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