Where concreting is carried out to a pole, muffing of the pole should also be carried out normally of
1˚ in diameter and up to a height of 1˚ above ground and finished smooth by cement slurry. This
will prevent the pole from rusting due to water.
III
(1)
EARTHING:
General:
For isolation of H.T. or L.T. line automatically on fault by means of a Circuit Breaker or a fuse
and for safety of the persons and animals our distribution system is required to be earthed
suitably. The earlier practice was to run a continuous earth wire along the line and every fifth
pole and end pole were earthed. However, this practice is now dispensed with and now
earthing to each H.T. or L.T. line pole individually is provided. In a P.S.C. pole, an earth wire
is already embedded with the top and bottom ends brought out for connections for the
earthing suitably. The top end of the embedded earth wire has to be connected tightly to all
the metal parts on the top of the pole, such as X-arms, top fittings, guard channel (if any)
etc. by a small pieces of a G.I. wire. Similarly, the bottom end of the embedded earth wire is
firmly fixed to a piece of G.I. wire which is connected on the other end to a suitable earthing
device.
Two types of earthings are standardized by Rural Electrification Corporation (R.E.C.). (i) Pipe
type earthing and (ii) Coil type earthing. Angle type earthing could also be utilized.
(2) Types of Earthings:
(i) Pipe Type Earthings:
In this method, 40 mm dia. G.I. pipe of 2500 mm length is used for earthing. This pipe
is perforated at each interval of 100 mm and is tapered at lower end. A pipe clamp is
fitted to this pipe at 100 mm below the top to fix the G.I. Earth Wire No. 8 SWG, the
other end of which is fixed firmly to the bottom end of the earth wire duly brought out,
which is embedded in the P.S.C. pole. This pipe is placed inside 2700 mm deep pit. A
600 mm dia. ‘farma’ of G.I. sheet or cement pipe is placed around the pipe. Then the
annular space between this ‘farma’ and the G.I. pipe is filled up by 300 mm alternate
layers of salt and coke/charcoal, while the remaining space outside the ‘farma’ is filled
up by loose earth. The ‘farma’ is gradually lifted up as the filling up progresses. Thus,
the pit is filled up up to 300 mm below the ground level, i.e. approx. up to the pipe
clamp. This remaining 300 mm is covered by constructing a small chamber of bricks so
that the top open end of the pipe and the pipe clamp connection remain accessible, for
attending when necessary. The chamber is closed by a wooden or stone slab cover.
Water is poured into the pipe through its open end periodically to keep the earthing
resistance within the specified limits.
(ii) Coil Type Earthing:
In this method of earthing, G.I. Wire No.8 SWG is used in the coil form instead of G.I.
pipe. The pit for earthing may be only 1800 mm deep and 600 mm section. The G.I.
earth wire is wound in the form of a coil of 50 mm dia. and 450 mm length (approx.115
turns) and placed at the bottom vertically in the centre. The G.I. wire from the top of
the coil extends right above the ground level and fixed firmly with the bottom end of
earth wire embedded in the P.S.C. pole. The surrounding space around the coil up to
350 mm dia. is filled up by 300 mm alternate layers of salt and coke/charcoal up to 1500
mm from the bottom. The top and surrounding remaining space of the pit is filled up
with loose earth.
It may be mentioned here that proper connections and rigid jointing must be done in
the earthing system, otherwise the purpose will not be served for which the earthing is
provided.