Page 463 - IRSEM_Main Book

19.11.5 Selecting Site for Earthing

(a) The site for earthing shall be chosen in the following order of preference:

(i) Wet marshy ground and grounds containing refuse, such as ashes,
cinders and brine waste.

(ii) Clay soil or loam mixed with small quantities of sand.

(iii) Clay and loam mixed with varying proportions of sand, gravel and stone;
and

(iv) Damp and wet sand and peat.

(b) A site which is naturally well drained shall be chosen. A water logged
situation, however, is not essential unless the soil be sand or gravel.
Perennial wells may also be used as sites for earth electrodes with
advantage where the bottom of the earth is rocky.

(c) Electrodes shall preferably be situated in a soil which has a fine texture
and which is packed by watering and ramming as tightly as possible.
Where practicable, the soil shall be sifted and all lumps broken up and
stones removed in the immediate vicinity of the electrodes.

(d) Where soil conductivity is poor, the chemical treatment may be resorted to
improve the same. Common salt together with charcoal in alternate layers
is generally used for this purpose and the addition of less than one part by
weight of salt to 200 parts of soil mass may reduce the resistivity by 80%
but there is little advantage in increasing the salt content above 3%.
Calcium chloride and sodium carbonate are also beneficial.

(e) Use should be made where possible of natural salts in soil produced by
bacteriological action on decaying plants. The resistivity of the soil on
which plants are growing will be less than that of a similar soil in the
absence of plants.

(f) As far as possible, the earthing arrangement shall be located in the natural
soil. The made-up soil which has not consolidated or is likely to be eroded
by weather, shall be avoided.

(g) The minimum clearance of equipment earths from system earths (e.g.
earthing of AT/Transformer etc.) provided by the Electrical Department
either of the Railways or of the other Administrations shall be 20 meter.

(h) Asphalt or concrete cover of about 50 mm thickness around the Earth for a
radius of 1 meter to retain the soil moisture is desirable.

Chapter 19: Installation, Testing & Maintenance of Signalling Equipment Page 411 of 535