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4.11.5 Design of grounding system
4.11.5.1 The design of the grounding earthing system depends on the following considerations
• Magnitude of fault current
• Duration of fault current
• Thermal stability limits of material
• Mechanical strength
• Corrosivity
The short time current carrying capacity of a conductor can be determined from the
following formula :-
time.
A = (12.15 X 10 -3 ) X 1 X √t for wedded joints and
A = (15.7 X 10 -3 ) X 1 X √t for bolted joint where
A is the cross-sectional area in mm 2 & 1 current in Amps and ‘t’ is fault clearance
To compensable the loss due to corrosion, the main earthing ring size shall be
increased by 100% and size of inner conductors by 50%.
4.11.5.2 The minimum number of pipe earth electrodes to be provided at traction
substation depends upon the soil resistivity. However, the approximate number of electrodes
can be determined from the following equation.
Number of electrodes =
The distance between the two earth electrodes shall not be less than twice the length
of the electrode. The earth electrode used at TSS is of 40 mm dia galvanised iron pipe of
four meter length.
4.11.6 Shielding wire/earth screen
Fault Current
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An overhead earth wire is provided in the switch yard of the sub-station connecting
the principal gantry masts with 7/9 SWG or 19/2.5 G.I. wire for protection against direct
lightening strokes. The shielding conductor is strung about 3.5 metres above the strung bus
(for 132 KV’s) so that all the conductors and equipments lies within the protection angle of
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