Substations Earthing System Design PART-1
Acc to
EUS (Electricity Utilities Specifications-27)
General Notes Acc to EUS (E27)
Grounding grid shall be buried at a depth ranging from 0.5 to 1.5 m
below final earth grade (excluding asphalt covering).
The grounding grid conductors shall preferable be laid at
reasonable uniform spacing. Depending upon site conditions typical
spacing of the main conductors generally range between 3 meters
to 15 meters.
A perimeter grid conductor shall also surround the substation control
building, at a distance of 0.5-1.5 meters. The grounding grid shall be
installed beneath the 66 KV building (if applicable).
General Notes Acc to EUS (E27)
The grounding grid shall encompass all of the area within the fence,
and shall extend at least 1.5 meters outside substation fence on all
sides (if space permits)
The ground rods shall have minimum dimensions of 16 mm x 3.0
meter However, for other space-limited installations extra long rods
may be considered.
It is recommended that rods be installed in the tops 50 cm minimum
below grade and bonded to the grounding grid by thermo weld
process.
General Notes Acc to EUS (E27)
A (0.15 m) layer of high resistivity surface material such as crushed stone
shall be spread on the earth's surface above the ground grid in the
switchyard are to provide a high resistance surface treatment to reduce
the hazard from step/touch potential to persons during a severe fault.
Maximum temperature (Melting/fusing) of the grid copper conductor is
1083° C , taking into account the exclusive use of exothermic connectors
on the buried ground grid.
The fault clearing time (tc) shall be taken as one second.
Maximum allowable conductor temperature during short circuit 300° C.
General Notes Acc to EUS (E27)
GROUNDING GRID DESIGN PARAMETERS TO BE CONSIDERED
General Notes Acc to EUS (E27)
Recommended Ground conductor Sizes
General Notes Acc to EUS (E27)
The minimum conductor size (in mm2)
Where:
I is the r.m.s current in kA
Amm2 is the conductor cross section in mm2
Tm is the maximum allowable temperature in °C.
Ta is the ambient temperature in °C.
Tr is the reference temperature for material constants in °C.
General Notes Acc to EUS (E27)
The minimum conductor size (in mm2)
Where:
0 is the thermal coefficient of resistivety at 0°C in 1/C°.
r is the thermal coefficient of resistivity at reference
temperature Tr in 1/C°,
r is the resistivity of the ground conductor at referenced
temperature Tr in micro ohms-cm.
=
General Notes Acc to EUS (E27)
Where:
K0= 1 1
𝑜𝑟 𝑘 = − 𝑇 𝑖𝑛 𝐶°
𝛼 𝛼
tc is the duration of fault current in sec.
TCAP is the thermal capacity factor per unit volume in J/cm3/°C.
=
General Notes Acc to EUS (E27)
Material constants for stranded, Annealed, Soft Copper Wire
Maximum allowable conductor temperature during short circuit 300° C
EUS-27 LINK
https://www.mediafire.com/file/m8wulddguj1jj56/E27.doc/file
Conductor Size Excel Sheet
https://www.mediafire.com/file/g0n6un4aatdrlgy/Main+Grid+Conductor+Sizing.xls/fil
e
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