Thumb Rules-13 ( Quick Reference Earthing -CPWD)


 

Earthing Strip for Sub-Station Equipment

CPWD-TABLE VIII

Type of Installation Earth Electrode Earth Strip
Indoor sub-station with HT panel, Transformer capacity up to  1600  KVA, LT panel, D.G Set. Copper Plate 25 x 5 mm Copper Strip
Indoor sub-station with HT  panel, Transformer  capacity  above  1600  KVA, LT panel, D.G Set Copper Plate 32 x 5 mm Copper Strip
HT Outdoor sub-station Copper Plate 25 x 5 mm Copper Strip
LT Indoor sub-station with generator Copper Plate 25 x 5 mm Copper Strip
LT   switch   room   with Main   LT  D.B Copper Plate 20 x 3 mm Copper Strip

 

Neutral Earthing of Transformers and Generators

CPWD-TABLE VIII

Type of Installation Earth Electrode Earth Strip for Neutral
Transformer  of  capacity  up  to  1600 KVA Copper Plate 25 x 5 mm Copper strip
Transformer  of  capacity  above  1600  KVA Copper Plate 32 x 5 mm Copper strip
Generating set of all capacity Copper Plate 26 x 5 mm Copper strip
Type of Installation Earth Electrode Earth Strip for Neutral
Transformer  of  capacity  up  to  1600 KVA Copper Plate 25 x 5 mm Copper strip

 

Earthing Strip for Bus Trunking and Rising Main

CPWD-TABLE VIII

Type of Installation Material of Main Conductor Earth Strip
Bus   trunking   up   to   2500   Amp   capacity Copper/ Aluminum 2 No 25 x 5 mm Copper Strip
Bus   trunking   above   2500   Amp   capacity Copper/ Aluminum 2 No 32 x 5 mm Copper Strip
Bus  trunking for  generating set and LT panel Copper/ Aluminum 2 No 25 x 5 mm Copper Strip
Rising main up to 400 Amp capacity Copper/ Aluminum 2 No 20 x 5 mm Copper Strip
Rising  main  above  400  Amp  and  up to 800 Amp Copper/ Aluminum 2 No  20 x 3 mm Copper Strip

 

The Size of Earthing conductors

As per CPWD

Size of phase conductor Size of Earthing conductor of the same material as phase conductor
Up to 4 sq.mm. Same size as that of phase conductor
Above 4 sq.mm. up to 16 sq.mm. Same size as that of phase conductor
Above 16 sq.mm. up to 35 sq.mm. 16 sq.mm.
Above 35 sq.mm. Half of the phase conductor

 

Materials and Sizes of Earth Electrodes

CPWD-TABLE IX

Type of Electrodes Material Size
Pipe GI medium class 40 mm dia 4.50 m long (without any joint)
Plate (i) GI 60 cm x 60 cm x 6 mm thick
(ii) Copper 60 cm x 60 cm x 3 mm thick
Strip (i) GI 100 sq. mm section
(ii) Copper 40 sq. mm section
Conductor (i) Copper 4 mm dia (8 SWG)
Note :  Galvanization of GI items shall conform to Class IV of IS 4736 : 1986.

 

Minimum Sizes of Earthing Conductors for Use Above Ground

CPWD- TABLE X

Material and Shape Minimum Size
Round copper wire or copper clad steel wire 6 mm diameter
Stranded copper wire 50 sq. mm or (7/3.00 mm dia)
Copper strip 20 mm x 3 mm
Galvanized iron strip 20 mm x 3 mm
Round Aluminum wire 8 mm diameter
Aluminum strip 25 mm x 3 mm

 

Minimum Sizes of Earthing Conductors for Use Below Ground

CPWD- TABLE XI

Material and Shape Minimum Size
Round copper wire or copper clad steel wire 8 mm diameter
Copper strip 32 mm x 6 mm
Round galvanized iron wire 10 mm x 6 mm
Galvanized iron strip 32 mm x 6 mm

 

Selection of Type of Earthing Electrodes

As per CPWD

Type of electrode Application
GI pipe Internal electrical installations like Distribution Board and Meter Boards (in residential quarters), feeder pillars and poles etc.
GI plate (i) For Fire Fighting pumps and water supply pumps.
(ii) Lightning conductors.
Copper plate Neutral earthing of transformers/ generating sets.
Strip/ Conductor Locations where it is not possible to use other types.

 

Number of Earth Electrodes

As per CPWD

Equipment No of Earthing
For neutral earthing of each transformer 2 sets
For body earthing of all the transformers, 2 sets
HT/LT Panels and other electrical equipment
in the Sub-station/ power house
For neutral earthing of each generating set 2 sets
For body earthing of all the generating sets, 2 sets
LT panels, other electrical equipment in the generator room

 

Size of protective conductor

As per CPWD

Size of phase conductor Size of protective conductor of the same material as phase conductor
Up to 16 sq.mm. Same as Phase Conductor  Size
16 to 35 sq.mm. 16 sq.mm.
35 sq.mm Half Size of Phase Conductor

 

Earthing Points

As per CPWD

Earthing Description
Location for Earth Electrodes Normally  an  earth  electrode  shall  not  be  located  closer  than  1.5  m  from  any  building.
Installation of Pipe Pipe electrode shall be buried in the ground vertically with its top at not less than 20 cm below the ground level
Installation of Plate Plate electrode shall be buried in ground with its faces vertical, and its top not less than 3.0 m below the ground level.
The strip or conductor The strip or conductor electrode shall be buried in trench not less than 0.5 m  deep
More Earthing Electrode When more than one electrode (plate/pipe) is to be installed, a separation of not less than 2 m shall be maintained between two adjacent electrodes.
Earthing Electrode  If the electrode cannot be laid in a straight length, it may be laid in a zigzag manner with a deviation upto 45 degrees from the axis of the strip. It can also be laid in the form of an arc with curvature more than 1 m or a polygon
Earthing Pit Cover A cast iron / MS frame with MS cover, 6 mm thick, and having locking arrangement shall be suitably embedded in the masonry enclosure.
Earthing Wire Protection  The  earthing  conductor  from  the  electrode  up  to  the  building  shall  be  protected  from mechanical injury by a medium class, 15 mm dia. GI pipe in the case of wire, and by 40 mm dia, medium class GI pipe in the case of strip. The protection pipe in ground shall be buried at least 30 cm deep (to be increased to 60 cm in case of road crossing and pavements)
No of Earthing Conductor Two protective conductors shall be provided for a switchboard carrying a 3-phase switch gear there on.
Earthing Electrode  No earth electrode shall have a greater ohmic resistance than 5 ohms as measured by an approved earth testing apparatus. In rocky soil the resistance may be up to 8 ohms.
Earthing Resistance Each   of   the   earth   stations   should   have   a   resistance   not   exceeding   the product  given  by  10  ohms  multiplied  by  the number  of  earth  electrodes  to be  provided  therein.  The  whole  of  the  lightning  protective  system,  including any  ring  earth, should  have  a  combined  resistance  to  earth  not  exceeding 
10 ohms without taking account of any bonding
More Earthing Resistance If the value obtained for the whole of the lightning protection system exceeds 10 ohms, a reduction can be achieved by extending or adding to the electrodes, or by  interconnecting  the  individual  earth  terminations  of  the  down  conductors  by  a  conductor

 

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