Earthing Strip /Wire / Pit Quick Reference (As per CPWD) (Part-2)
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 |