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Saturday, 28 April 2012

EARTHING PRACTICE

EARTHING  PRACTICE ,GIST  IS: 3043 CODE  
                       
All medium voltage equipment shall be earthed by two separate and distinct connections with earth In the case of.
  • High and extra high voltage the neutral points shall be earthed by not Less than two separate and distinct connections with earth, each having its own electrode at the generating station or substation and may be earthed at any other point provided ‘no interference is caused by such earthing.
  • If necessary, the neutral may be earthed through suitable impedance.
  • In cases where direct earthing may prove harmful rather than provide safety (for example, high frequency and main. frequency coreless induction furnaces). Relaxation may be obtained from the competent authority.
  • As far as possible, all earth connections shall be visible for inspection.
  • No cut-out, link or switch other than a linked switch arranged to operate simultaneously on the Earthed or earthed neutral conductor and the live conductors shall be inserted on any supply System.
  • This however does not include the case of a switch for use in controlling a generator or a transformer or a link for test purposes.
  • Grounding is not likely to reduce the total magnitude of over-voltage produced by lighting or switching surges. It can, however, distribute the voltage between phases and reduce the possibility of excessive voltage stress on the phase-to-ground insulation of a particular phase.
  • Plate electrodes shall be of the size at least 60 c m X 60 cm, Plates are generally of cut iron not less than 12 mm thick and preferably ribbed. The earth connection should be joined to the plate at not less than two separate points. Plate electrodes, when made of GI or steel, shall be not less than 63 mm in thickness.
  • Plate electrodes of Cu shall be not less than 3.15 mm in thickness.
  • Plate electrodes shall be buried ouch that ii. Top edge is at a depth not less than 15m from the surface of the ground. However, the depth at which plates are set should be such as to ensure that the surrounding soil is always damp.
  • Pipes may be of cast iron of not less than 100mm diameter, 2.5 to 3 m long and 13 mm thick. Such pipes cannot be driven satisfactorily and may, therefore, be more expensive to install than plates for the same effective Area.
  • Water pipes shall not be use as consumer earth electrodes.
  • Under fault conditions, the earth electrode is raised to a potential with respect to the general mass of the earth that can be calculated from the prospective fault current and the earth resistance of the electrode. The results in the existence of voltage. In the soil around the electrode, that may be injurious to telephone and pilot cables, whose cores are substantially at earth potential, owing to the voltage to which the sheaths of such cables are raised.
  • The voltage gradient at the surface of the ground may also constitute a danger to life, especially where cattle are concerned. The former risk arises mainly in connection with large elect rode systems as at power stations and substation.
  • Earth electrodes, other than the used for the earthing of the fence itself, should not be installed
  • In proximity to a metal fence to avoid the possibility of the fence becoming live and thus. Dangerous at points remote from the substation.
  • The materials used for making connections have to be compatible with the earth rod and the copper earthing conductor so that galvanic corrosion is minimized. In all cases, the connections have to be mechanically strong.
  • The cross-sectional area of every protective conductor which does not form part of the supply cable or cable enclosure shall be  in any case, not less than
  • (a) 2.5 mm2, if mechanical protection is provided and
  • (b) 4 mm2, if mechanical protection ii not provided.
  • Joints of protective conductors shall be accessible for inspection and testing except m compound-filled or encapsulated joints.
  • No switching device  is inserted in the protective conductor, but joints which can be disconnected for test purposes by use of a tool may be provided.
  • An auxiliary earth electrode shall be provided electrically’ independent of all other earthed metal, for example, constructional metalwork, pipers, or metal-sheathed cables. This requirement is considered to be fulfilled if the auxiliary earth electrode is installed at a specified distance from all other earthed metal (value of distance under consideration).
  • The earthing conductor leading to the auxiliary earth electrode shall be isolated to avoid contact with the protective conductor or any of the parts connected thereto or extraneous conductive parts which are, or may be, in contact with them.
  • In TN systems, for cables in fixed installations having a cross sectional area not less than 10 mm2 for copper and 16 mm2 for  aluminium, a single conductor may serve both as protective conductor and neutral conductor, provided that the part of the installation concerned is not protected by a residual current-operated device.
  • However, the minimum cross sectional area of a conductor may be 4 mm2, provided that the cable is of a concentric type conforming to Indian Standards and that duplicate continuity connections exist at all joints and terminations in the run of the concentric conductors.
  • When the source of energy is privately owned, there should be no metallic connection with the
  • General public supply unless there has been consultation with the electricity authority concerned.
  • It should be emphasized that an installation together with its source of energy may not consist entirely of one particular type of system. In such cases, each part of that installation may be required to be treated separately without detriment to other parts of the same installation.
  • RCD’s having minimum operating currents greater than 30 mA – These devices are intended to give indirect shock risk protection.
  • The neutral points of each separate electricity system which has to be earthed at the power station or substation.
  • GI/Copper Plate Size: 500mmX500mmX10mm.
  • Wood coal powder and salt are in same quantity.
  • Size of GI Strip: 300mmX10mm
  • Size of GI Pipe: 2.5” Diameter.
  • Minimum 1Ω Resistance should be available at a distance of 15mt.
  • IR value of Earth resistance is less than 10Ω.
  • Earthing resistance of earthing rod is changed from 0.3Ω to 0…8Ωbetween summer to winter.
  • Distance between two earthing pit is 2 X Length of earthing electrode.
  • If ground resistance is less than plate earthing (if hard rock) than Pipe earthing shall be used.
  • Resistance between two earthing pit is negligible.
  • Earthing of lighting protection should not mix with power system earthing.
  • Lighting protection earthing should be 10 time stronger than normal earthing (use copper bus strip instead of wire)
  • Jointing of earthing strip shall be overlap of min 50mm and for earthing wire overlapping shall be min 40mm
Type of earthing
Plate Earthing Electrode
  • For copper shall be 600X600X31mm and
  • For Hot dip GI shall be 600X600X63mm.
Pipe Earthing Electrode.
  • Earthing electrode shall consist of a GI pipe (class B of approved make), not less than 40 mm dia. and 3 meters long. CL pipe electrode shall be cut tapered at the bottom and provided with holes of 12 mm dia. drilled at 75 mm interval up to 2.5 meters length from bottom. The electrode shall be buried vertically in the ground as far as practicable below permanent moisture level, but in any case not less than 3 meters below ground level. The electrode shall be in one piece and no joints shall be allowed in the electrode.
Size of Excavation:
  • Size of 1 meter diameter and 3 meter length shall be excavated after depth of 3 meter the size of excavation shall be 900X300X900mm depth.
  • Plate / Pipe Electrode shall be in vertical position.
  • GI/PVC pipe  for Watering shall be used of 40mm Diameter, length of 3 meter ( contain hole of 12mm Diameter in Zigzag manner starting from  15cm  away from bottom to 2 meter height ).
  • At bottom 150mm layer of Salt and charcoal power shall be installed than Plate shall be installed.
  • Min 120kg of charcoal power and 120kg of salt shall be used for each earthing pit.
  • The plate \ pipe electrode, as far as practicable, shall be buried below permanent moisture level but in no case not less than 2.5 M below finished ground level.

3 comments:

  1. Thanks for nice sharing. You can also find best Cast Iron material at Laxmi Iron & Steel Industry. The main object of LAXMI IRON & STEEL INDUSTRIES is to provide best quality and cost effective Cast Iron. For more information Visit Cast Iron Earthing Pipe.

    ReplyDelete
  2. can u show the photos of strip earthing under the packaged substation

    ReplyDelete