EARTHING

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All system earthing and equipment earthing shall comply with Electricity Regulations 1994 and relevant parts of IEC 60364. All protective conductors shall be of high conductivity copper conductor either stranded or solid, continuous throughout the whole lengths and without joints, except by means of approved mechanical clamps. They shall be robust design and protected from mechanical damage and corrosion. Before connecting the protective conductor, the metal works, the conductors and the metal work at the paint of contact, including the clamps, shall be thoroughly cleaned of surface corrosion or paint and tinned to ensure that good electrical contact is made.
Every circuit of a switchboard, distribution board, control board and tap-off units, all lighting points, lighting switches, fan points, fan switches, three pin switched or unswitched socket-outlets, power point etc. shall be provided with circuit protective conductors. All exposed conductive parts and extraneous conductive parts shall be effectively bonded to earth. Unless otherwise specified the minimum cross sectional area of the protective conductors shall be in accordance with the following Tables/Clause in the MS IEC 60364-5-54: –
(a) Earthing conductors – Table 54A (except for solid earthing)
(b) Circuit protective conductors – Table 54(neednot exceed 75 sq. mm)
(c) Bonding conductors – Clause 547
Where connections are made at switchgear and such items of electrical equipment the protective conductors shall terminate in a cable socket or other approved means. In the case of MICC cables, the copper outer sheath may be utilised as earth protective conductor provided that at the termination of each cable run the copper sheath is effectively bonded to earth.
In the case of solid earthing, main earthing terminals or bars mounted on porcelain insulators shall be provided at a position near to the switchboard as indicated in the Drawings. Main earthing bar shall be of tinned copper bar of dimension as in Table 4 with respect to the prospective earth fault current indicated in the Drawings. Main earthing bar shall be of sufficient length to accommodate termination for all protective conductors, earth conductors and main equipotential bonding conductors of electrical installation, lightning protection system and generator installation. Means shall be provided for disconnecting the earth conductors to permit measurement of the resistance of the earthing arrangements. The joints shall be disconnectable only by means of a tool, and shall be mechanically strong to ensure electrical continuity. A transparent protective cover of sufficient thickness and dimensions with label engraved and marked in red legibly with words ‘Main Earthing Terminals – Safety Electrical Connections – Do Not Remove’ shall be installed to cover full length of the main eathing bar. All connections to the main earthing bars shall be soundly made and electrically satisfactory by means of bolts and nuts with spring washers and jam nuts.
Two earthing conductors of tinned copper tape of dimension as in Table 4 below shall be provided to connect the main earthing bar to two different earth electrodes. The earthing conductors shall be protected, where necessary, by means of galvanised steel conduit and buried in the ground at a depth of not less than 450 mm below finished ground level.
Protective conductors between main switchboard or sub-switchboard and main earthing bar shall be effected by tinned copper tape of same dimensions as the main earthing bars and they shall be protected by running in insulated PVC rectangular channel flushed with the floor slab.
TABLE 4: Dimensions of Main Earthing Bars and Earthing Conductors

 

 

 

 

 

 

 

Earth electrodes shall be of copper-jacketed steel core rods with 16 mm diameter and supplied in 1500 mm length and shall have provision for screw coupling with another standard length. The copper jacket of 99.9 % purity electrolytic copper shall be of minimum radial thickness 0.25 mm and shall be molecularly bonded to the steel core to ensure that the copper jacket and steel core are non separable. Each earth electrode shall be driven 3000 mm in depth. Where the desired earth resistance value cannot be achieved after the first earth electrode have been driven, sufficient number of earth electrodes in
parallel shall be installed outside the resistance area until required value is reached.
Mutual separation between two earth electrodes shall be equal todriven depth of the earth electrode. Earth electrodes shall not be installedclose to a metallic fence. Unless the metallic fence is separately earthed, the fence shall be separated from the electrical earthing system by at least 2000 mm. Interconnection between different earth electrodes shall be by means of 25 mm x 3 mm annealed copper tape.
Two test earth electrodes namely potential test probe and current test probe shall be provided. The test earth electrodes shall be one length of 1500 mm in depth. The current test probe shall be placed 30 m from the first earth electrodes with potential test probe midway between. Test leads of 2.5 sq. mm PVC insulated cable connecting test earth electrodes shall be terminated independently on the porcelain insulators next to the main earthing terminals or bars.
The test leads shall be protected by means of non metallic conduit and buried in the ground at a depth of not less than 450 mm below finished ground level. Termination shall be identified with permanent labels durably and legibly marked with words ‘Potential Earth Test Probe – Do Not Remove’ and ‘Current Earth Test Probe – Do Not Remove’. Similar labels of not less than 4.75 mm high shall be permanently fixed in a visible position at earth electrodes
The connection of the earthing conductor and/or the earth electrode to the earth electrode shall be soundly made by the use of plumbed joints, either by brazing using zinc-free material with a melting point of at least 600 0C or by thermic welding or by cold pressure welding. Each earth electrodes shall be provided with heavy duty type inspection chamber with removable cover. The compressive strength of the inspection
chamber and cover shall be minimum 6 N/mm2. Lifting hook shall be provided
in the cover.

Residual current operated circuit breakers shall be provided with minimum cross-sectional area 16 mm2 PVC insulated copper conductor cable as earth conductors, one set of earth electrodes as described above, inspection chambers and removable covers.
However, if they are connected to a main earthing system, separate earth electrode need not be provided. A permanent label durably marked with words ‘Safety Electrical Connection – Do Not Remove’, in legible type not less than 4.75 mm high, shall be
permanently fixed in a visible position at or near: –
(a) the point of connection of every earthing conductor to an earth electrode, and
(b) the point of connection of every bonding conductor to extraneous conductive parts.
In addition each earthing point shall be identified by permanent label legibly marked with the words `MSB Earth’, ‘SSB Earth’, ‘RCCB Earth’ or any other appropriate words permanently fixed to the point of connection of every earthing conductor and earth electrode.


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