EARTHING

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9.0 EARTHING
The neutral point of the low voltage system of the transformer shall be solidly earned to achieve an earth resistance not exceeding one ohm. The frameworks and all non-current carrying metal parts of the transformer shall be earthed similarly to achieve an earth resistance not exceeding one ohm. An earth bar of flat hard drawn copper with dimension not less than 25 mm x 6mm shall be installed around the four walls of the transformer room at a height of 300 mm from the finished floor level. The earth bar shall be bolted to the frame earth of the transformer. All joints of the earth bar shall be tinned and bolted. The earth bars shall be painted with an approved green enamel. Unless otherwise specified, the neutral earth and the frame earth of the transformer shall be separately connected to its own group of earth electrodes. The neutral earth and the frame earth shall be connected to its electrodes by means of two numbers green PVC insulated copper cable of cross sectional area not less than 70 sq.mm. The two groups of earth electrodes after having been hang verifies by the S.O. to be each of earth resistance not exceeding one ohm, shall be linked together by means of PVC cables as mentioned above unless otherwise specified. At least two points of the frame earth system
of the transformer shall be connected to the earth electrodes. Earth electrodes shall be of copper jacketed steel core rods with 16 mm diameter and supplied in 2.4 m length and shall have provision for screw coupling with another standard length. The copper jacket shall be of minimum thickness 0.25 mm and shall be metallically bonded to the steel core to ensure that the copper jacket and steel core are not separable. Where the desired earth resistance value cannot be driven, sufficient number of sets of earth electrodes shall be installed outside the resistance area
and linked together by PVC copper cables as mentioned above until the required value is reached. Each set of earth electrode shall be provided with brass connecting clamp and approved type of precast heavy duty concrete inspection chamber with removable cover. The earthing points shall be identified by permanent label legibly marked with the words `Transformer Frame Earth’ or `Transformer Neutral Earth’ permanently fixed at the point of connection of every earthing conductor to an earth electrode.


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