EARTHING

All system earthing and equipment earthing shall comply with Electricity Regulations 1994 and relevant parts of MS IEC 60364. Earthing methods shall base on and comply with BS 7430.

Main earthing bar for the generator set (hereinafter as “main earthing bar”) shall be provided at a position near to the generator set switchboard as indicated in the Drawings. Main earthing bar shall be of tinned copper bar mounted on porcelain insulators and of dimension as in Table 1 with respect to the prospective earth fault current indicated in the Drawing. If prospective earth fault current is not specified in the Drawing, then the value shall base on the calculation with respect to the rated capacity, the transient reactance and sub-transient reactance of the generator set. The calculation shall be made available by the generator set manufacturer, and the Electrical Contractor shall provide such calculation to the S.O.’s Representative before the construction of the main earthing bar. Main earthing bar shall be of sufficient length to accommodate terminations for all protective conductors, earth conductors, main equipotential bonding conductors and neutral earthing conductors of the generator set 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 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 ‘Generator Main Earthing Terminals – Safety Electrical Connections – Do Not Remove’ shall be installed to cover full length of the main earthing 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 the Table 1 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 generator set control board, generator set switchboard and main earthing bar for the generator set shall be effected by tinned copper tape of same dimensions as the main earthing bar and they shall be protected by running in insulated PVC rectangular channel flushed with the floor slab.

TABLE 1: Dimensions of Main Earthing Bars for Generator Set 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 more than, but less than twice, the driven depth of the earth electrode. Earth electrodes shall not be installed close 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 bar. 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. 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 `Generator Earth’ or any other appropriate words permanently fixed to the point of connection of every earthing conductor and earth electrode. Protective conductor between the generator set frame and the main earthing bar shall be provided for each generator set and shall be effected by 2 x 3 mm x 25 mm copper tape or single core 150 mm2 pvc green cable. In addition bonding conductor between the main earthing bar and the main earthing bar for the electrical installation shall also be provided and shall be effected by a single core 150 mm2 pvc green cable. The bonding conductors shall be protected by running in insulated pvc rectangular channel/circular tube flushed with the floor slab and/or clipped along the inside wall of the cable trench. Termination at the main earthing bar shall be identified with permanent labels durably and legibly marked with words ‘Generator # Frame’ and ‘MSB Bond’ respectively.

The neutral earthing of each alternator shall be brought out and connected via a single core 150 mm2 pvc insulated black cable to main earthing bar. The cable shall be protected by running in insulated pvc circular tube flushed with the floor slab, clipped to the side walls of the cable trench or clipped along the cable tray or cable ladder as the case may be. Termination at the main earthing bar shall be identified with permanent labels durably and legibly marked with words ‘Generator # Neutral’

Four (4) sets of earthing test results, duly certified by the Electrical Contractor’s competent person of qualification in accordance with the requirements of the Electricity Supply Act 1990 and Electricity Regulations 1994, shall be submitted to the S.O.’s Representative before testing and commissioning of the generator set.

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