The metallic sheath and armour of multicore cables shall be bonded together and earthed.
Cable earthing conductors shall be copper strip with appropriate section laid in formed cable trenches or ducts, or fixed to walls, concrete or steel work, by means of clamps or claw type cleats, appropriately secured into suitable wall plugs or tapped holes. The spacing of fixings shall not be greater than 1 m. Where earth strip is buried or laid in positions in which there is danger of corrosion it shall be provided with a corrosion-proof serving of extruded thermo-plastic or self-adhesive PVC tape which for short lengths may be hand applied on Site.
The Contractor shall connect, unless otherwise instructed, all cable sheaths and armour, supporting steel work and the metal enclosure of sealing boxes, joints, etc., to the main earth bar by suitable branch connections which may be of bare or insulated stranded conductor as required to suit the cable system.
Concentric type bonding leads of suitable impedance shall be employed where necessary. The bonding and earthing system shall be subject to the approval of the Engineer and shall be designed to meet the subsequent maintenance requirements.
Bonding of Metallic Screens of Multicore Cables
Power loss in a multicore cable depends on the currents flowing through the metallic sheath which has to be minimized through proper methods of bonding to the earthing and therefore, overall current carrying capacity of the cable can be maximized. Following are the methods to be used:
Both-ends Bonding For Multi Core Cable
Both-ends bonding method applies to mainly in trefoil formation taking advantage of low power loss. Both of the ends of the cable to be bonded using cable sheath.
Single-Point Bonding For Multi Core Cable
Single point Bonding method is being used where cable sheath provide no path for the external faults or circulating currents. There will be induced voltage between sheath and earth depending its length of the cable but no current flowing.
Cross-Bonding for Multi Core Cable
This method used where continuous sheath runs from earthed termination to another earthed termination. Since it is sensationalized, there will not be much current flowed through the bonding. This method permits a cable current-carrying capacity as high as with single-point bonding but longer route lengths than the latter. It requires screen separation and additional link boxes.