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Component parts of earthing system are to include the following:
- Earth electrode (rods, tapes etc.)
- Main earthing terminals or bars
- Earthing conductors
- Protective conductors
- Equipotential bonding conductors
- Electrically independent earth electrodes for special systems (clean earth)
- Accessories and termination fittings, bonding, welding kits and other materials
1. Earth or Ground Electrode
- Earth electrode is to consist of one or more earth rods (also earth plate or earth matt), interconnected by buried earthing tape or cable, which is to have a total combined resistance value, during any season of the year and before interconnection to other earthed systems or earthing means, not exceeding I ohm. Distance between 2 rods is not to be less than 6 meters.
- Main ring earthing resistance should not exceed 1 ohm.
- Ring type earth electrode is to consist of earthing conductors, in a closed loop, buried in exterior wall foundations underneath the water-proofing, or alternatively at 0.6 m around the perimeter of the building foundations, as shown on the Drawings. Connect all earthing conductors to this ring. Insulated connection flags into the building, of same material as earthing conductors, are to be located at positions of service entrance and main switchboard rooms, terminating in bolt-type earth points (studs) or test links for connection of main earth bar(s). Additional earth rods connecting with the earth ring are to be provided, as necessary, to bring down earth electrode resistance to an acceptable value.
- Functional earth electrode is to be provided separately from, but interconnected to, other earth electrode(s) through suitably rated (470 V) spark gap. Functional earth electrodes are to be used for earthing electronic equipment (communication equipment, digital processors, computers etc.) as required by the particular Section of the Specification and recommendation of the manufacturer.
- Alternative Earth Electrode: Other types of earth electrode may be used, after approval, including:
- Copper plate(s)
- Tape mats (strips)
2. Main Earthing Terminal or Bar
- Main earthing bar is to be provided at point of service entrance or main distribution room, and as described in the Specification or shown on the Drawings. Connect all earthing conductors, protective conductors and bonding conductors to the main earthing bar. Provide 2 insulated main earthing conductors, I at each end of the bar, connected via testing joints to the earth electrode at 2 separate earth pits. Conductor is to be sized to carry maximum earth fault current of system at point of application with final conductor temperature not exceeding 160 deg. C (320 deg. F) for at least 5 seconds. Main earthing conductors are to be minimum 120 mm2 or as otherwise required by the particular Section of the Specification. Main earthing bar shall be positioned at an accessible location within the electrical room and clearly labeled.
- The main earth bar shall be in the form of a ring or rings of bare conductors surrounding or within an area in which items to be earthed are located. Where 2 or more rings are installed they shall be interconnected by at least two conductors, which shall be widely separated.
- Testing joints (test links) are to be provided, in an accessible position, on each main earthing conductor, between earthing terminal or bar and earth electrode.
3. Earthing or Grounding conductors
- Protective conductors are to be separate for each circuit. Selection of sizes is to be in accordance with Table 54F of IEE Regulations.
- Protective conductors are not to be formed by conduit, trunking, ducting or the like.
- Continuity of Protective Conductors: Series connection of protective conductor from one piece of equipment to another is not permitted. Extraneous and exposed conductive parts of equipment are not to be used as protective conductors, but are to be connected by bolted clamp type connectors and/or brazing to continuous protective conductors which are to be insulated by molded materials. Conductor sheaths shall be of yellow-green coloured PVC to meet the requirements ofBS 6004 or IEC 60502-1 Grade ST1 with a minimum thickness of 1.5 mm.
- Bare strip conductors only shall be used for earth electrodes or voltage control meshes.
- Conductors buried in the ground shall normally be laid at a depth of 1000 mm below the underground power cables in an excavated trench. The backfill in the vicinity of the conductor shall be free of stones and the whole backfill shall be well consolidated. All conductors not buried in the ground shall be straightened immediately prior to installation and supported clear of the adjacent surface.
- Earth Fault Loop Impedance: For final circuits supplying socket outlets, earth fault impedance at every socket outlet is to be such that disconnection of protective device on over-current occurs within 0.4 seconds. For final circuits supplying only fixed equipment, earth fault loop impedance at every point of utilization is to be such that disconnection occurs within 5 seconds. Use appropriate tables and present same for approval by the Engineer (IEE Regulations: Tables 41A1 and 41A2, Appendix 7 and Regulation 543).
4. Equipotential Bonding
- Supplementary Equipotential Bonding: Connect all extraneous conductive parts of the building such as metallic water pipes, drain pipes, other service pipes and dueling, metallic conduit and raceways, cable trays and cable armor to nearest earthing terminals by equipotential bonding conductors. Cross-section of protective bonding conductor shall not be less than 1/2 of the protective conductor connected to respective earthing terminal with a minimum of 4 mm2.
- Individual components of metallic structures of plant shall be bonded to adjacent components to form an electrically continuous metallic path to the bonding conductor.
- Small electrically isolated metallic components mounted on non-conducting building fabric need not be bonded to the main earth bar.
- Bolted joints in metallic structures including pipework, which do not provide direct metallic contact shall be bridged by a bonding conductor or both sides of the joint shall be separately bonded to earth unless the joint is intended to be an insulated joint for cathodic protection or other purposes.
- Main Equipotential Bonding: Main incoming and outgoing water pipes and any other metallic service pipes are to be connected by main equipotential bonding conductors to main earth terminal or bar. Bonding connections are to be as short as practicable between point of entry/exit of services and main earthing bar. Where meters are installed, bonding is to be made on the premise side of the meter. Cross-sections of conductors are not to be less than 1/2 that of the earthing conductor connected thereto, and minimum 6 mm2.
5. Identification and Protection of Earthing Components
- Connection of every earthing conductor to earthing electrode and every bonding conductor to extraneous conducting parts is to be labeled as follows, in accordance with the Regulations. SAFETY ELECTRICAL CONNECTION - DO NOT REMOVE.
- Protective and earthing conductors are to be identified by combination of green-and-yellow colors of insulation or by painting bar conductors with these colors, as approved.
- Source earthing conductor (or neutral earthing conductor) is to be identified along its entire length by continuous black insulation labeled 'neutral earthing'. The neutral earthing ground rod pit is to be also clearly identified.
- Exposed external earthing or grounding conductor connection joints are to be protected from corrosion with grease caps or Denso tape (bituminous tape) or approved equal.
- The earthing and lightning protection system for any new extension shall be suitably connected to the existing system.
- In general earthing conductor connections to structures, connections within the lightning protection system conductors, are to be exothermic copper-weld type unless stated otherwise.