Earthing and earth conductor sizing play a crucial role in ensuring the safety of electrical systems. Below, you will find important details about earthing and conductor sizing, along with an example:
1. Earthing
It is essential to permanently and effectively ground electrical equipment, metallic structures, vessels, conduits, manholes, cable trays, and fencing. For lightning protection, the tallest columns, stacks, steel structures, cable trays, and pipe supports should be earthed. In hazardous classified areas, process equipment, motors, and control stations should be earthed to prevent static discharge.
1.1 Maximum Resistance to Earth:
- For systems over 600 V: The maximum resistance to earth should not exceed 1 ohm.
- For systems below 600 V: The maximum resistance to earth should not exceed 5 ohms.
- Unless otherwise specified by the system supplier, computer systems should maintain a maximum resistance of 1 ohm.
1.2 Earthing System Components:
- The earthing system should include earthing electrodes, earth loops, earthing grids (where required), and connections to the equipment requiring earthing.
1.3 Main Earth Loop Cable:
- The main earth loop cables should be a minimum of 95 mm², bare, stranded, tinned copper.
1.4 Cable Size for Outdoor Equipment Earthing:
- For outdoor equipment earthing, the cable size should be a minimum of 16 mm².
1.5 Minimum Earthing Cable Size:
- The minimum cable size for earthing should be 6 mm².
1.6 Earth Connections for Electrical Equipment above 600 V:
- Electrical equipment rated above 600 V should have two connections to the earth system, one at each end of the equipment.
1.7 Earthing System Cable Specification:
- The earthing system cable should be stranded copper, with green or green with yellow striped PVC insulation.
1.8 Buried Connections:
- Buried connections should be of the exothermic welded type. Above ground, approved compression type connectors may be used.
1.9 Earth Electrodes:
- Earth electrodes should be made of copper or copper-clad steel, with a minimum length of 3048 mm and a minimum diameter of 19 mm.
1.10 Burial Depth for Earth Loop Cable:
- The earth loop cable should be buried at a minimum depth of 750 mm.
1.11 Non-Metallic Raceway Systems:
- In non-metallic raceway systems, a single insulated earth wire should be pulled through each section of the system to be used for equipment earthing.
1.12 Cable Tray Usage:
- Cable trays should not be used as an earth to other equipment or as an earthing conductor. Cable trays should be made electrically continuous, both section to section and within the cable tray system. Earth path continuity between sections should be maintained using the cable tray manufacturer's certified splice plates for earthing or by installing a bonding jumper. Continuous cable tray systems should have bonding jumpers installed across expansion joints and should be connected to the earthing system at intervals not exceeding 30 m.
1.13 Interconnection of System and Equipment Earthing Electrodes:
- System and equipment earthing electrodes should be interconnected using the main ground loop.
1.14 Earthing Grid:
- An earthing grid consisting of 3048 mm copper-clad steel ground rods and 95 mm² copper cables forming a continuous loop or grid should be used. The grid should be installed in a uniform pattern.
1.15 Installation of Earth Rods:
- Earth rods should be installed through a single 914 mm length of 13 mm diameter concrete pipe with a removable cover, allowing periodic inspection and access to the ground rod clamps. The cover of the concrete pipe should be located flush with the finished grade.
1.16 Earthing of Fences and Gates:
- The earthing of fences and gates should comply with the specified requirements.
1.17 Design Calculations:
- Design calculations should be based on measured soil resistivity values.
1.18 Grounding Resistance Verification:
- The grounding resistance should be calculated and verified after installation to ensure the required values are met.
1.19 Ground Resistance Measurement:
- Ground resistance measurements should be provided for each installed ground rod, both before and after bonding.
1.20 Cable Armor Usage:
- The use of cable armor as the sole means of grounding is not permitted.
1.21 Connection to Existing Ground Grid:
- The new plant grounding grid should be connected to the existing system ground grid at a minimum of two separate points.
Example:
- Earth Electrode & Earth Conductor Material: Galvanized Iron (GI)
- Earth Fault Duration: 1 second
- Standards and Specifications Followed: IS-3043-1987, Indian Electricity Rules & Act, and Contract Specifications
Earth Grid Conductor Size Calculation:
- Assuming a fault current of 50 kA in the system
- Using the formula from IS-3043: I = S. k. (1/√t)
- I = Max. Earth Fault Current (14.83 kA = 14830 A)
- S = Conductor Area (mm²)
- t = Fault Duration (1 second)
- k = Factor for conductor of steel (Table 6A of IS 3043) (80)
Conductor Size Calculation:
- Size of conductor = I/k (1/√t) = 50000 × 1/80 = 625 mm²
Earth Grid Conductor Specification:
- For a single run, a 75x10 mm GI flat conductor shall be used, as stipulated in the contract.