Earthing or Grounding Electrode for an effective and reliable electrical earthing system requires proper survey and analysis of project location, type of structures installed and electrical / electronic equipments used in the entire system. The design calculation of the earth electrode systems shall be based on the approved earth resistivity data and the system's fault currents and their duration.
Design Calculation Of The Earth / Ground Electrode
The design calculations shall be to the approval of the Engineer and shall be based on the methods given in the standards listed. The calculations shall include the following parameters:
- Earth resistance of the whole system of its components
- Earth potential rise
- Step, touch and mesh potentials inside and outside the perimeter fence (see fence earthing and bonding)
- Requirements for a high resistance surface layer
- Conductor ratings
The earth potential rises shall not exceed the ITU limits appropriate to the classification of the system unless special precautions are taken to cater for transferred potentials
Step, touch and mesh potentials shall be within the limits calculated in accordance with the standards given in IEEE 80 and BS 7430 for the proposed surface layer. The formula for allowable body current shall be used for 50 Kg body weight.
Ground or Earth Electrode
The earth electrodes shall comprise a system of bare conductors forming a mesh buried near the surface of the ground supplemented, as necessary, by one or more of the following electrodes:
- A system of interconnected steel rods driven into the ground
- Structural steel metalwork in direct contact with the ground.
- Reinforcing steel in buried concrete
Mesh System as Earthing or Grounding Electrode
The mesh system shall be designed in accordance with above sub-title "Design calculations" above to limit touch, step and mesh potentials taking into account the combined length of the mesh conductors, other buried conductors and rods but excluding any buried conductors outside the perimeter fence. Due regard shall be given to non-linear distribution of the fault current giving rise to the highest potentials at mesh corners.
The rating of the mesh conductors shall be compatible with the fault currents after allowing for parallel paths with a minimum conductor size of 300 mm 2 copper.
The current division factors = 0.8 of given fault current shall be taken for design calculations of ground mesh.
If the placement of earthing mesh is designed for outside of the building, the depth of mesh shall be 2 m below the ground level or this shall be at least 1 m below the power cables of standard depth.
Interconnected Earth Rods or Ground Rods
If the design calculations show that a mesh alone is unable to limit the potentials to the required values, then the mesh shall be supplemented by the use of interconnected earthing rods driven into the ground or installed in bored holes.
Rods shall be installed inside the perimeter fence to enclose the maximum possible area compatible with the earthing of any metallic fence. (The spacing between rods shall not be less than their length, unless rating considerations determine otherwise). The rods shall be interconnected in groups of 4 to I 0 rods by yellow-green coloured PVC insulated stranded copper conductors to form a ring. Each group shall be connected to the mesh by duplicate insulated copper conductor via disconnecting test links.
Individual rods may be connected directly to the mesh provided the conductor to the rod can be disconnected for testing the rod.
Rods installed in bored holes may be used to reach lower resistivity ground strata at depths beyond the reach of driven rods or where rock is encountered and it is not possible to drive rods. After installing the rod the bored hole shall be back-filled with a low resistivity liquid mixture that shall not shrink after pouring to ensure good contact between the rod and the ground for the life of the installation.
The resistance and rating of individual rods and the combined resistance of the groups of rods in the proposed design shall be calculated and the rating of the interconnecting conductors shall not be less than that of the main grid conductor.
The calculation of potentials in the design of the complete installation shall be made without the group of rods with the lowest estimated resistance to simulate the condition with the group disconnected for testing.
Reinforcing Steel of the Building as Earth / Ground Electrode
The reinforcing steel in the foundations of buildings containing the primary electrical equipment may be used as auxiliary electrodes subject to the approval of the Engineer. The Contractor shall show in the design calculations that the fault currents and d.c. stray currents will not damage the structure.
Steel reinforcing mesh in the floors of the building may also be used for the control of step and touch potentials within the building subject to the approval of the Engineer.
Conductors Outside Perimeter Fence
If the design calculations show that the step and touch potentials outside the perimeter fence, gate or wall exceed the limits then additional bare conductors shall be buried in the ground outside the fence in the form of rings encircling the whole site.
The distance of the conductors from the fence and the depth shall be determined in the design to ensure that step and touch potentials are within the limits.
The minimum conductor size shall be 70 mm2 and shall be connected to the fence or the mesh with 70 mm2 conductors at each corner of the site and at intervals of not more than 100 m. These conductors shall not be included in the calculations called for above.
Earthing or Grounding of Gas Insulated Switchgear (GIS)
The earthing of gas-insulated switchgear (GIS) installations shall be subject to special considerations regarding step and touch potentials in accordance with the standards listed. However, the earthing conductor shall be yellow-green colored PVC insulated stranded copper conductor of minimum overall conductor size of 300 mm2 The connection shall be at multiple points.
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