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Abstract of IEEE-80/ IS 3043 / BS 7430 for earthing
The installation process of earth mats in under ground metro stations includes square grids of copper rods and parallel connected copper electrodes.
Factors affecting the design of the earth mat
- Soil resistivity – The size and no of electrodes required in an earth mat system strictly depends upon the soil resistivity value. Soil resistivity varies from 12-300 ohms normally and for larger values we must use satellite earth mat systems.
- Step touch voltages – The size of one grid depends on the allowable step and touch voltages in that area. Under normal condition the size of grids are 1m X 1m to 4.5m X 4.5m depending upon the step and touch voltage requirement. For more on step and touch voltage please see the post “Step and touch voltage – GPR”
- Use of copper material ensures longevity of the installation, and estimated life of these earth mats are 80-100 years. (For GI earthing, it is 15-20 years only). And no extra provision of water sprinkling or charcoal layer is required for copper earth mats, it is maintenance free installation and in a long run of its use it is economically better option for earthing in under ground metro stations.
How to start the work
- Design work (as per IEEE 80/2000) – To do the calculation work we would need fault current, fault current time Kf, length of the electrode (usually 3m – refer IS 3043) , dia of the copper electrode (after calculated permissible current carrying capacity of the rod BS 7430), Soil resistivity, resistivity of the floor above the mat to calculate the step and touch voltage.
- While designing we should keep
- Electrodes should be minimum 6m apart from each other (in Delhi metro underground stations, due to shortage of area the electrodes are to be placed 3m apart from each other. But, if there is enough space we would install them 6m from each other to minimize the ground potential saturation during fault)
- The copper earth mat should be surrounded by at least 300 mm virgin soil at every point i.e. there should be 300 mm soil above the mat, then only any RCC should come above it. If the mat is being installed in a rocky soil strata, it is advised to pour 600 mm virgin soil below the mat and a 200 mm dia hole for installation of electrodes filled with virgin soil upto a depth of 3m below the earth mat level. The resistance of the electrodes should be less than 10 ohm in normal condition and before installation we can estimate the resistance of a single electrode by the formula R = (100 X P / 2 X 3.14 X L ) X ( Loge (4L/D))
where P = Soil resistivity
L = Length of electrode in cm
D = Dia of electrode in cm
- The copper grid (to be made of copper rod of same dia as of the electrode) will be square in shape. The rods should be brazed (for cross joints and over lap joints ) with copper silver brazing alloy approved for brazing of copper material. The brazing for copper overlap should be more than 75 mm, for copper earth mat risers it should be more than 350 mm of brazing. For additional strength, copper U clamps at every joint should be clamped.
- Designer should make a design which minimizes the no of joints, bends in the rods.
Note : For more details on earthing subject, search in the study material category.