Explain the criteria to be applied in deciding the earthing system for an electrical installation.
1 Answer
Answer :
Following criteria (factors) to be applied in deciding the earthing system for an electrical installation:
(1) Temperature of soil:
(2) Soil Condition: * Different soil conditions give different soil resistivity. Most of the soils are very poor conductors of electricity when they are completely dry.( Soil resistivity is measured in ohm-meters or ohm-cm.) * Soil with low resistivity is highly corrosive. If soil is dry then soil resistivity value will be very high.
(3) Moisture: * Moisture has a great influence on resistivity value of soil. The resistance of soil drops quickly to a more or less steady minimum value of about 20% moisture content in soil.( A dry soil has high resistivity if it contains no soluble salts)
(4) Dissolved salts: * Resistivity of soil depends on resistivity of water; Small quantity of salts in water reduces soil resistivity by 80%.
(5) Climate Condition: * Increase or decrease of moisture content determines the increase or decrease of soil resistivity.
* Thus in dry whether resistivity will be very high and in monsoon months the resistivity will be low.
(6) Physical Composition:
* Different soil composition gives different average resistivity. Based on the type of soil, the resistivity of clay soil may be in the range of 4 – 150 ohm-meter, whereas for rocky or gravel soils, the same may be well above 1000 ohm-meter.
(7) Location of Earth Pit:
* The location also contributes to resistivity to a great extent. Therefore, choose a site of earth pit that is naturally not well drained.
(8) Effect of grain size and its distribution:
* Grain size, its distribution and closeness of packing are also contributory factors, since they control the manner in which the moisture is held in the soil.
(9) Area Available:
* Single electrode rod or strip or plate will not achieve the desired resistance alone.
* If a number of electrodes could be installed and interconnected the desired resistance could be achieved.
(10) Obstructions:
* The soil may look good on the surface but there may be obstructions below a few feet like virgin rock. In that event resistivity will be affected. Obstructions like concrete structure near about the pits will affect resistivity.
(11) Depth of electrode embedded in the earth. (Depth: As a ground rod is driven deeper into the earth, its resistance is substantially reduced. In general, doubling the rod length reduces the resistance by an additional 40%)
(12) Size and spacing of earth plate and size of conductor. (Size: Increasing the diameter of the rod does not materially reduce its resistance. Doubling the diameter of the ground rod reduces resistance by less than 10%)
(13) Metal of earth plate and earth wire.
(14) Quality of Coal / Charcoal used in the earth electrode pit.
(15) Leakage Current Magnitude:
* A current of significant magnitude and duration will cause significant drying condition in soil and thus increase the soil resistivity.
16) The resistance of the grounding is made up of the following components:
* Resistance of the electrode itself and that of the connection to it
* Contact resistance of the surrounding earth to the electrode
* Resistance of the earth immediately surrounding the grounding electrode or resistivity of earth, which is often the most significant factor
17) Cost of the earthing pit