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Examoogle • User • info@examoogle.com • EE-2024-8821
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Which type of earthing is done in rocky soil?
Plate earthing
Pipe earthing
Coil earthing
Strip
Strip
Quick Summary: Strip or wire earthing is used in locations with rocky soil where excavation for plate or pipe earthing is difficult due to hard sub-strata. In this method, strip or conductor electrodes are buried in horizontal trenches at a shallow depth of about 0.5 meters to maximize the contact area with the soil.
Strip or wire earthing is used in locations with rocky soil where excavation for plate or pipe earthing is difficult due to hard sub-strata. In this method, strip or conductor electrodes are buried in horizontal trenches at a shallow depth of about 0.5 meters to maximize the contact area with the soil.
R=ρAL — Where R is earth resistance, ρ is soil resistivity, L is length, and A is cross-sectional area
Rtotal=2πLρln(d4L) — Resistance of a strip electrode of length L and depth d
The principle relies on spreading the earthing conductor horizontally over a large surface area rather than deep vertical insertion. Since rocky soil has high resistivity, increasing the surface area of contact between the metallic conductor and the ground significantly reduces the total earth resistance (R=ρAL). By laying strips, we overcome the limitations of the high-impedance rocky substrate.
Used specifically when rocky soil makes digging deep pits impossible.
Strip electrodes are usually galvanized iron or copper.
Horizontal trenches are dug to accommodate the length of the conductor.
Ensures low resistance path by maximizing electrode contact surface.
Ideal for rocky terrains with high ground resistivity.
Requires less vertical excavation depth.
Efficient for large fault current dissipation.
Requires a larger lateral area of land.
More susceptible to mechanical damage near the surface.
Longer conductors are needed to achieve equivalent resistance compared to deep-driven pipes.
Mountainous and rocky electrical substation sites.
Transmission tower earthing.
Temporary installations where deep drilling is not feasible.
Strip earthing is typically installed at a minimum depth of 0.5 meters below ground level.
Plate and pipe earthing are unsuitable in rocky soil because the high resistivity of stone prevents the grounding grid from effectively dissipating fault current.
D is correct — Strip earthing is the preferred method in rocky soil because it facilitates ground contact through horizontal layout rather than deep vertical boring.
Always remember: If you see 'rocky soil' or 'shallow depth' in an earthing question, think 'Strip' or 'Wire' earthing. If you see 'high fault current' and 'normal soil', think 'Pipe' earthing.