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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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The size of the earth or ground wire is based on the
maximum fault current carrying through the ground wire
rated current carrying capacity of the service line
depends on the soil resistance
both A and C
both A and C
Quick Summary: The size of an earthing conductor (ground wire) is determined by its ability to carry the maximum fault current without exceeding thermal limits and its effectiveness in maintaining low impedance to the ground to ensure rapid operation of protective devices.
The size of an earthing conductor (ground wire) is determined by its ability to carry the maximum fault current without exceeding thermal limits and its effectiveness in maintaining low impedance to the ground to ensure rapid operation of protective devices.
A=kI⋅t — Minimum cross-sectional area calculation where I is fault current, t is time, and k is a material-dependent factor.
Zs=Rearth+Rline — Earth fault loop impedance which must be low enough for protective operation.
The earth wire acts as a low-impedance path during a ground fault. Its cross-sectional area must be sufficient to withstand the thermal stress of the short-circuit current for the duration of the fault clearance time. Simultaneously, the earth-fault loop impedance (influenced by soil resistivity) must be low enough to allow sufficient fault current to flow, ensuring that circuit breakers or fuses trip instantaneously according to IS 3043 standards.
The thermal limit of the wire prevents fusion or excessive heating during high-current fault conditions.
Soil resistivity directly affects the 'Earth Resistance', which dictates how easily the fault current disperses into the ground.
IS 3043 is the primary Indian Standard governing the design and selection of grounding systems.
If soil resistance is high, more elaborate electrode systems (like grid or multiple rods) are required rather than just thicker wires.
Ensures safety of personnel by preventing dangerous touch and step potentials.
Provides a stable reference point for system voltage.
Soil resistivity is subject to seasonal changes due to moisture content fluctuations.
Corrosion of earth conductors over time can increase loop impedance.
Domestic electrical installations.
Large industrial power plants and substations.
Lightning protection systems.
Option B (rated current of the service line) is incorrect because the service line current pertains to normal operational load, whereas earth wire sizing is primarily a fault-handling requirement.
A properly sized earth wire prevents the chassis of equipment from reaching a lethal potential during a phase-to-ground fault.
D is correct — The sizing of an earthing conductor is governed by both the thermal withstand capability under maximum fault currents and the overall loop impedance dictated by the soil's resistivity.
Always remember that earthing is a safety-first design; whereas cable sizing is based on Load Current (Thermal/Voltage drop), Earthing is based on Fault Current (Safety/Time-Current characteristics).