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What should be the value of earth resistance?
More
Less
Infinity
Measurable
Less
Quick Summary: The earth resistance should be as low as possible to ensure the safety of equipment and human life. A low-resistance path facilitates the rapid dissipation of fault current into the ground, triggering protective devices like fuses or circuit breakers effectively.
The earth resistance should be as low as possible to ensure the safety of equipment and human life. A low-resistance path facilitates the rapid dissipation of fault current into the ground, triggering protective devices like fuses or circuit breakers effectively.
R=ρAL — Resistance of an earth electrode where ρ is soil resistivity, L is depth, and A is effective area.
Vtouch=If×Re — Potential rise during fault proportional to earth resistance Re.
According to Ohm's Law (V=IR), the potential rise (V) during a fault is directly proportional to the resistance (R) of the earthing system. By maintaining a low earth resistance, the touch potential during a ground fault is minimized, and the current flow is maximized, ensuring that fault detection relays or breakers operate instantaneously to isolate the circuit.
Ideal earth resistance is effectively zero ohms.
Lower resistance reduces step and touch potentials during faults.
Soil moisture, salt content, and electrode depth significantly influence resistance values.
IS 3043 provides guidelines for permissible earth resistance values based on the electrical system.
Improved safety for operating personnel
Efficient operation of protective relays
Reduction in equipment damage from surges
High cost for achieving extremely low resistance in dry/rocky soil
Requirement of frequent maintenance to prevent corrosion
Power transmission substations
Industrial distribution boards
Domestic electrical installations
For small residential installations, earth resistance is typically kept below 5-8 ohms.
In large power stations, values may be required to be less than 0.5 ohms.
Option 'Infinity' is physically impossible for a functioning earthing system; 'Measurable' is technically true but not the desired target for safety.
B is correct — Earth resistance should always be as low as possible to allow fault currents to flow safely to the ground and to keep the potential of grounded metal parts near zero.
Always remember that in grounding systems, low impedance is the priority to minimize surge voltage reflections and ensure rapid fault clearance.