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The size of earth wire is determined on the basis of
Voltage capacity of service line
Current carrying capacity of service line
Atmospheric condition
None of above
Current carrying capacity of service line
The size of an earth wire (or earthing conductor) is determined by the maximum fault current that the system may be subjected to during a short-circuit condition. It must be robust enough to carry this fault current for a sufficient duration until the protective device (fuse or circuit breaker) operates to isolate the circuit.
The size of an earth wire (or earthing conductor) is determined by the maximum fault current that the system may be subjected to during a short-circuit condition. It must be robust enough to carry this fault current for a sufficient duration until the protective device (fuse or circuit breaker) operates to isolate the circuit.
A=kIтЛЕtтАЛтАЛ тАФ Standard formula for determining cross-sectional area (A) based on fault current (I) and time (t)
I2tтЙдS2k2 тАФ The fundamental constraint for adiabatic temperature rise in earthing conductors
During an insulation failure, the fault current flows through the earth wire to the ground. The conductor size is chosen to ensure that the temperature rise due to I2Rt losses does not damage the cable insulation or the conductor itself. The Indian Electricity Rules specify that the earthing conductor must be at least half the size of the largest current-carrying conductor in the installation.
Earthing conductor size is independent of the load voltage rating.
It must be capable of carrying the prospective fault current without exceeding its thermal limits.
Mechanical strength is also a factor for larger installations.
Material conductivity (copper vs aluminum) affects the 'k' factor.
Ensures safety against touch potential during faults
Prevents fire hazards due to overheating of earthing paths
High fault current requirements lead to thicker, more expensive copper cables
Corrosion of earth electrodes can increase impedance over time
Domestic wiring earthing
Industrial power distribution grounding
Substation earth mats
In India, according to IS 3043:1987, the minimum size of the earthing conductor is generally decided based on the thermal withstand capacity.
Option A is incorrect because voltage does not determine the current-carrying requirement during a fault.
Option C (Atmospheric condition) affects the choice of material (corrosion resistance) but is not the primary factor for current sizing.
B is correct тАФ The size of an earth wire is determined based on the current-carrying capacity required to safely dissipate maximum expected fault currents.
Always remember that the protective earthing conductor is sized for the 'worst-case' fault current, not the steady-state load current.