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Dielectric strength of air under normal condition is
30 kV/cm
100 kV/cm
150 kV/cm
200 kV/cm
30 kV/cm
The dielectric strength of air, also known as the breakdown strength, is the maximum electric field intensity that a sample of air can withstand without undergoing electrical breakdown. Under normal conditions (at standard temperature and pressure), this value is approximately 30 kV/cm (peak).
The dielectric strength of air, also known as the breakdown strength, is the maximum electric field intensity that a sample of air can withstand without undergoing electrical breakdown. Under normal conditions (at standard temperature and pressure), this value is approximately 30 kV/cm (peak).
EmaxтАЛ=dVтАЛ тАФ formula for electric field intensity between parallel plates
gmaxтАЛ=rln(D/r)VтАЛ тАФ formula for maximum voltage gradient in a cylindrical system
When the electric field intensity between two electrodes exceeds 30 kV/cm, the air molecules undergo ionization. Free electrons gain sufficient kinetic energy to collide with neutral air molecules, liberating more electrons in a process known as Townsend discharge or avalanche breakdown. This creates a conductive path, leading to the formation of an arc or corona discharge.
Dielectric strength is dependent on pressure, temperature, and humidity.
Corona discharge occurs when the voltage gradient exceeds the disruptive critical voltage of air.
Air is considered a self-restoring dielectric material.
Increasing humidity typically increases the dielectric strength of air.
Low cost and easily available
Self-restoring properties after electrical breakdown
Low dielectric strength compared to insulating oils or gases like SF6тАЛ
Susceptible to atmospheric conditions (pressure, moisture, dust)
Overhead transmission lines
Switchgear insulation
Air-core inductors and capacitors
The 30 kV/cm value is specifically for the peak voltage gradient; for RMS, it is often cited as approximately 21.2 kV/cm.
Options B, C, and D represent values significantly higher than the standard breakdown field of air at 1 atm and 25┬░C.
A is correct тАФ The dielectric strength of air at normal conditions (Standard Temperature and Pressure) is approximately 30 kV/cm.
Remember that air's dielectric strength decreases with an increase in altitude (lower air density), which is why corona effects are more prominent at higher elevations.