In humid weather, corona occurs at a voltage

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Option A: Much less than needed in fair weather

Explanation & Concepts:
Corona discharge is a phenomenon of ionization of air surrounding a conductor. In humid or rainy weather, the air contains moisture and suspended particles, which reduce the dielectric strength of the air, causing corona discharge to occur at a lower critical disruptive voltage compared to dry, fair weather.

Working Principle: The critical disruptive voltage ($V_c$) is directly proportional to the air density factor ($\delta$) and the dielectric strength of air ($g_0$). During humid weather, the air density factor is effectively reduced or the surface conditions of the conductor become irregular due to water droplets, which increases the electric field intensity at the surface. Consequently, the breakdown of air occurs at a lower applied potential.

Key Formulas: $V_c = m_o \delta g_o r \ln(\frac{d}{r})$ — Disruptive critical voltage formula; $\delta = \frac{3.92b}{273+t}$ — Air density factor calculation

Key Points: Corona loss is higher in humid weather than in fair weather.; The presence of moisture lowers the breakdown strength of air ($g_0$).; Corona occurs when the potential gradient exceeds the dielectric strength of air.; Higher frequency and larger conductor surface irregularity increase corona.

Advantages: Provides visual warning of insulation failure; Acts as a damping mechanism for surges

Disadvantages: Significant power loss; Radio interference; Ozone production causes chemical corrosion; Audible noise

Applications: Used in electrostatic precipitators; Used in ozone generators; Used in Kirlian photography

Additional Info: For fair weather, the surface factor $m_o$ is approximately 1.0, while for dirty or wet conductors, $m_o$ ranges from 0.8 to 0.9.. Option B is incorrect because moisture acts as an ion source, facilitating ionization rather than inhibiting it.

Answer: A is correct — Humid weather lowers the dielectric strength of air, requiring less voltage to initiate ionization compared to fair weather conditions.

Exam Tip: Always remember that corona is inversely related to the dielectric strength of the medium; if conditions (like humidity) lower the dielectric strength, the voltage required to initiate corona also decreases.

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Option B: Much higher than that needed in fair weather

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Option C: Equal to that needed in fair weather

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Option D: None of above

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