Dielectric strength of air is

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Option A: Proportional to barometric pressure

Explanation & Concepts:
The dielectric strength of air, which determines its ability to withstand electric stress without breaking down into a conducting path, is directly proportional to its density. Since air density increases with higher barometric pressure and decreases with higher absolute temperature, the dielectric strength follows these variations accordingly.

Working Principle: According to Peek's Law and Paschen's Law, the breakdown voltage of a gas gap is a function of the product of pressure and gap distance. As pressure increases, the density of gas molecules increases, resulting in a higher number of collisions per unit length for free electrons, thereby requiring a stronger electric field to initiate an electron avalanche.

Key Formulas: $g_v = g_0 \delta (1 + \frac{0.3}{\sqrt{\delta r}})$ — Peek's formula for disruptive critical voltage gradient; $\delta = \frac{3.92P}{273+t}$ — Relative air density factor

Key Points: Dielectric strength is directly proportional to air density ($\delta$).; Air density is directly proportional to atmospheric pressure ($P$).; Air density is inversely proportional to absolute temperature ($T = 273+t$).; Corona loss increases as the air density decreases at high altitudes.

Advantages: Predicting the onset of corona on transmission lines.; Designing insulation levels for high-voltage outdoor equipment.

Disadvantages: Increased corona loss at high altitudes due to low barometric pressure.; Variation in performance across different climatic conditions.

Applications: Power transmission line design.; High-voltage switchgear and transformer insulation planning.

Additional Info: The standard dielectric strength of air at NTP (Normal Temperature and Pressure: 76 cmHg, 25°C) is approximately 30 kV/cm (peak).. Option B is incorrect because dielectric strength is inversely proportional to temperature, not directly proportional.

Answer: A is correct — Dielectric strength increases as barometric pressure rises due to the resulting increase in air density.

Exam Tip: Always remember that corona losses are higher in hilly areas (low pressure) compared to plains, because the dielectric strength of air is lower at higher altitudes.

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Option B: Proportional to absolute temperature

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Option C: Inversely proportional to barometric pressure

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Option D: None of above

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