Visual critical voltage is

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Option B: Higher than disruptive critical voltage

Explanation & Concepts:
Visual critical voltage ($V_v$) is the minimum phase voltage required to initiate visible corona glow around a conductor. It is always higher than the disruptive critical voltage ($V_d$), which is the threshold at which the air surrounding the conductor begins to ionize and become conductive.

Working Principle: Disruptive critical voltage represents the onset of dielectric breakdown in air. However, the visual appearance of corona (glowing discharge) requires a higher localized electrical stress to sustain the excitation of air molecules to an energy level where they emit visible light. Thus, after the air begins to conduct at $V_d$, the voltage must be increased further to $V_v$ for the characteristic blue-violet glow to appear.

Key Formulas: $V_d = g_0 \delta m_0 r \ln(\frac{D}{r})$ — Disruptive critical voltage formula; $V_v = V_d (1 + \frac{0.3}{\sqrt{\delta r}})$ — Visual critical voltage formula (Peek's law)

Key Points: Corona onset occurs in stages: ionization first, then light emission.; The constant $0.3$ in Peek's formula accounts for the additional intensity required for visible glow.; Factors like air density ($\delta$), surface condition ($m_0$), and conductor radius ($r$) affect both voltages.; Visual corona leads to power loss and ozone production.

Advantages: Provides visual warning of electrical stress on lines.; Helps in designing conductor diameters to avoid audible noise and RI.

Disadvantages: Causes significant power loss (Corona loss).; Results in electromagnetic interference (RI/TVI).

Applications: High Voltage transmission line design.; Insulation coordination studies.

Additional Info: Peek's empirical formula is the standard reference for calculating these critical voltages.. For stranded conductors, the surface irregularity factor $m_0$ is less than 1, which reduces the $V_v$ required compared to a perfectly smooth conductor.

Answer: B is correct — Visual critical voltage is always higher than disruptive critical voltage because visible light emission requires greater excitation energy than the initial ionization of air.

Exam Tip: Always remember that ionization (disruptive) happens first, followed by visible glow; therefore, $V_v$ must be greater than $V_d$.

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Option A: Lower than disruptive critical voltage

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Option C: Equal to disruptive critical voltage

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Option D: None of above

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