Breakdown in Gases and Liquids

Paschen’s law states that breakdown voltage is a function of pd (pressure × gap distance), producing a U-shaped curve with a minimum. At very low pd there are too few collisions to build an avalanche; at high pd the mean free path is too short for electrons to gain ionising energy — both raise the required voltage.

Townsend’s theory describes the electron avalanche via the primary ionisation coefficient α and the secondary emission coefficient γ; self-sustained breakdown occurs when γ(e^{αd} − 1) = 1.

(breakdown vs pd product)

Relation 1 —

(breakdown vs pd product)

Write this relation with symbols exactly as in High Voltage Engineering — Rakesh Das before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 2 —

Write this relation with symbols exactly as in High Voltage Engineering — Rakesh Das before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 3 —

Write this relation with symbols exactly as in High Voltage Engineering — Rakesh Das before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

For short uniform gaps the Townsend mechanism applies; for long gaps and non-uniform fields the faster streamer mechanism dominates, where a single avalanche creates enough space charge to propagate a self-sustaining channel.

Corona is a partial discharge that appears on sharp conductors before full breakdown; it causes power loss, audible noise and radio interference on transmission lines, and is reduced by bundling and corona rings.

Liquid dielectrics (transformer oil) break down through streamers seeded by moisture, particles or gas bubbles; purity and degassing raise the breakdown strength significantly.

State assumptions explicitly before using any relation for breakdown in gases and liquids — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.

Wrong assumptions invalidate the entire solution even when the formula is correct. In High Voltage Engineering viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

1. Read the question and list given data with SI units (common in High Voltage Engineering papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to breakdown in gases and liquids.
4. Use equation 1: .
5. Use equation 2: .
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Breakdown in Gases and Liquids appears in substations and cable systems. In Indian electrical curricula this topic is tested because it connects theory to insulation, breakdown, and testing.

GATE and semester exams often combine breakdown in gases and liquids with earlier units — revise prerequisites before attempting mixed problems.

Industry interview panels sometimes ask: "Where did you use breakdown in gases and liquids?" — answer with a lab, mini-project, or plant visit example if possible.

• Treating breakdown voltage as depending on gap distance alone (it is the pd product)
• Placing the Paschen minimum at zero pd (it occurs at a specific finite pd)
• Confusing corona onset with full breakdown voltage
• Ignoring the strong effect of contamination on liquid breakdown strength

Before attempting breakdown in gases and liquids problems, confirm you can:

1. Minimum breakdown voltage at specific pd in Paschen curve
2. Corona onset before full breakdown on conductors
3. Liquid breakdown: streamers, particle contamination, bubble mechanism

Revise the solved examples in High Voltage Engineering — Rakesh Das and one previous-year GATE or university paper for this unit.