Notes on Arc Production in Circuit Breakers

Introduction to Arc Production

• Arc Definition: A column of ionized gas; consists of ions (positively charged) and free electrons (negatively charged).
• Ion Formation: Ions are created when atoms/molecules lose electrons, resulting in positive charge.
• Weight Comparison: Ions are heavier than electrons, affecting their movement speed in an electric field.

Movement of Charges

• Electron Movement: Electrons accelerate quickly towards the positive terminal due to their low mass.
• Collision Effects: As electrons move, they collide with atoms, dislodging more electrons and increasing current.
• Current Contribution: Negative charges (electrons) primarily contribute to the current in the arc.

Characteristics of Arc Production

1. High Voltage Gradient at the Cathode

• Contact Area Reduction: As contacts separate, the area for current flow decreases.
• Increased Current Density: High current through a small area (order of 10^6 A/cm²) raises resistance and voltage drop.
• Voltage Breakdown: If voltage exceeds breakdown strength of air (~30 kV/cm), it leads to ionization and arc formation.

2. Thermionic Emissions

• Temperature Effect: Increased temperature leads to molecular vibrations, causing electrons to be released from atoms.
• Material Consideration: Copper is used in contacts due to its conductivity and resistance to high temperatures.

Arc Maintenance Mechanisms

• Kinetic Energy: Increased collisions result from the kinetic energy of electrons and vibrating molecules, generating more ionized particles.
• Voltage Gradient Impact: Higher potential gradients increase electron velocity, resulting in more collisions and ionization.

Arc Voltage and Energy

• Arc Voltage Behavior: At current zero, arc voltage is at its maximum; as current increases, voltage decreases.
• Energy Calculation: Arc energy = Integration of (Current * Voltage * Time).
• Ayrton's Equation: Describes the voltage drop across the contacts:
  • EB = A + B/IA
  • Where A = alpha + gamma * L (alpha = 30 V, gamma = 10 V/cm for copper).*

Conductance of the Arc

• Ionized Electron Density: Conductance is proportional to the number of ionized electrons per cm³.
• Length and Diameter Effects: Conductance decreases with increased arc length and diameter.

Arc Extinguishment Techniques

1. Cooling: Reduces temperature, leading to recombination of electrons with atoms.
2. Carrier Removal: Using materials (e.g., SF6 gas) that absorb electrons or blowing air to remove ionized particles.
3. Narrow Channel Creation: Adjusting contact area to decrease arc diameter and increase length.

Summary and Next Steps

• Reviewed arc generation, maintenance, and extinguishment techniques.
• Next class will focus on practical procedures for extinguishing arcs and related theorems.
• Questions can be posted in the comment section for further clarification.