Electrical Circuit Breaker Arc Formation Process

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Electrical Circuit Breaker Arc Formation Process :
Q. What is arc formation process?
Q. Define circuit breaker. Explain arcing phenomenon in circuit breaker.

Arc Formation Definition :

  • Refer Figure (a) circuit breakers contacts are in closed position. And entire cross section of conductor carries the current. Now when fault occurs on system a heavy current flows through the contacts of the circuit breaker before they are opened to protect the system is called as arc formation process.
  • Refer Figure (b) when the contacts begin to separate the contact area decreases rapidly and large fault current flows through reduced cross section, which causes increased current density and hence rise in temperature.
  • Thus the heat produced in between the contacts is sufficient to ionize the medium (usually the medium is oil or air) between the contacts.

(a) Contact pressed at high pressure in closed position.
(b) Pressure reduced hence contact are reduced current concentrated on a few spot of high current density.
(c) Contacts separated, arc formation, are is surrounded by ionized gas and hot column of gas.
(d) The voltage gradient = V/∆L
                 If V = 1 KV , ∆L = 1 mm
                 Voltage gradient = 1000 V/mm

  • In circuit breakers the contact space is ionised by the following causes :
  1. Thermal ionization of gas.
  2. Ionization by collision of particles.
  3. Thermal emission from surface of contact.
  4. Secondary emission from surface of contact.
  5. Field emission from the surface of contact.
  • The ionised air or vapour acts as a conductor and an are is struck between the contacts. The potential difference between the contacts is quite small and is just sufficient to maintain the arc. The are provides a low resistance path and consequently the current is the circuit remains uninterrupted so long as the arc persists.
  • The volt-ampere characteristic of a steady are is given by an equation.
Figure (e)
  •  medium The voltage across arc reduces as the current increases as shown in the following Figure (e).

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