Eliminating the arc

Typography
Opinion

Jeson Pitt, marketing, D&F Liquidators, explores how to achieve arc extinction in circuit breakers.

When current carrying contacts in a circuit breaker open, the medium in between the contacts becomes highly ionised. The interrupting current gets a low resistive path through this highly ionised medium and it continues to flow even when the contacts are physically separated. When the current flows from one contact to another, the path becomes so heated that it glows. This is called an arc. 

The current through the circuit breaker will not be interrupted as long as this arc is sustained between the contacts. This is because of the conductive nature of the arc.  In order to totally interrupt the current in the circuit breaker, the arc must be extinguished immediately. Not doing so can lead to serious dangers.  Listed below are two ways which can help eliminate the arc.  

1. High resistance 

In this method, you can increase the electrical resistance many times to a high value. This would force the current to reach zero and in turn can avoid the arc from restriking.  You must ensure that the rate at which the resistance is increased or decreased is not abnormal, as it may lead to the generation of harmful induced voltages in the system. 

The arc resistance is increased with time so that the current can be reduced to a value that can no longer maintain the arc. You can increase the arc resistance by several methods. Let’s take a look at 4four of them.

  • Lengthening: Since the resistance of the arc is directly proportional to its length, you can increase its length by enlarging the gap between the contacts. 
  • Cooling: You can de-ionise the medium between the contacts by cooling with a gas blast.  
  • Reducing cross-section area: When you reduce the cross-section portion of the arc, the voltage necessary to maintain the arc and the resistance its path will be increased. You can then reduce the arc by allowing it to pass through a narrow opening or having a smaller area of contacts. 
  • Splitting: Another way is to split the arc into multiple smaller arcs. Every small arc experiences the effect of lengthening and cooling. You can also split the arc by introducing some conducting plates between the contacts.

2. Low resistance or zero current method

This method can only be applied to AC circuit due to the presence of natural zero level of current. The arc will get extinguished at the natural zero level of the AC wave. Moreover, it is prevented from restriking by a rapid building of dielectric strength of the contact space. 

You can interrupt the arc by de-ionising the medium between contacts as soon as the current becomes zero. This is to ensure that the rising contact voltage or restriking voltage cannot breakdown the space between contacts.

Methods for de-ionisation of medium

Lengthening of the gap: The dielectric strength of the medium is directly proportional to the length of the gap between contacts. By increasing this gap, you can achieve higher dielectric strength of the medium.  

High pressure: When you increase the pressure in the vicinity of the arc, the density of the particles constituting the discharge increases. This increased density will cause increased de-ionisation, thus boosting the dielectric strength. 

Cooling: The natural amalgamation of ionised particles will be faster if they are allowed to cool. So, by cooling the arc you can amplify the dielectric strength effectively.  

Blast effect: You can also raise the dielectric strength significantly by replacing the ionized particles with the unionized ones. This can be achieved by gas blast or forcing oil into the space between contacts.