Arc in Circuit Breaker provides low resistance path to the current flow. This post will discuss about Arc in Circuit Breaker, how it works, two methods of extinction (High Resistance and Low Resistance Method).
What is Arc in Circuit Breaker
Arc in Circuit Breaker provides low resistance path to the current flow which helps in keeping the current in the circuit constant. When a short circuit occurs, a heavy current flows through the contacts of the Circuit Breaker before they are opened by the protective system.
Fig. 1 – Introduction to Arc in Circuit Breaker
When the contacts begin to split, the contact area reduces quickly. As the contact area reduces; temperature and current density increases. The heat produced in the medium between contacts (usually the medium is oil or air) is sufficient to ionize the air (Ohmic heating) or vaporize and ionize the oil. The ionized air or vapor acts as conductor and an Arc is struck between the contacts.
The Electric Arc produced increases Electrical conductivity and hence current continues to flow without any interruption. In other words, Electric Arc can be defined as an electrical breakdown of gas producing plasma discharge due to the current flow in non-conducting medium.
Fig. 2 – Image of Arc Flash/ Blast
Electric Arc has to be quenched in a minimal time else it would damage the Circuit Breaker and eventually the entire system as shown in the Fig. 2. Arc Resistance is inversely proportional to the flow of current during Arcing time. As the Arc Resistance increases, current flow between the contacts decreases. The Arc Resistance depends on three major factors, namely:
- Ionization Degree
- Arc Length
- Arc Cross Section
Degree of Ionization refers to the ratio of Neutral Particles that are ionized in gaseous medium or liquid solution to the charged particles. The ionized molecules/particles are responsible in maintaining the Arc. Increase in the Ionization of the medium, decreases the Arc Resistance.
Arc Length refers to the distance of separation between the contacts of Breaker. If Arc length increases, Arc resistance also increases.
Arc Cross Section
The Arc resistance boosts with the decline in area of cross-section of the Arc.
How does Arc in Circuit Breaker Work
Arc in Circuit Breaker refers to a flash of electric discharge produced when the contacts of circuit breaker is separated. Moving Contact separates itself and moves away from the Fixed Contact which results in the decrease of contact area. It refers to low impedance connection in an electrical system which aids the movement of unwanted electric discharge through air.
It consists of column of ionized gaseous molecules with negatively charged electrons. These electrons get attracted towards positive contact. Similarly the positive ions get attracted towards negative contact.
Fig. 3 – Schematic Representation of Arc in Circuit Breaker
The separation of the contacts of Circuit Breaker creates potential difference between them. The difference in potential leads to sudden rise in temperature and fault current. Low resistance path occurs due to decrease in the contact area. The heat generated ionizes the air or oil which acts as a conducting path for the current flow which produces Arc.
The voltage across the contacts of Circuit Breaker during Arcing is known as Arc Voltage. If ‘V’ is the potential difference between the contacts and ‘d’ is the distance of separation between the contacts; then Electric Filed ‘E’ is:
Fig. 4 – Electric Field between the Contacts
Electric Arc Extinction Methods
There are two methods of Electric Arc Extinction. They are:
- High Resistance Method
- Low Resistance or Zero Current Method
High Resistance Method
In this method, Arc resistance is made to rise up with time so that current is decreased to a value inadequate to sustain the Arc in Circuit Breaker. Accordingly, the current is intermittent or the Arc is quenched. The disadvantage of this method is that massive energy degenerates in the Arc. Hence, High Resistance Arc Extinguishing method is used only in low capacity AC Circuit Breakers.
The resistance of the Arc may be amplified by:
- Arc Lengthening
- Arc Cooling
- Arc Splitting
The Arc Length is directly proportional to its resistance. By widening the gap between contacts, Arc length can be increased.
Cooling is essential for deionization of the medium between the contacts. Cooling also helps in boosting the Arc resistance. A gas blast along the Arc can give efficient cooling.
Arc resistance can be improved by dividing the Arc into a series of smaller Arc’s. All the Arc’s will experience lengthening and cooling effect.
Low Resistance or Zero Current Method
The Low Resistance method is also called as Current Zero method. This method is used in AC circuits for Arc extinction. Modern high power AC Circuit Breakers use Low Resistance method. The Arc resistance is kept lowest and as the current reaches zero, the arc turns off and prevents from emission instead of high voltage across the contacts. Current automatically drops to zero after every cycle in an AC system. When the current is zero, the Arc turns off for a short duration.
The point between the contacts has ions and electrons to facilitate a small dielectric strength that can be easily conked out by the rising contact voltage. If such a collapse happens; the Arc will keep on for another half cycle. If at once after the current zero, the dielectric force of the medium between contacts is developed more quickly than the voltage across the contacts, the Arc falls short to re-strike and the current will be conked out. The quick boost of dielectric strength of the medium near current zero can be attained by :
The real difficulty in AC Arc disruption is to quickly deionize the medium between contacts as the current becomes zero at the rising contact voltage. The de-ionization of the medium can be attained by:
- Gap Lengthening between the contacts.
- Increasing pressure in the surrounding area of Arc.
- Cooling of the particles.
- Blast Effect resulting in replacement of ionized particles by unionized particles by blowing air.