How do Circuit Breakers Extinguish Electric Arc?

Pictures showing a circuit breaker in operation

Figure 1: A circuit breaker in operation.

The arc is a free gas with high temperature and high conductivity, which not only has a great destructive effect on the contacts, but also prolongs the time of breaking the circuit. One of the protective functions of circuit breakers is to extinguish electric arc. Here are the principles and four methods of circuit breakers’ arc extinguishing.

How do Vacuum Circuit Breakers Extinguish Electric Arc?

When the vacuum circuit breaker is disconnected, the capacitance between the two contacts causes the insulation between the contacts to break down, resulting in a vacuum electric arc. When the current interrupted by the circuit breaker is close to zero, the temperature and pressure of the arc between the contacts drop sharply, so that the arc cannot continue to persist and is suspended.

2D drawings showing the principles of circuit breakers’ arc extinguishing

Figure 2: The principles of circuit breakers’ arc extinguishing.

Within a few μs after the arc is suspended, the vacuum vacancy withstand voltage level between the two contacts is restored. At the same time, there is a certain distance between the contacts and can withstand high voltages. Therefore, usually after the current crosses zero, the arc will not reignite and be interrupted. This is the principle of its arc extinguishing.

4 Methods of Electric Arc Extinguishing

1. Mechanical arc extinguishing: The arc is elongated through extreme equipment. This method is mostly used in switching appliances.

2. Magnetic blowing arc extinguishing: Under the effect of a magnetic field generated by a magnetic blowing coil connected in series with the contact, the arc is elongated by the effect of electromagnetic force. Then, the arc is blown into the arc extinguishing cover made of solid medium, and when it touches the solid medium, it is cooled and suspended.

2D drawings showing the principles of arc blow-out

Figure 3: Principles of arc blow-out.

3. Narrow slit (longitudinal) arc extinguishing: Under the effect of the magnetic field electromotive force formed by the arc, the arc is elongated and enters the narrow (longitudinal) slit of the arc extinguishing cover. A few longitudinal slits can cut the arc into segments and touch the solid medium, and then, the arc is stopped. This structure is mostly used for communication touch devices.

4. Grid arc extinguishing: When the contacts are separated, the generated arc is pushed into a group of metal grids under the effect of electric power and is cut into several sections. Each of the mutually insulated metal grids is equivalent to an electrode, so there are many cathode and anode voltage drops.

A sample of grid arc extinguishing components of circuit breakers

Figure 4: Grid arc extinguishing components of circuit breakers.

For the communicating arc, when the arc crosses zero, there will be a dielectric strength of 150V ~ 250V near the cathode. This stops the arc from continuing to hold. Because the arc-extinguishing effect of the grid is much stronger than that of the DC, the grid is often used for arc-extinguishing of communicating device.