The main contacts, which are placed concentrically outside the arcing contacts, are in Fig a open and the current has commutated to the arcing contacts. These are also about to open in Fig b and when they do, an arc ignites. A nozzle made of PTFE is fixed to the moving contact and guides the compressed gas out of the puffer volume and onto the arc just as the arcing contacts separate.
The flow is, to a certain degree, dependent on the amplitude of the current being interrupted. At large short-circuit currents the arc cross-section may be larger than the nozzle throat diameter, blocking the gas flow. This is called current clogging.
The gas pressure in the puffer volume then continues to increase, mainly because of the mechanical movement, but also due to the transfer of heat from the arc, leading to a rapid temp rise. As the arc approaches its current zero crossing, the arc cross-section decreases and the compressed gas in the puffer volume flows out of the nozzle creating a powerful blast onto the arc,(Fig c) After the arc is extinguished, the gas particles recombine, the dielectric strength is quickly restored and the contact movement stops,(Fig.d) The gas pressure then levels out inside the CB.
