It has become a widely adopted practice to determine the interrupting capability of circuit breakers in kiloamperes symmetrical. The rated short-circuit current in rms kiloamperes is referred to the rated maximum voltage in kilovolts.

The ratings structure and tables of ratings for ac high-voltage circuit breakers are found in IEEE C37.042 and IEEE C37.063.

The short-circuit current interrupting process is characterized first by an arc appearing between the breaker contacts. The arc contains a high conductivity plasma column originating from the high temperature and related gas ionization (in the case of gas-blast interrupters).

Interruption will occur at current zero and in this case is first determined by successful cooling of the arc (through gas flow) to eliminate the ionized gas conductive path, and then the race to build up dielectric strength of the open contact gap faster than the rise of the power system recovery voltage.

Several specific problems are encountered during the interrupting process of gas-blast interrupters:

1. Arc plasma temperatures exceeding 20,000 K.

2. The turbulent supersonic flow of the quenching gas in a changing flow geometry with speeds ranging from a few hundred meters per second to several thousand meters per second.

3. The interrupter-moving system and its drive accelerates the moving masses in the few thousandths\ of a second to speeds as high as 10 m/s while simultaneously compressing the quenching gas.

4. The stress places on the network system by the current interruption and the recovery voltage.

The interrupting principle of an SF6 puffer-type interrupter is sketched in Fig. 10-60. On opening, the fixed and moving contacts are pulled apart by the operating mechanism. Thus, the fault current is forced to flow along the arc plasma.

The contact movement combined with the compression cylinder movement in the opposite direction compresses the quenching gas inside the cylinder. The quenching gas is consequently forced to flow through the contact system, and the insulated nozzle toward the exhaust.

This intensive flow of quenching medium along the arc rapidly removes the energy converted within the arc plasma and transforms the path between the open contacts into an insulating gap.

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