Based on the thermodynamics and isentropic flow equations

a simulation program was developed. The gas pressure and temperature inpuffer cylinder in the period of arcing and the spacial distribution of Pressure

flow velocity and temperature along the nozzle axis after current zero were calculated for ELFSL4-2 circuit breaker. It is shown that the ratio of upstream pressure to downstream pressure γ should be increased as high as 3 or more in order to move the normal shock wave in the nozzle to the exit of nozzle so as to get the supersonic flow in the full diverging section of nozzle. It is alos found that from the calculation results γ could be increased to 3 or more by use of the effect of arc clogging in the arc chamber of a ELF SL4-2circuit breaker under its rating breaking currents 45-50kA and arcing time 15-23ms. The arc chamber will have an excellent interruption capability.