The SLF(Short Line Fault) breaking capability test for high voltage class $SF_6$ GCB(Gas Circuit Breaker) was conducted. Simplified LC resonant circuit test facility was used for SLF breaking test. During test, Test current was measured by Rogwski coil and arc voltage was measured by voltage divider. Arc conductance was calculated by using these test results before 200ns at current zero. Critical arc conductance value at rated voltage 145kV class is about 2.3mS regardless of breaking current magnitude and arc conductance value at rated voltage 170kV class is about 2.6mS.