With increasing concerns about the global warming potential (GWP) impact of SF₆ gas, new high voltage switchgears are under development with support by research on switching technology in environment-friendly gases, in which C4-FN gas mixtures are identified so far to be one of the most desirable and practical choices. In the research of improving the interruption performance, the arcing behaviors, especially the arc voltage and nozzle ablation, were studied in a 145kV/40kA self-blast circuit breaker with C4-FN gas mixtures under short-line fault condition, and compared to those with SF₆ gas. Behaviors of mass losses of PTFE nozzles were assessed in aspect of arcing energy, diameter and length of nozzle throat and gas mediums. The rate of nozzle ablation appears to be less with C4-FN gas mixtures than that with SF₆ under the same condition. This study can be useful for developing arc models of current interruption with C4-FN gas mixtures, and predicting the electric endurance of PTFE nozzle in high voltage circuit breakers.