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An improved empirical model for circuit-breaker arcs is presented. The model makes it possible to predict the breaker thermal short-circuit interruption performance within the entire range of practical interest, using only a small number of measurements from synthetic tests of a production SF6 breaker. The procedure is based on the cybernetic model developed by Hochrainer et al. The key improvement is that the response time characteristic is expressed as a function of the dynamic conductance and not of the externally imposed current. The numerical method consists of an iterative process for fitting the characteristic functions to the measured data by means of truncated Chebyshev series, and does not require manual intervention. This procedure has a significant advantage over most previous treatments in that it does not require any smoothing of the measured data, and has proved to be insensitive against noise.