Measurements of particle temperatures and concentrations are performed for a transient and unstable plasma arc in a circuit breaker operated in a 3 atm SF6 gas. The large current intensity causes an important vaporization of the electrodes so that the plasma develops in a mixture of SF6 and Cu. Time resolved photographs of the arc obtained with interference filters show a compact and homogeneous zone dominated by a S+−F plasma, and an inhomogeneous zone close to the electrodes dominated by Cu. Average electron densities and temperatures and also the total number of the different emitting atoms or ions are determined in the two zones by spectroscopical methods using simultaneous measurements with time resolution at high and low spectral resolution. Particle concentrations are obtained after an estimate of the average volumes of the different plasma zones.