The physical modelling of the interaction of arc electric current with a high speed flow of gas in circuit-breakers is presented. The flow model is based on the Euler equations for compressible flow and an energy source term is added to represent the electric arc power input and the radiation transport. The discrete equations are formulated to include the motion of the boundaries and solved using an extension of Roe's scheme for moving grids. The related stability criteria is discussed and the algorithm is validated for a Rayleigh line flow. The simulation of a circuit-breaker operation is performed and the numerical results compared with experimental data.