Thermal plasma generated either by transferred arc or by plasma torches are involved in many industrial processes including, for example, cutting, welding, powder synthesis or coating elaboration by plasma spray techniques. The common feature is that electrical energy is converted into thermal energy, which must be transferred differently following the goal of the technique. Cutting requires a high thermal flux density to the anode in order to optimize both the accuracy and the speed of cutting. On the opposite, heat transfer to the anode of a plasma spray torch needs to be reduced, as much as possible, in order to slow down the nozzle erosion which is responsible, among other problems, of the monotonic deviation of the torche performances over a long time scale. This presentation is devoted to a general approach of the physical processes concerning an electric arc, thermally constricted and confined in the channel of a plasma torch nozzle. Experimental results characterizing both stationnary and transient behaviour of the generated plasma flow will be described. General tendancies observed for the shift of the torch performances and connected to the anode erosion, will be commented.