This article describes direct numerical simulation of the first three milliseconds following ignition of the arc in a low-voltage circuit-breaker using a computational-fluid-dynamics code adapted for electric-arc modelling. The mobile electrode is allowed for by a moving mesh. The results describe the evolution of the are with time in terms of its detailed electrical, thermal and fluid dynamic properties. They allow the identification of several phases during the overall arc development process studied here: arc initialisation in the widening electrode gap, arc-thermal expansion, displacement of the arc towards the tip of the mobile electrode, and the beginning of commutation to the fixed electrode. (C) 2008 Elsevier Ltd. All rights reserved.