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This paper presents an improved dynamic model based on the finite element method (FEM) for predicting ac critical flashover voltage (FOV) of ice-covered insulators during the melting period. The first improved FEM model is based on the Obenaus/Rizk model, and deals with the case of a single arc established on an ice surface. The arc is modelled by its root which is considered to be an equipotential surface for which a voltage boundary condition is used, and whose propagation is provided by the validation of Hampton's criterion. The second improved FEM model developed is an extension of the single arc model, but takes into account two arcs in series in order to be able to compute the critical FOV for insulators having an arcing distance up to two meters. The simulation results obtained by the two improved FEM models are in agreement with those obtained experimentally. This confirms that using the Hampton criterion as a propagation condition is a very good approach for designing simple dynamic models which can be easily implemented in commercial FEM software.