The single-phase arc grounding fault is a main fault form in medium voltage distribution networks. It is essential to build an accurate grounding arc model for fault analysis. In this paper， several existing arc models are compared and a new grounding arc model is built based on the dynamic arc model with the arc gap energy-balance theory. And then a model including 10 kV distribution networks and the arc grounding fault model is established with PSCAD. Comparing the simulation waveform with the experimental waveform， the results show that this model can reflect the characteristics of arc grounding fault effectively. Then this model is used to simulate two different intermittent arc grounding faults， the result shows that their over-voltage multiple is compliance with the actual situation， which proves the validity of the arc grounding fault model indirectly.