The most common type of short circuit in a three-phase system is a single-phase-to-ground short circuit, accounting for 70 to 90% of electrical faults. In this paper simulation of the protective device from single-phase-to-ground short circuit with automatic change of current setting in electrical networks of 6-10 kV voltage on the basis of Coloured Petri Net is considered. The complexity of elaborated technical systems makes the problem of their modelling actual at the stage of development with the purpose of obtaining estimations of prospective and achievable characteristics. Now the modelling theory of the dynamic discrete systems, based on the formalism of Coloured Petri Nets has a wide application. Petri Net, describing the device operation, was designed and analysed using CPN Tools Programm. The resulting Petri net is correct, as it is live, reversible and safe. Thereby designed device can provide the selective protection from single-phase-to-ground short circuit.