Magnetic devices such as transformers and rotating electrical machines are key components of modern power systems and the simulation of transient events involving them is fundamental. In this paper, a method for strongly coupling a power systems transients program with a finite element field solver is proposed, which eliminates the time step delay in the solution of the two separate domains, and therefore avoids the instability which otherwise could arise. The field model provides an accurate computation of the magnetic field distribution in the device, taking into account the ferromagnetic core nonlinearity and spatial effects, while the electrical network is represented by a circuit model. The transients program used for the coupling is the Alternative Transients Program (ATP), and the field solver is FLD. The simulation scheme and its implementation have been verified by comparison with a directly coupled circuit-field solver.