The most important element in lightning protection system is grounding. A good grounding system is a system that dissipates the lightning current into the ground efficiently. For designing optimal and effective grounding systems better understanding of the behavior of grounding systems under transient currents is essential. An optimal design is important for achieving Electromagnetic Compatibility (EMC) requirements as well as protection against high currents and voltage hazards. In this work, a model for calculating the response of a vertical electrode to lightning current is presented. The model is based on electromagnetic field theory, which is the most rigorous method for approaching the problem. The model takes into account the radius of the electrode, the conductivity and permittivity of the ground.