A large number of electromagnetic transient studies have so far reported findings related to the overvoltage behavior of systems within a broad frequency range. However, in most cases, actual grounding effects have been either not taken into account or have been just partially considered. Although an accurate methodology to study grounding effects exists, a detailed analysis of the grounding effects has not yet been fully performed. Moreover, many test applications are performed in laboratories where the grounding is close to ideal. In this paper, the grounding system modeling methodology is described, and its capability is demonstrated for an existing complex grounding system. First, the complex grounding system is modeled in the frequency domain by the TRAGSYS program, which represents the grounding system as an equivalent multiport network. Next, the equivalent network parameters are represented in a broad frequency range enabling time domain computations in ATP-EMTP. Finally, the protection level of the chosen lightning arresters is discussed under most severe conditions.