This paper conducts an electromagnetic analysis of the closed cabinet used for instrumentation and control in nuclear power plants for the case of a parasitic electric-line current that is undesirably generated onto an interior digital module. Based on the Helmholtz equation in conjunction with both the separation of variables and superposition principle, we employed a mode-matching method to obtain the analytic solution to an electromagnetic interference problem. Using modal coefficients from a set of simultaneous equations by enforcing boundary conditions, we computationally derive electromagnetic-field distributions in terms of various geometric parameters. The results provide us with information useful for estimating the level of influence of the undesired current source on the adjacent digital modules.