In many electrical and electronic systems, metallic enclosures are used to provide electromagnetic shielding. These enclosures normally contain thin wires, thin slots, and frequency-selective slots that degrade the shielding effects. The capacitance thin-slot formalism (C-TSF) by [2] has been around for quite some time in the finite-difference time-domain analysis (FDTD) of thin-slot penetration. An improved CTSF (I-TSF) has been proposed using the conformal mapping technique in [3]. The TSFs is used in the analysis of shielding effectiveness (SE), and it is verified that I-TSF gives a better result than C-TSF in Shielding Effectiveness analysis.