Transient faults (TFs) are increasingly affecting micro-electronic devices as their size decreases. During the design phase, the robustness of circuits for high reliability applications with respect to this kind of faults is generally validated through simulations. However, traditional HSPICE like simulators are too slow for the task of simulating the effects of TFs on large circuits. In this paper, we present a novel mathematical model to accurately estimate the possible propagation of transient fault-due glitches through a CMOS combinational circuit, which is suitable to be used into a new simulation tool able to provide good accuracy, while significantly speeding up simulations, with respect to HPSICE. In particular, our model allows approximately 90% accuracy with respect to HSPICE simulations.