Since desaturation voltage rise can be seen with a sudden surge in current, it can be used to protect the power device against possible short circuit fault that might occur. However, simply measuring the voltage is difficult, because the voltage can go extremely high. Fortunately, these can be done easily with GD IC such as ACPL-332J from Avago. But the CC is fixed, not suitable for dealing with various different switches that call for different charging times. Theoretical analysis of commonly found circuits is often complex. This paper discusses these circuits and analyzes them theoretically in order to calculate their blanking time, or sensitivities, accurately.