This paper presents a comprehensive investigation into the practical application of the One Hundred Millivolt (100 mV) Cathodic Polarization Criterion to assess Cathodic Protection effectiveness by means of coupon on underground pipelines in a Gas Distribution Network. The One Hundred Millivolt Cathodic Polarization Criterion is a widely recognized criterion for assessing the effectiveness of CP systems particularly in those areas where the -850 mV Potential criterion is difficult to respect due to the higher soil resistivity or the network configuration. When using the criterion by evaluating the 100 mV shift by measuring the depolarization after disconnecting the CP current, the instant-off potential to consider as the reference value is often measured too late, thus loosing completely the overvoltage generated by activation polarization that generally disappears in the order of few milliseconds. In this study, a deep investigation on applying the criterion by measuring depolarization on coupons is given, evaluating on real field measurements the effect of the waiting time on taking the reference value from coupon disconnection, as well as the effect of changing in soil resistivity considering different periods of the year.