In case of single phase earth fault in medium voltage (MV) isolated neutral systems, the voltages of the remaining two phases to the earth raise from normal phase to neutral voltage to full line value, i.e., 3 times their normal value. This causes the stress, e.g., thermal stress on the inner insulation and also on the outer polyethylene (PE) jacket, which is the weakest point in the cable. Differential scanning calorimetry (DSC) monitors heat effects associated with phase transitions, e.g., crystallization, and chemical reactions as a function of temperature. The knowledge of the crystallization mechanisms of polymers presents a fundamental importance, since the degree of crystallinity influences the mechanical and thermal properties of these materials. In this paper the percentage of crystallinity of an underground cable outer PE jacket have been calculated from a peak of exothermic crystallization of differential scanning calorimetry (DSC) thermograms.