Electrical systems, equipments and materials are subjected to failures that can cause the total destruction of equipments and severe power outages. For a better understanding failures of equipments and materials and the mechanism of these failures is important to understand the concept of Mean Time Between Failures (MTBF)that is used to evaluate reliability of equipments. MTBF is the predicted elapsed time. Design errors may be a consequence of incorrect, insufficient and/or incomplete data regarding the characteristics, the duty cycle and the expected functioning of the installation, provided by the Owner. Design will not be correctly carried out, calculations will be. Apart from natural ageing of materials and unexpected causes (very strong winds, fall of trees, animals actions, lightning, functioning under severe transient conditions, malfunction of protection systems, careless excavations, communications blackout,. Even in qualified and certified manufacturers with installations that meet all the requirements manufacturing errors and deficiencies may happen. Several factors contribute for those.