The electrical systems are subjected constantly to failures caused by various causes. Disturbances in these systems may originate from acts of God (lightning, high winds, rain, frost, etc.) or human actions (fire, vandalism, project errors, operation errors, miscommunications) and intrinsic failures to the own equipment that compose them. Regarding the type of disturbance, single or multiple contingencies may occur.

The systems are planned by the reliability criterion n-1, i.e. they must be able to withstand any single contingency without interrupting the supply. However, in real-time operation, the systems are subject to contingencies involving the loss of two or more components. When the additional investment in reliability is lower than the social and / or economic impact caused by any multiple failures in the supply, the adoption of a more restrictive reliability criterion can be justified. ONS has been working continuously in order to add additional security for the performance of the BIPS, through various initiatives.