The scope of the work covers back-up protection policies, including breaker failure protection, for lines, transformers and busbars of a primary transmission network. it defines input data and describes a theoretical and reasonable approach to evaluate the necessity and performance of a general back-up protection system assuming the input data is known. A description of the moste wide-spread and near-future back-up protection systems and the principal advantages and disadvantages of each solution. Application of the previous theoretical approach to the different solutions described.