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EHV transmission systems of power utilities use the N-1 principle for planning purposes. With very long distances the application of two full systems is limited because of reliability reasons and economical limits. Among others, one measure for increasing the reliability of a single three phase system is the use of an additional spare phase. In order to achieve approximately the same reliability of a double system GIL only a further spare phase with a suitable switching unit has to be applied. This enables the chance of reduced investment and lifecycle costs without reduction of reliability. The sensibility of this solution and the savings achievable depend on the boundary conditions of each project.