The current that flows through an element of a power system is a parameter that can be used to detect faults, given the large increase in current flow when a short-circuit occurs. For this reason, a review of the concepts and procedures for calculating fault currents will be made in this chapter, together with some calculations illustrating the methods used. Although the use of these short-circuit calculations in relation to protection settings will be considered in detail, it is important to bear in mind that these calculations are also required for other appli cations, for example, calculating the substation earthing grid, the selection of conductor sizes and for the specifications of equipment such as power circuit breakers.