The basic principle of n-1 security in grid planning states that if a component – e.g. a transformer or circuit – should fail or be shut down in a grid operating at the maximum forecast levels of transmission and supply, the grid security must still be guaranteed. This means that in this case undue interruptions in supply or the spreading of a failure must not occur. Furthermore, the voltage must remain within the permitted limits and the remaining resources must not be overloaded. This engineering principle is generally accepted throughout all grid levels. Interruptions in supply are, however, tolerated to a certain extent in the distribution grid depending on client structure and as long as they can be rectified within a defined time period. On the other hand, there are more delicate areas of the transmission grid where another measure in addition to the n-1 criterion is implemented when particularly sensitive clients like factories in the chemical or steel industry have to be supplied or where a failure could result in a more large-scale disruption or a very hazardous situation. In this case, the grid is constructed so that grid security can still be guaranteed even if one element is shut down for operational reasons and another should fail at the same time (n-2 case).