In an electrical grid, contingency is an unexpected failure of a single principal component (e.g., an electrical generator or a power transmission line)[1] that causes the change of the system state large enough to endanger the grid security.[2] Some protective relays are set up in a way that multiple individual components are disconnected due to a single fault, in this case, taking out of all the units in a group counts as a single contingency.[3] A scheduled outage (like maintenance) is not a contingency.[4]
The choice of term emphasizes the fact that a single fault can cause severe damage to the system so quickly that the operator will not have time to intervene, and therefore a reaction to the fault has to be defensively pre-built into the system configuration.[5] Some sources use the term interchangeably with "disturbance" and "fault".[2]