Long interruptions are always due to component outages. Component outages are due to three different causes:
I. A fault occurs in the power system which leads to an intervention by the power system protection. If the fault occurs in a part of the system which is not redundant or of which the redundant part is out of operation the intervention by the protection leads to an interruption for a number of customers or pieces of equipment. The fault is typically a short-circuit fault, but situations like overloading of transformers or under frequency may also lead to long interruptions. Although the results can be very disturbing to the affected customers, this is a correct intervention of the protection. Would the protection not intervene, the fault would most likely lead to an interruption for a much larger group of customers, as well as to serious damage to the electrical equipment.
As distribution systems are often operated radially (i.e., without redundancy) and transmission systems meshed (with redundancy), faults in transmission systems do not have much influence on the reliability of the supply, but faults in distribution systems do.
2. A protection relay intervenes incorrectly, thus causing a component outage, which might again lead to a long interruption. If the incorrect tripping (or maltrip) occurs in a part of the system without redundancy, it will always lead to an interruption. If it occurs in a part of the system with redundancy the situation is different. For a completely random maltrip, the chance that the redundant component is out of operation is rather small. Random maltrips are thus not a serious reliability concern in redundant systems. However maltrips are often not fully random, but more likely when the system is faulted. In that case there will be two trips by the protection: a correct intervention and an incorrect one. The maltrip trips the redundant component just at the moment that redundancy is needed. Fault-related maltrips are a serious concern in redundant systems.
3. Operator actions cause a component outage which can also lead to a long interruption. Some actions should be treated as a backup to the power system protection, either correct or incorrect. But an operator can also decide to switch off certain parts of the system for preventive maintenance. This is a very normal action and normally not of any concern to customers. There is in most cases at least some level of redundancy available so that the maintenance does not lead to an interruption for any of the customers. In some low voltage networks there is no redundancy present at all, which implies that preventive maintenance and repair or changes in the system can only be performed when the supply to a part of the customers is interrupted. These interruptions are called "scheduled interruptions" or "planned interruptions."
The customer can take some precautions that make the consequences of the interruption less than for a nonscheduled interruption. This of course assumes that the utility informs the customer well in advance, which is unfortunately not always the case.