|  | 8.11 | Network Power Supply Issues: Surge Suppressors and Uninterruptible Power Supply (UPS) Functions
| 8.11.4 | UPS: for certain electrical problems
Sags and brownouts are usually power outages that are of a relatively short duration, and are caused by something, such as a lightning strike. This creates a power overload, and trips a circuit breaker. Because circuit breakers are designed to automatically reset, they can work from the surrounding power grid to where the source of a short is located in order to re-establish power. This usually occurs within seconds or minutes. Longer power outages can occur, however, when an event, such as a severe storm or flood, causes physical disruption of the power transmission system. Unlike shorter power outages, this type of disruption in service is usually dependent on service crews for repair. An uninterruptible power source is designed to handle only short-duration power outages. If a LAN requires uninterrupted power, even during power outages that could last several hours, then a generator would be needed to supplement the backup provided by a UPS. Can you think of situations where LANs might need the added backup of a generator?