Computers, communication networks and other modern electronic devices need not only power, but also a steady flow of it to continue to operate. If the source voltage drops significantly or drops out completely these devices will fail, even if it is for a fraction of a second. Because of this, even a generator back-up does not provide protection because of the start-up time involved.

To achieve this, extra equipment such as surge protectors, inverters, or a sometimes a complete uninterruptible power supply (UPS) is used. UPS systems can be local or building wide. A local UPS is a small box that fits under a desk or a telecom rack and powers a small number of devices. A building wide UPS can take on several different forms, depending on the application. It directly feeds a system of outlets designated as UPS feed and can power a large number of devices.

Since telephone exchanges use DC, the building's battery room is generally wired directly to the consuming equipment and floats continuously on the output of the rectifiers that normally supply DC rectified from utility power. When utility power fails, the battery carries the load without needing to switch. With this simple though somewhat expensive system, some exchanges have never lost power for a moment since the 1920s.