If you must have continuous operation in the face of line voltage interruption lasting more than a half second or so, then your only choice is to go to an uninterruptible power supply (UPS). The basic UPS consists of a rectifier/battery charger which takes AC line power and rectifies it to DC, an inverter which takes the DC and converts it back to AC power of the proper voltage and frequency, and a battery bank which takes the place of the rectified DC power source if line power fails. UPS systems are complex, expensive, somewhat inefficient, have a high output impedance, and frequently require special installation facilities and increased air conditioning capacity to dissipate the heat. But if assured continuity of operation is imperative, then you must pick the best UPS for your situation and live with it. The proper UPS will protect your load equipment from all types of power line disturbances.
When power fails, the size of the battery bank determines the length of time the inverter will supply normal power to the load. For a large computer installation this time is usually about 15 or 20 minutes, since this is the length of time the computer will operate without air conditioning before the high-temperature trip causes shutdown. An alarm is activated when 5 minutes of battery time remains to permit orderly shutdown. If more extended protection is required then you must add further backup via an engine-driven generator. Here, battery time is usually only about 5 minutes - long enough to get the engine-driven generator up and synchronized with inverter output. Of course, such an installation is very expensive and requires special facilities and substantial maintenance. It should only be considered if extended power outages are frequent and continuity is critical.
If the load equipment does not require special air conditioning (most process control systems and medical electronics, for example) then it is practical to size the battery bank to cover more extended outages. In this situation, up to an hour of running time is common and much longer periods are possible.
