Almost all firms use UPS systems. They maintain the continuity of supply in case of power outages. But all UPS equipments regardless of manufacturer should be completely de-energized for preventive maintenance at least once per year. This limits the availability of the power system.

But now a ?days certain configurations of the UPS systems are there to permit concurrent maintenance so that we can overcome the above problem.

Each of the system configurations described in this paper has its own advantages and disadvantages. The single module system will continue to be the simplest, least-expensive approach for applications that can comfortably schedule downtime for preventive maintenance and system alterations.

The parallel-redundant system improves system availability and reliability, and simplifies maintenance of individual UPS modules. The isolated redundant configuration enables the facilities manager to add redundancy to an existing single-module installation.

The distributed- redundant, selective-redundant, Power-Tie and hybrid ac-dc systems provide continuous availability of conditioned power.