Which UPS configuration allows a branch-circuit breaker to be changed without disconnecting the load?

The correct choice is the configuration known as "System + System." This setup allows for more than one Uninterruptible Power Supply (UPS) unit to work together, providing redundancy and increased availability. In this configuration, if one UPS needs maintenance or if a branch-circuit breaker needs to be changed, the load can continue to receive power from the other UPS unit. This seamless transition ensures that operations are not interrupted, which can be crucial in environments where uptime is critical, such as data centers or hospitals.

In contrast, other configurations like 1N, N+1, or Backup Power System may not offer the same level of flexibility. For example, in an N+1 configuration, there is generally one additional UPS unit for redundancy, but switching out a branch-circuit breaker while the load remains connected may still require careful handling. Similarly, a Backup Power System is primarily designed for providing power in case of failure, and may not facilitate maintenance without load interruption. Thus, System + System is particularly advantageous when considering maintenance operations without affecting the load.