When power fails, all generators start. The first to reach operating conditions connects to the paralleling gear. The highest-priority transfer switches operate and power the life safety and critical branches. The remaining generators move toward synchronization with the paralleling bus and, one by one, are connected. As each new generator comes online, system controls enable another block of transfer switches, in order of priority, to connect to the essential system.

Synchronization takes time and is, at least partially, a stochastic process. Synchronization times cannot be precisely predicted. Depending on control system technology and generator characteristics, as well as load behavior, it can take more than 30 seconds for each successive generator to connect.

After all generators are connected, the system determines whether all are necessary to support the load with an adequate margin and de-energizes unnecessary units. If a generator fails, the lowest-priority transfer switches are disconnected from the essential system, any de-energized units are restarted and resynchronized, and the system seeks a new optimal operating point. If adequate capacity is unavailable, the lowest-priority loads will remain de-energized.

If the hospital essential system is to behave toward the data center as an N + 1 system, the data center’s load must be inserted into a load block ahead of the lowest-priority block. Calculations of uninterruptible power supply runtime requirements must include allowance for synchronization delays in accordance with the system characteristics, and should include a margin of safety to allow for the probabilistic nature of those delays.