What must AC power sources be before paralleling?

Before paralleling AC power sources, they must be synchronized. Synchronization ensures that the voltage levels, frequencies, and phases of the AC sources match. If these parameters are not aligned, connecting the sources can cause significant electrical disturbances such as voltage spikes, circulating currents, or even equipment damage.

In a synchronized state, both power sources operate in unison, allowing for a seamless transfer of load between them. This is crucial in maintaining system stability and efficiency. Paralleling unsynchronized sources can lead to immediate issues like system instability and potential safety hazards.

The other options do not accurately describe the requirement for successfully paralleling AC power sources. While isolation can be important in certain scenarios for safety or maintenance, it does not pertain to the requirement for paralleling. Similarly, conversion of AC to another form (like DC) or adjusting frequency does not apply to the principle of synchronizing AC sources for parallel operation.