What action must be taken when paralleling secondary windings internally?

When paralleling secondary windings internally, marking abandoned bushings is a critical step. This action is essential because it helps to avoid confusion during future maintenance or operation by clearly indicating which bushings are not in use. Properly identifying these bushings ensures that any personnel working on the equipment can quickly ascertain the status and avoid potential errors that could lead to unsafe conditions or equipment damage.

Marking abandoned bushings also assists in maintaining clear documentation and communication among linemen and other technical staff, which is vital for safety and coordination. Proper identification contributes to effective troubleshooting and maintenance practices in the future, promoting overall system integrity.

The other choices, although they may be important actions in different contexts, do not directly address the need for clarity and safety in the specific scenario of paralleling secondary windings internally.