When connecting three transformers for three-phase use with a delta secondary, what is the correct voltage reading before closing the secondary?

When connecting three transformers for three-phase use with a delta secondary configuration, the correct voltage reading before closing the secondary will show the rated secondary voltage value. This is because, in a delta connection, the transformer's secondary windings are connected in a closed loop, which allows the voltage to be present across the phases as designed.

Before the secondary is closed, the transformers are energized on the primary side, which will induce the rated voltage across the secondary windings. Each transformer will have a specific rated secondary voltage that corresponds to the input voltage from the primary. This voltage will remain present until the secondary circuit is closed, allowing current to flow.

The other options reflect misunderstandings of transformer operation and the behavior of the secondary circuit. For instance, reading zero volts would indicate that there is no potential difference across the secondary windings, which contradicts the operational principles of transformers under these conditions. Likewise, values like twice the rated voltage or the sum of the three transformer primaries do not accurately represent what occurs in a delta configuration where the voltage is maintained at its rated value across the windings. Thus, the rated secondary voltage value is the correct answer reflecting typical transformer behavior before the load is applied.