Which action completes the safe de-energization of the old transformer after paralleling?

• A. Break parallel on old fmr. Disconnect hot leads on secondary and safety off in the clear leaving existing secondary neutral in place, to eliminate backfeed potential. Open cut out doors to de-energize old xmr. Then remove primary, neutral last, and remove old xmr from service.
• B. Turn off the main switch and remove all connections at once.
• C. Disconnect the new transformer first.
• D. Do nothing until the customer reports an outage.

Answer: (A)

When transformers are operating in parallel, the safe way to take one unit out of service is to isolate it from the parallel connection first so there’s no backfeed or energized path through the other transformer. The goal is to remove the old transformer from service without energizing any part of it or allowing energy to circulate back through the shared circuits.

First, break the parallel connection on the old transformer. This stops it from sharing load with the remaining transformer and prevents current from flowing between the two units. Then disconnect the hot leads on the secondary while leaving the secondary neutral in place. This isolates the low-voltage windings and prevents energizing the old transformer while still keeping a reference point in the circuit, which helps eliminate backfeed potential.

Next, verify and implement proper safety measures and ensure the area is clear. Open the high-voltage cut-out doors to de-energize the old transformer and confirm there is no voltage present. Only after the transformer is safely de-energized should you proceed to remove the primary winding, doing the neutral last, so there’s no energized path remaining during the final disconnection.

Finally, remove the old transformer from service. Following this sequence minimizes the risk of backfeed, maintains safety for personnel, and ensures a clean shutdown of the old unit before removal. The other options don’t provide the same level of isolation and can leave energizing paths or safety gaps that could lead to backfeed or accidental energization.