A secondary disconnected from a transformer should be what to ensure no backfeed?

• A. Left disconnected
• B. Floating
• C. Tested and grounded
• D. Uninsulated

Answer: (C)

When a transformer secondary is disconnected, it must be tested and grounded to prevent backfeed and potential shock hazards. Grounding provides a defined earth reference and a clear path for fault currents, so any stray voltage on the isolated secondary is directed to ground rather than back into the system. Testing verifies that the secondary is truly de-energized and safe to work on, ensuring there is zero voltage present before touching or coupling equipment. Leaving the secondary floating or ungrounded can let induced or capacitive voltages build up, creating a backfeed risk that could energize the line unexpectedly. Grounding the disconnected secondary after confirming zero voltage is the safest and most reliable way to eliminate backfeed.