What are some ways a 'de-energized' system can become energized?

• A. Backfeeds from potential transformers only
• B. Undocumented alternate power sources only
• C. All of the above, including backfeeds, undocumented sources, missed lockouts, test equipment failure, lightning, energized lines contacting de-energized lines, and insulation failure
• D. None of the above

Answer: (C)

A de-energized system can become energized through several real-world paths, not just by someone flipping a switch. Energy can re-enter via backfeed from measurement and protection devices, like potential transformers that energize a circuit they’re monitoring when there’s a source elsewhere feeding the system. Unknown or undocumented power sources, such as unreported generators or customer equipment, can also energize a circuit if they’re connected to it. If lockout and tagging were missed, equipment can be re-energized while someone is working, because there’s no approved isolation preventing it. Test equipment itself can fail or introduce unintended connections, momentarily energizing parts of the system. A lightning strike can drive surge energy into lines, creating energized paths where none were expected. When an energized line contacts a de-energized line, current can flow into the latter and energize it. Insulation failure can create a conductive path that allows voltage to appear on conductors or equipment that were supposed to be de-energized. Because each of these are plausible energizing paths, the best answer is that all of the above cover the ways a de-energized system can become energized. In practice, this is why thorough switching procedures, proper lockout/tagout, and verification of absence of voltage with tested equipment are essential before any work.