Which transformer should be disconnected, de-energized, marked, or short-circuited if connected to an energized circuit?

• A. Arc-flash hazard
• B. Unused current transformer
• C. Maximum available fault-current
• D. Enclosure-type number mark

Answer: (B)

When a current transformer is present in a circuit but not used for measurement, its secondary must not be left open while the primary is energized. An energized primary drives current in the CT’s secondary, and if that secondary is open, the CT can generate a dangerously high voltage across the open terminals. To prevent this hazard, the correct action is to disconnect or de-energize the device, or short-circuit the CT secondary and mark it so it cannot be mistaken for a live secondary. This keeps the transformer from developing dangerous voltages and protects people and equipment.

The other options describe hazards or labeling concepts that don’t address the specific safety requirement for an unused current transformer in an energized circuit. Arc-flash hazard is a general risk in energized work, and maximum available fault-current or an enclosure-type number mark refer to system data or identification, not the immediate action needed for an unused CT.