A dangerously high voltage is produced when someone opens the secondary circuit of a

• A. Distribution transformer
• B. Auto-transformer
• C. Isolation transformer
• D. Current transformer

Answer: (D)

A current transformer is designed to transfer the load current from the primary conductor into its secondary winding, with the secondary current scaled to the primary. That design relies on having a burden (a connected load) so the secondary current has a path to flow and the core flux stays controlled. If the secondary is opened, there’s no path for that secondary current to flow. The result is that the transformer tries to push current through whatever paths are available (stray capacitances, insulation, gaps), and the voltage across the open secondary climbs very high. That high open-circuit voltage can be dangerous to touch or arc across insulation.

Other types of transformers are not in the same situation. They do deliver a voltage on the secondary when energized, but opening the secondary doesn’t produce the same uncontrolled spike because the primary current is not forced to become an uncontrolled source of current into an open circuit. The current transformer, by contrast, is specifically meant to operate with a closed secondary to keep voltages and flux in check; opening it creates a hazardous, high-voltage condition on the secondary terminals.