Which device’s secondary circuit can produce a dangerously high voltage if opened?

• A. Transformer
• B. Auto transformer
• C. Isolation transformer
• D. Current transformer

Answer: (D)

The thing this question is getting at is how a current transformer operates and why its secondary must stay closed. In a current transformer, the current in the secondary is forced to track the primary current (scaled by the turns ratio) and the secondary is intended to carry that current into a burden or measuring instrument. The magnetic flux in the core is kept nearly zero by the opposing magnetomotive forces of the primary and the secondary currents. If the secondary path is opened while current is flowing in the primary, there is no secondary current to balance the primary’s effect. The transformer then drives up the voltage on the secondary in an attempt to push current through the open circuit. That voltage can be dangerously high, limited only by the insulation and construction, so touching the terminals can be hazardous.

Other transformers don’t have this vulnerability in the same way. An ordinary power transformer or an isolation transformer is designed so that opening the secondary simply means no load and no current flows, but the voltage on an open secondary is limited to the rated secondary voltage and doesn’t spike uncontrollably. The danger described is specific to current transformers, where keeping the secondary closed is essential for safe operation.