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The National Electrical Code (NEC) specifies that unused current transformers should be short-circuited. This practice is essential for several reasons. Leaving a current transformer open can lead to dangerously high voltages at its terminals when it is not loaded, potentially causing insulation failure or electric shock. Short-circuiting ensures that the transformer does not pose a safety hazard to both the electrical system and maintenance personnel.

In certain applications, such as metering and protective relaying, unused transformers need to be handled properly to maintain the integrity and safety of the entire system. Only by short-circuiting unused current transformers can you ensure that they do not create electrical imbalances or contribute to equipment malfunction.

The other options do not provide the necessary safety measures. Leaving them open could be hazardous, while ignoring them would neglect the potential risks. Removing them is unnecessary if proper precautions can be taken by simply short-circuiting them. This makes short-circuiting the most sensible and safety-conscious choice according to NEC guidelines.