The CT’s secondary winding should never be open-circuited!
Since, Open-circuiting a CT’s secondary winding may result in a dangerous amount of voltage developing between the secondary terminals because the CT will attempt to
drive a substantial current through a very high resistance.
This is why you will never see fuses in the secondary circuit of a current transformer. Such a fuse, when blown open, would pose a greater hazard to life and property than a closed circuit with any amount of current the CT could muster.
While the recommendation to never short-circuit the output of a PT makes perfect sense to any student of electricity or electronics who has been drilled never to short-circuit a battery or a laboratory power supply, the recommendation to never open -circuit a powered CT often requires some explanation.
Since CTs transform current, their output current value is naturally limited to a fixed ratio of the power conductor’s line current. That is to say, short-circuiting the secondary winding of a CT will not result in more current output by that CT than what it would output to any normal current-sensing instrument! In fact, a CT encounters minimum “burden” when powering a short-circuit because it doesn’t have to output any substantial voltage to maintain that amount of secondary current. It is only when a CT is forced to output current through a substantial impedance that it must “work hard” (i.e. output more power) by generating a substantial secondary voltage along with a secondary current.
The latent danger of a CT is underscored by an examination of its primary-to-secondary turns ratio. A single conductor passed through the aperture of a current transformer acts as a winding with one turn, while the multiple turns of wire wrapped around the toroidal core of a current transformer provides the ratio necessary to step down current from the power line to the receiving instrument. However, as every student of transformers knows, while a secondary winding possessing more turns of wire than the primary steps current down, that same transformer conversely will step voltage up. This means an open-circuited CT behaves as a voltage step-up transformer.
Given the fact that the power line being measured usually has a dangerously high voltage to begin with, the prospect of an instrument transformer stepping that voltage up even higher is sobering indeed. In fact, the only way to ensure a CT will not output high voltage when powered by line current is to keep its secondary winding loaded with a low impedance.
Reference and complete PDF article: Short Handbook on Instrument Transformers Safety
Related Downloads:
- Potential Transformers Handbook
- Current Transformers Handbook
- Transformer Polarity Handbook
- Short Handbook on Instrument Transformers Safety