Why CT's shall not be kept open?
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Answer Posted / richard harris
There is an inaccurate theory that has been in the electrical discipline for decades, and that is - if you open circuit a Current Transformer (CT) under load it can produce millions or infinite voltage.
It is the character of a CT to develop the voltage that will drive its current through its load.
(Current transformer circuits are generally a low impedance load.)
When one opens a CT circuit, to the CT - it sees an infinite resistance, and tries to develop an infinite voltage across an infinite resistance, therefore, the voltage across an open circuit CT circuit increases until the core of the CT is saturated. Once the core of the CT is saturated - it can not produce a higher voltage, - this is true for air core or iron core CTs.
There is a limit to the voltage that can be developed across a CT.
In the case of an air core CT the voltage increases until the air of the CT is saturated.
In the case of an Iron core CT, the voltage increases until magnetic flux saturates the iron.
The limiting factor in an iron core of a CT.
When the iron core of a CT becomes flux saturated, the CT like all other transformers, can NOT produce a higher voltage. The voltage that is produced is a function of how much iron is in the core of the transformer. That is true for CTs and Power Transformers.
In some cases, a higher voltage is needed to push the current through a high impedance load. To have a CT produce a higher, the CT core will need more iron in its core.
Current Transformers are rated in classes - and the higher class - the more iron in the class of the CT, and the higher voltage it can produce. That is why CTs have to be tested to determine their Saturation Voltage against their impedance load.
Plus they have to be sized not to go into saturation during a short circuit or system fault event.
Since 1978 - I have been teaching at a college, university, and professional level. I was a NETA technician level 3 for many years, and every thing that I have taught I have had field hands on experience.
The reason that I am responding to this question, as I have never had anyone accurately explain about why not to open a CT circuit "underload." - yes you can open a CT or CT circuit ONLY if it has no load or current going through its circuit.
In teaching an Electrical Safety class on a telephone conference with 25 individuals - some when back and told the Operations Supervisor and he came and confronted me. Even Electrical Engineers and management technical have never explained it or understood it to the point that they could explain it to me.
I hope this short explanation answers your question as to why a Current Transformer, or its circuit should not be opened under load.
Should you have any additional questions, do feel free to ask.
Richard Harris
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