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CT theory

Johno12345
Im just reading about CTs, just for interest really. I have always known that you short CTs when they arent connected, but assumed it was just so you couldn't get a shock from the cable which would rise to the voltage on the cable its clamped around. I know this isn't quite right now.


Evidently, the voltage can rise to thousands of volts and damage the CT, I assume this happens instantly, rather than a gradual rise


The other thing, and this is my question really, if one side is grounded and commoned, as the drawings of them show, and the other side is open, does the voltage rise still occur? I just cant quite fathom it in my head
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    if one side is grounded and commoned, as the drawings of them show, and the other side is open, does the voltage rise still occur?

    Basically yes. CT transformers - like any other transformer - divide or multiply the supply voltage and current into the secondary according to the ratio of the turns. In normal operation you're not bothered about the output voltage so arrange a suitable division of the current to suit the instrumentation - which usually means a 1:rather-a-lot ratio (the straight conductor the CT is wrapped around being a single turn primary) - which divides the current, but at the same time multiples the voltage. However the relatively low impedance of the current measuring instruments on the secondary together with the low currents involved means the actual voltage on the secondary collapses to something quite modest in normal operation. Leave the secondary open circuit and the theoretical full voltage can develop between the ends of the secondary winding.

       - Andy.
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  • 0
    if one side is grounded and commoned, as the drawings of them show, and the other side is open, does the voltage rise still occur?

    Basically yes. CT transformers - like any other transformer - divide or multiply the supply voltage and current into the secondary according to the ratio of the turns. In normal operation you're not bothered about the output voltage so arrange a suitable division of the current to suit the instrumentation - which usually means a 1:rather-a-lot ratio (the straight conductor the CT is wrapped around being a single turn primary) - which divides the current, but at the same time multiples the voltage. However the relatively low impedance of the current measuring instruments on the secondary together with the low currents involved means the actual voltage on the secondary collapses to something quite modest in normal operation. Leave the secondary open circuit and the theoretical full voltage can develop between the ends of the secondary winding.

       - Andy.
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