while normal transformers are considered, the open circuit tests are conducted as mandatory test. however in case of current transformer the secondary being open circuited is never preferred. why?
Hello,
A current transformer (CT) is basically a step-up transformer: the buss bar, whose current you wish to measure, is the primary winding (1 turn) of the CT whose secondary (a Rogowski coil or toroid) has multiple turns. During installation, the two leads from the secondary of the CT are connected to two terminals on a shorting block with a shorting strap in place, i.e., shorting the secondary of the CT for safety reasons due to the high voltage it can produce in the open circuit condition; the instrument monitoring the CT is also connected to the same two terminals on the shorting block as the CT. Once the installation is finished, the shorting strap is disconnected. If it turns out that the CT secondary leads have been connected to the monitor instrument backwards ( a very common occurrence), the technician simply uses an insulated screwdriver to reconnect the shorting strap and then reverses the two lead going to the instrument - not the two leads coming from the CT as this would expose the technician to a electrical shock hazard since the CT leads would now be open circuited. Note, the safest way to correct an incorrectly wired CT is to turn off the power to the buss, connect the shorting strap, and then reverse the instrument leads. In any event, you always connect the shorting strap in the interest of safety.
I have to go back to my files, but I do have a picture of a shorting block and other information about CT's. I will post them when I locate them.
With regard to your question about testing the CT, the best answer I can give is as follows. The instrument connected to the CT presents a finite resistance as the load to the CT. The calibration factor of the CT is matched to the resistance the CT 'sees' when connected to the instrument. In other words, the CT does not 'see' an open circuit (infinite resistance) when it 'looks' back at the instrument, so why would you test the CT in the open circuit configuration - the calibration factor would be incorrect.
Regards,
Tom Cuff
Dear colleague!
What do you want to measure at opened secondary winding of the current transformer? Measuring of current requires use of ammeter in secondary. It is not possible to match no-load regime you wrote about with ammeter regime (almost short-circuit).
Hello,
I just realized that you may be talking about RF (Radio Frequency) current transformers (CT's) such as the Tektronix CT-1 or CT-2, and not CT's used for measuring current at 50 or 60 Hz in power systems. In the case of RF CT's, which usually use ferrite cores instead of the laminated iron cores, the secondary is connected to the measuring instrument (oscilloscope) via a coaxial cable with a characteristic impedance of 50 ohms. The secondary of the RF CT is calibrated with the assumption that the coaxial cable will be terminated in 50 ohms. Hence the load the CT is assumed to see is 50 ohms, not the infinite resistance of an open circuit.
I have included a photo of a shorting block. The silver colored screws on one side of the block are used to connect the leads from the CT to the instrument, whose leads are connected to the silver colored screws on the opposite side of the block. To short the leads, the two corresponding copper colored screws are screwed in, thus effectively placing the copper colored shorting strap across the leads.
Regards,
Tom Cuff
A quite simple answer is that if you leave the secondary of a current transformer open, you end up with an overvoltage at the secondary. which could be dangerous either to people or to the transformer insulation.
In fact, if you take the current transformer as a perfect tansformer, when there is a current at primary side, there must be a proportional amount of current at the secondary side. Now you multiply the secondary current with the infinite impedence of an open circuit, the secondary voltage becomes infinite.
Risaana,
Like already stated, you don't want an overvoltage on the secondary. Hence the shorting, because the short circuit current will always be small at low VA.
-SAnjay
Hello,
Can you tell us something about what you are working on. It would help if we knew the specifics. In other words, are you using CT's to measure current in power systems, or are you using RF CT's to examine currents in high-speed circuits? Or are you interested in the calibration of CT's? Or is it strictly the use of CT's in an industrial setting, i.e., how to use them safely? Everyone who has answered has made some kind(s) of assumption(s), but you are not giving us much to go on. Perhaps all of our assumptions are wrong.
Regards,
Tom Cuff
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