CT Reference Voltage

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Jameson
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Location: Santa Cruz, CA
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Re: CT Reference Voltage

Post by Jameson » Tue Aug 02, 2016 4:18 pm

Im not sure to be honest. I have not found ohms to be a reliable method of determining if a CT is bad or not.

Here is a little experiment that I just conducted on 3 of the BCT-015-200 and 3 of the BCT-025-200 Current Transformers in ohms. These are new out of the drawer:

BCT-015-200:

580 ohms
564 ohms
582 ohms

BCT-025-200:

580 ohms
564 ohms
582 ohms

I do not know what these results mean they were taken on an old Fluke 8840A for your reference.

I would test your CTs using the Omnimeter, you can connect all 3 CTs to the same Primary Current line and connect all 3 voltage inputs together with jumpers. That way all 3 inputs are seeing the same voltage and all 3 CTs should be measuring the same current. You can then look at the watts on each line. I would do this on a steady 100 watt load, a steady 1000 watt load, and a steady 5000 watt load. Let me know what you get. If there is something that jumps out as being odd you can swap CTs inputs around to see if the anomaly follows the CT.

Just a heads up for you and others that are reading this. One way you can damage a CT is if it is connected to a primary wire and the secondary wires of the CT are either not connected to a meter or they are not shunted together. You should always shunt the CT white wire to the black wire if you are going to leave the CT connected to a primary wire with current in it.
Jameson
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BlueHornet
Posts: 56
Joined: Fri Dec 16, 2011 10:42 pm
Location: Victoria, Australia

Re: CT Reference Voltage

Post by BlueHornet » Wed Aug 03, 2016 2:37 am

Im not sure to be honest. I have not found ohms to be a reliable method of determining if a CT is bad or not
This why the CT's were first tested using a known resistive load and as I pointed out in my post the resistance value is not the ideal method but it is the next best thing and certainly starts to put the results into some perspective once the Power readings were established and which appear to be giving erroneous values.
I would test your CTs using the Omnimeter, you can connect all 3 CTs to the same Primary Current line and connect all 3 voltage inputs together with jumpers. That way all 3 inputs are seeing the same voltage and all 3 CTs should be measuring the same current. You can then look at the watts on each line.
This is exactly how they were tested with connections to CT1, CT2 & CT3 and L1, L2 & L3 on a common rail primary and the CT's were separated by about 8" when the CT's were moved around between CT1-CT3 the errors followed the CT''s, only difference is that they were only loaded with 5000W, I'm sure that the results using 100W & 1000W loads will be proportionally similar according to the load, but it's something that I can easily test again when I get some spare time over the next few days . The current readings were not used as they have always been erroneous and the reason for this whole exercise, but I will note them when I run a more thorough test, I also have other CT's that I can further use for tests, its just that these were the ones that appeared to be out of tolerance. The other CT's I have have resistance values of between 533 ohms and 550 ohms and appear to return correct readings with the test load connected. I should probably point out that 3 of the suspect CT's are BCT-015-200 and the other is a BCT-013-200.

Now having said that 3 appear to outside of the sample readings that you were able to take and only 1 manages to fall within that sample reading set being the 580.0 ohm CT which returned a value of 5086 Watts, coincidentally this is also the on which reflects the closest correct value to what I was expecting to see 5000Watts +/- 0.5% or even 1% but they all fall outside of that tolerance.
One way you can damage a CT is if it is connected to a primary wire and the secondary wires of the CT are either not connected to a meter or they are not shunted together. You should always shunt the CT white wire to the black wire if you are going to leave the CT connected to a primary wire with current in it.
Totally understand that this can occur and can guarantee you that the CT's have never been subjected to this condition. Power is never flowing to either the meter or through the primary when I make any connections or adjustments.

I looks like I've managed to get a loan of a calibrated shunt type power monitor for use over the weekend so it will be a big help in accurately determining and cross checking values.

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