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CT Testing and Connections

CT Testing and Connections

CT Testing and Connections


Long time reader, first time poster. We were testing some bushing CT's today. 20 of them tested fine, 1 had some weird results. I have attached the results of a good test and the test in question. The CT would not saturate, we drove 5 Amps into the secondary and could not get a knee point. Has anybody ever seen this before? Comments? Failed CT? Ratio was decent no great but close. Smallest tap was 50:5 and error was 20%.

Also I know that IEEE C57.13.3 recommends grounding the secondary circuits of CT/PT's at the first "point of application". I have generally used the first TB the instrument transformer touches. Is this not acceptable? Should I be using the first TB inside the control room? If so do you run the risk of losing your ground if you have multiple shorting blocks or FT switches ahead of this point.

Thanks in advance.


RE: CT Testing and Connections

Hi Nathan,

H2 results are strange indeed.  Are these metering CT's?  The Kneepoint Voltage doesn't even meet a C50 for either CT (H2/H3).  Obvious guestions, but I have to ask...  Are the other bushing connections to the transformer isolated?  Have you tried to chase down the CT secondary leads from the terminal block in the cabinet to the access points (generally a round cover on side / top of xfmr), to test from there (look for partial short somewhere)?  

The CT in question could have a turn to turn short, looks like in the common part of the winding - this is seen from the high current draw in all taps at a given voltage.  You should be able to see the V/A data points in the Vanguard SW, to export to Excel.  Compare apples to apples (Given V/A points Ct to CT).

As far as your second question goes, I have seen it both ways (in control cabinet or in building / relay panel).  I always like to see it closest to the relay, rather than outside.  Trying to explain that to a utility that has done it the other way for 60 years is another thing.  :)


RE: CT Testing and Connections

Saw a similar result where an arrester bracket created a ground loop on a breaker. Make sure your primary is open and you don't have an inadvertent tertiary through the CT window.

A terminal block is not an application. Think first meter or relay, so the control room is where you likely want the ground. If multiple blocks ahead of this location cause risk, perhaps some should be eliminated.

RE: CT Testing and Connections

Inadvertent ground loops are common when testing for ratio, but in this case I believe his stated problem is with the excitation test.  There may be something (as I suggested) in the leads (although megger tests look good), where there is leakage to ground.  Get your test leads up to the CT secondary bushing (where the studs are) and test from there.

A ratio error (20%) in the lowest tap setting is likely due to the voltage drop on the leads (excessive current).  In my experience, the lower taps aren't always that accurate, tested either by voltage method or primary current injection.  Inter taps are just as bad.  

Although performing the excitation test on a CT is not rocket science, you must eliminate all possibilities of any other problems.  This is a bushing CT on (in) a transformer (unless it is external).  The amount of work required to replace a bad CT si significant.  The CT leads going to the relays / meters are lifted correct?

RE: CT Testing and Connections

Ground loops affect excitation as well, since you don't have the open circuit needed for the test.

RE: CT Testing and Connections

I have found that the existence of a shorting turn has a much greater effect on the saturation than the ratio test.  We used to see more of these on CT's mounted internal to OCB's due to the mechanical activity in OCB operation.

old field guy

RE: CT Testing and Connections

Thanks for the input everybody.

The results posted were a test from the CT junction box. The first connection box after leaving the HV turret. They are metering CT's 1.2 B 0.5. The manufactures factory test results for this CT are very suspicious. They looked faked to me. H1, H2 and H3 tests for this CT are all exactly the same down to three decimal places. We are now awaiting an answer.

Any manufacture who has built a piece of equipment with CT's in it will have a shorting block in the field. To me if you do not put a ground on the first CT shoring block at some point in the future you risk shorting the CT for a period of time without a proper ground on your circuit. To me this poses more of a hazard to personnel then "stray voltages" in the metering panels. But that's just my opinion.

Again thanks for your time.

RE: CT Testing and Connections

Is this a new install?
If so does your input current suddenly spike as your ramping up?

I've had a bushing CT do this to me and come to find out that one the bare ring terminals on the backside of the barrier board was pressed against the adjecent wire. It wasn't metal to metal contact but against the frayed end of the cloth insulation. I could ratio it but when I applied current for saturation it ramped up normal for about a second and then pegged out.

We dropped the oil level enough to pull it out and found the above. I just pryed it back over. What a waste of $$$$ on a simple QC measure.

The only other thing I can think of is the factory smoked that CT during the heat run. If you can't find any shorting devices in the control cabinet then I wouldn't rule this out.

Just a couple ideas as I've ran into this myself.

Good luck,


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