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Proper connection for overcurrent relay with parallel CT's

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  1. Join Date
    Jan 2015


    Quote Originally Posted by Vernon View Post
    Usually they are all subtractive polarity as shown in your drawing. My take was that they would be connected so the currents were additive and if you had 1 A out of CT#1 and 1 A from CT#2 you get 2A in the relay. Since they didn't use the term "polarity" I went with the currents adding and not canceling. Connect the polarity marks together so that the secondary currents are in phase (I think that is what I posted). The same polarity might be a correct answer but, yes, both subtractive is how it would usually be. I have never seen an additive polarity CT - it is current in on H1 is current out in X1 as shown in your attachment.

    The relay is looking at the total current in the two feeders and 400 total amps always gives you 5 A secondary. If just feeder one is loaded to 400A you get 5 relay amps. If just feeder 2 is loaded to 400A - you get 5 A in the relay. if you have a balance and 200 A on each feeder then each secondary is sourcing 2.5 amps for a total of 5A. So the CT value in the relay should be set to 400 to 5 because 400 total amps primary always gives you 5 amps secondary just like it would if you just have one feeder. If you had 800 amps you would get 10 amps regardless of the distribution of load between the cables - but the ratio is supposed to be to 5, not 10. If you had one feeder with a 400:5 you would put 400:5 in the relay for the CT ratio and this is the same thing. 400 primary amps gives you 5A. www.cttestset.com

    This is a duplicate of my post in the other thread.
    Thank you. I finally understood the question and could make an educated decision. You are right, If he CT is 400:5 that is what the relay needs to see.

  2. #12
  3. Join Date
    Apr 2010
    United States
    Quote Originally Posted by Vernon View Post
    Answer A is close but there is no correct answer. It should be parallel additive with a ratio of 400:5.
    Answer A has been updated to reflect the correct ratio. Thank you.

  4. #13
  5. Join Date
    Apr 2017

    Parallel CT

    Quote Originally Posted by marvinray View Post
    Two parallel feeder cables with individual 400:5 CT’s per parallel run are connected to one overcurrent relay per phase. The proper connection would be:

    a. Parallel additive with relay set to 800:5
    b. Parallel subtractive with relay set to 800:5
    c. Series additive with relay set to 400:5
    d. Series subtractive with relay set to 400:5

    Please help me solve this question.
    Answer is A, https://ctlsys.com/support/measuring...el-conductors/

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