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Proper connection for overcurrent relay with parallel CT's

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    Proper connection for overcurrent relay with parallel CT's

    Two parallel feeder cables with individual 400:5 CTís 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.
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    Quote Originally Posted by marvinray View Post
    Two parallel feeder cables with individual 400:5 CTís 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.
    My guess would be series additive. I'm thinking the subtractive would be polarity swapped on one ct which would constantly cancel out the other ct I would think? which leaves me with additive in series and parallel. I wouldn't think parallel ct's for protection would be very accurate,however in series circuits current adds up. so if you had 400 amps on each primary of each ct your output would be 10 amps,which is what you would see if you had a single 400 and both primary conductors were routed through it. That's how I think it would work, hopefully some others can chime in.....
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    Quote Originally Posted by lester mcmanaway View Post
    My guess would be series additive. I'm thinking the subtractive would be polarity swapped on one ct which would constantly cancel out the other ct I would think? which leaves me with additive in series and parallel. I wouldn't think parallel ct's for protection would be very accurate,however in series circuits current adds up. so if you had 400 amps on each primary of each ct your output would be 10 amps,which is what you would see if you had a single 400 and both primary conductors were routed through it. That's how I think it would work, hopefully some others can chime in.....
    If the installation means that the CTs are installed on each cable and monitoring different currents on the same phase, then the ratio would need to be 800:5. Assuming same polarity it would be additive.
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    Quote Originally Posted by lester mcmanaway View Post
    My guess would be series additive. I'm thinking the subtractive would be polarity swapped on one ct which would constantly cancel out the other ct I would think? which leaves me with additive in series and parallel. I wouldn't think parallel ct's for protection would be very accurate,however in series circuits current adds up. so if you had 400 amps on each primary of each ct your output would be 10 amps,which is what you would see if you had a single 400 and both primary conductors were routed through it. That's how I think it would work, hopefully some others can chime in.....
    Think about it, do currents in series really add? Draw a circuit with two current sources in series. Will the current measure differently at any point in that circuit?
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    Correct answer is A
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    Quote Originally Posted by marvinray View Post
    Two parallel feeder cables with individual 400:5 CTís 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.
    It would seem that the CTs would be connected in parallel, polarity marking connected together - parallel additive. If you had 200 A on feeder 1, that CT would source 2.5 Amps. If you had 200A on feeder 2, you also get 2.5 A from that CT. 400A total gives you 5 A so the relay should be set to 400:5. Answer A is close but there is no correct answer. It should be parallel additive with a ratio of 400:5. Connecting CT secondaries in series would never work because a CT with no load, like if that feeder was out of service, would act as an open. That is my take here at www.cttestset.com
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