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Percentage differential relay - What is the calculated imbalance?

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  1. jbren81 is offline Junior Member Pro Subscriber
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    Percentage differential relay - What is the calculated imbalance?

    A percentage differential relay with a 25 percent slope characteristic has 4.9 A in one restraint coil and 4.2A in the other. What is the calculated imbalance?

    I have the answer from the help link of 16.6% but need help on the formulas used and calculation process.

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    Kalbi_Rob is offline Experienced Member Pro Subscriber
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    Quote Originally Posted by jbren81 View Post
    A percentage differential relay with a 25 percent slope characteristic has 4.9 A in one restraint coil and 4.2A in the other. What is the calculated imbalance?

    I have the answer from the help link of 16.6% but need help on the formulas used and calculation process.
    First determine the difference between the restraint coils:
    4.9A - 4.2A = 0.7A

    Then calculate the % slope by using the smaller restraint coil current (can use either but the higher % slope is from the smaller restraint current):
    0.7A / 4.2A = 0.166 * 100% = 16.6%.

    16.6% < 25% so the relay will not operate.

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  5. jbren81 is offline Junior Member Pro Subscriber
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    Quote Originally Posted by Kalbi_Rob View Post
    First determine the difference between the restraint coils:
    4.9A - 4.2A = 0.7A

    Then calculate the % slope by using the smaller restraint coil current (can use either but the higher % slope is from the smaller restraint current):
    0.7A / 4.2A = 0.166 * 100% = 16.6%.

    16.6% < 25% so the relay will not operate.
    Thank you, I appreciate the help!

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    Quote Originally Posted by Kalbi_Rob View Post
    First determine the difference between the restraint coils:
    4.9A - 4.2A = 0.7A

    Then calculate the % slope by using the smaller restraint coil current (can use either but the higher % slope is from the smaller restraint current):
    0.7A / 4.2A = 0.166 * 100% = 16.6%.

    16.6% < 25% so the relay will not operate.
    Since it's percent differential, don't you use the biasing current to determine the slope %S=Id/Ib? Where Ib=(I1+I2)/2?

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