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# Percentage differential relay - What is the calculated imbalance?

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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.

2. Originally Posted by jbren81
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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Originally Posted by Kalbi_Rob
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!

4. Originally Posted by Kalbi_Rob
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?