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Unbalance CT: Why do we use "measurement" type class 1 and not "protection" type (5P10,...)?

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Commander Commander

Unbalance CT: Why do we use "measurement" type class 1 and not "protection" type (5P10,...)?

This good question came to me from Li Ling (Schneider Electric China).

Here are some explanations:

For unbalance protection of MV capacitor bank we must use a "Measurement" type of CT.

The reason is that the unbalance protection relay must operate at a current smaller or equal to the rated secondary current of the CT.

Only the "Measurement" type of CT can have its accuracy level guaranteed between 25% to 100% of its secondary rated current.

For "Protection" type of CT the accuracy is guaranteed for values higher than the rated secondary current.

Feel free to add comments and/or complete this answer.