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The knee point voltage of a protection current transformer is the secondary voltage at which

  • A.the ratio error of the transformer first exceeds its class limit
  • B.a ten per cent rise in voltage raises the exciting current by fifty per cent
  • C.the winding insulation begins to break down under the applied stress
  • D.the secondary current reaches its rated value with the specified burden

Answer: B

It is the standard definition of the onset of saturation, taken from the excitation curve. Above the knee the transformer can no longer force its secondary current through the burden, so a relay scheme must be designed to operate below it.

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