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A current transformer is used with a one ampere secondary rather than five amperes when

  • A.the primary current is very much larger than usual for such a transformer
  • B.the accuracy required is lower than a five ampere secondary can provide
  • C.the transformer is intended for protection rather than for metering
  • D.the leads to the instrument are long

Answer: D

The burden of the connecting leads is proportional to the square of the secondary current, so dropping from five amperes to one reduces it twenty-five-fold. In a large substation where the instruments are far from the transformer this is decisive.

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