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A current transformer and a potential transformer are both connected to a wattmeter. The overall error in the power reading is

  • A.the largest of the three errors
  • B.the product of the three individual errors expressed as fractions
  • C.the error of the wattmeter alone
  • D.the sum of the errors of the two transformers and the instrument

Answer: D

Small fractional errors add, both the ratio errors and the phase angle contributions, so the measurement is worse than any single element in the chain. Precision work applies the correction factors of each transformer rather than accepting the accumulated class limits.

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