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An operator finds a wattmeter reading low on a circuit metered through a current transformer whose secondary leads are long and thin. The likely explanation is

  • A.the core of the transformer has been permanently magnetised
  • B.the burden of the leads has increased the ratio error
  • C.the phase angle error of the transformer has reversed in sign
  • D.the transformer has been connected with its polarity reversed

Answer: B

Long thin leads impose a substantial burden, which raises the core flux and the exciting current and so makes the secondary current fall further short of the nominal ratio. Shorter or thicker leads, or a one ampere secondary, is the remedy.

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