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The capacitance of a cable rises with age in service chiefly because

  • A.moisture entering the insulation raises its permittivity
  • B.the conductor expands and reduces the insulation thickness
  • C.the sheath corrodes and increases the effective outer radius
  • D.the dielectric strength of the insulation falls with time

Answer: A

Water has a relative permittivity of about eighty, so even a small ingress raises the effective permittivity of the insulation appreciably. A rising capacitance measured on a routine test is therefore one of the indications of a cable taking in moisture, alongside a falling insulation resistance.

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