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The guard terminal of a digital voltmeter is connected to

  • A.the low potential point of the source being measured
  • B.the earth of the mains supply feeding the instrument
  • C.the case of the instrument
  • D.the high input terminal

Answer: A

The guard is an internal shield surrounding the input circuit, and connecting it to the source's own low point makes the common mode current flow in the guard rather than through the measuring circuit. Connected wrongly it can make matters worse rather than better.

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