Red Seal Electrician Practice
TransformersQuestion 1096 of 1506

A 250 V DC source raises the secondary circuit of a transformer above ground. What does a DC voltmeter show for this voltage on the primary circuit?

  1. 250 V, because DC passes through the core
  2. Zero, because the transformer isolates the primary from it
  3. 125 V, because the voltage is shared by the windings
  4. The voltage is multiplied by the turns ratio
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Correct answer: Zero, because the transformer isolates the primary from it

The transformer isolates the primary from the common-mode voltage. The voltage stays on the secondary side.

Source: Lessons in Electric Circuits II: AC (Kuphaldt), 10.3: Electrical Isolation

Practice transformers

Questions last reviewed: 18 October 2026