Red Seal Electrician Practice
Signalling and communicationQuestion 117 of 1506

A 4-20 mA loop shows an unsteady reading near AC power wiring. What is the likely cause?

  1. Noise coupled from the power wiring is read as a change in level
  2. The tank level is really changing at 60 Hz
  3. The loop resistance is too high for the transmitter
  4. The transmitter output is shorted to the case of the tank
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Correct answer: Noise coupled from the power wiring is read as a change in level

Any noise in an analog signal is read as a change in the measured quantity.

Source: Lessons in Electric Circuits IV: Digital (Kuphaldt), 14.1: Introduction to Digital Communication

Practice signalling and communication

Questions last reviewed: 18 October 2026