In a current loop, what indicates a transmitter fault when monitoring the loop current?

• A. Current rises to 30 mA
• B. Current drops to 0 mA
• C. Current remains exactly at 4 mA
• D. Current fluctuates between 2 and 3 mA

Answer: (B)

In a current loop, the transmitter modulates the loop current to represent the measured variable, and normal operation keeps the current between 4 and 20 mA. A transmitter fault is most clearly indicated when the current drops to zero, meaning the loop is open or the transmitter is not driving current at all. Other situations—such as current rising above the normal range, staying at the minimum, or fluctuating below the minimum—show abnormal conditions as well, but zero current is the definitive sign of a transmitter fault.