How would you test a 4-20 mA loop in a DDC lab?

• A. Verify transmitter supply, measure current with a multimeter, ensure transmitter outputs within range, check loop resistance, confirm DDC reads.
• B. Disconnect transmitter and measure open-circuit voltage.
• C. Only measure the loop's resistance.
• D. Replace the transmitter and assume the loop is fixed.

Answer: (A)

Testing a 4-20 mA loop in a DDC lab hinges on confirming power, current, load, and sensing. Start by verifying the transmitter is supplied with the correct voltage and is enabled, since the loop relies on that power to drive the current. Then measure the loop current with a multimeter placed in series; the current should fall between 4 and 20 mA and should respond as the process variable changes. Next, check the loop burden—the total resistance in the loop, including the DDC input and any shunt resistor—because too high a resistance can rob headroom and prevent the loop from reaching the full 20 mA. Finally, confirm that the DDC reads the loop current correctly (or the corresponding voltage across the burden), ensuring the control system sees the right signal. This approach covers power availability, actual current, load compatibility, and the receiving device’s sensing, which are all essential for diagnosing and validating a 4-20 mA loop. Open-circuit voltage tests don’t reveal how the loop behaves under load, measuring only resistance ignores the active current path, and swapping the transmitter without checking the rest of the loop can mask underlying issues.