Why is 4 mA used as the lower end of the loop current in a 4-20 mA signaling system?

• A. It ensures a minimum output even when the process variable is at the minimum and helps detect a broken loop.
• B. It is chosen to maximize voltage drop across wiring.
• C. It corresponds to 0% of measurement.
• D. It is arbitrary.

Answer: (A)

The main idea here is that a 4-20 mA loop uses a live-zero signaling scheme. Having 4 mA as the lower end means the minimum measurement still produces a real, non-zero current through the loop. This guarantees a detectable signal when the process variable is at its minimum and, importantly, allows the system to recognize faults. If the loop current dropped to near zero or went open, the receiver could clearly flag a fault like a broken wire or failed transmitter. At the same time, keeping a non-zero minimum current provides enough voltage across the wiring and load for the transmitter to regulate the current reliably, even with cable resistance and load variations. So, the 4 mA lower limit ensures both a meaningful minimum signal and a robust fault-detection capability, rather than serving to maximize voltage drop or being arbitrary.