In a 4-20 mA loop, what do 4 mA and 20 mA represent?

• A. They are arbitrary currents with no significance
• B. They are the lower range value (LRV) and upper range value (URV)
• C. They indicate fault conditions
• D. They determine loop impedance

Answer: (B)

The key idea is that a 4-20 mA loop uses current to represent a measured quantity, and the two numbers define the signal range for that measurement. The lower end, 4 mA, corresponds to the minimum value of the process variable (often zero), while the upper end, 20 mA, corresponds to the maximum or full-scale value. Any current in between is proportional to the measured variable, so a higher current means a higher process value.

Choosing 4 mA as the minimum gives you a live-zero, which also helps detect faults: if the loop current drops toward zero, you can recognize a possible wiring or sensor fault rather than misreading a true zero. The emphasis here is on signaling range rather than arbitrary current levels, fault conditions, or loop impedance.