Why is the 4-20 mA signal commonly described as tolerant of wiring resistance on long runs?

Why is the 4-20 mA signal commonly described as tolerant of wiring resistance on long runs?

• A. It allows the loop to carry digital data over long distances.
• B. It ensures the signal remains within 4-20 mA despite drop across long wiring, enabling long-distance measurement.
• C. It prevents electromagnetic interference completely.
• D. It eliminates the need for calibration.

Answer: (B)

The key idea is that 4-20 mA is a current loop, not a voltage signal, which makes it robust to wiring losses over long distances. The transmitter controls the current flowing through the loop to represent the measured value. Any resistance in the wiring causes a voltage drop (V = I·R), but as long as the loop has enough supply voltage to keep the current in the 4-20 mA range, the current stays within that band. A burden resistor at the receiving end converts that current to a voltage the readout can use, so the same current produces the same indication regardless of how long the wires are. This design enables accurate long-distance measurement because the critical signal is current, not voltage, and is only limited by supply headroom and loop integrity. The other options aren’t accurate: the 4-20 mA loop isn’t primarily for digital data, it doesn’t completely eliminate EMI, and calibration requirements aren’t removed by using a current loop.

The key idea is that 4-20 mA is a current loop, not a voltage signal, which makes it robust to wiring losses over long distances. The transmitter controls the current flowing through the loop to represent the measured value. Any resistance in the wiring causes a voltage drop (V = I·R), but as long as the loop has enough supply voltage to keep the current in the 4-20 mA range, the current stays within that band. A burden resistor at the receiving end converts that current to a voltage the readout can use, so the same current produces the same indication regardless of how long the wires are. This design enables accurate long-distance measurement because the critical signal is current, not voltage, and is only limited by supply headroom and loop integrity. The other options aren’t accurate: the 4-20 mA loop isn’t primarily for digital data, it doesn’t completely eliminate EMI, and calibration requirements aren’t removed by using a current loop.