What is a primary advantage of the 4-20 mA current range for long cable runs?

• A. The current can be transmitted with relatively constant voltage drop independent of length, maintaining the 4-20 mA signal.
• B. The data rate increases with cable length.
• C. It eliminates the need for a transmitter.
• D. It converts to a voltage signal automatically at the receiving end.

Answer: (A)

The key idea is that the 4-20 mA signal uses current as the information carrier, not voltage. The transmitter forces a controlled current through the loop, and the current stays essentially the same even as cable length increases. Long runs add more resistance, so the voltage drop along the wiring grows, but that doesn’t change the current in the loop as long as the power supply has enough headroom to overcome the total drop. The receiver simply measures the current by converting that current to a voltage across a known resistor, which means the measurement is largely unaffected by how long the cable is. This makes the 4-20 mA approach robust for long distances. The other statements don’t hold: data rate isn’t improved by length; you still need a transmitter to establish the current loop; and while the receiver converts current to voltage, that conversion is already part of the loop design and isn’t the primary advantage.