Why 4 mA and not 0
A 4–20 mA current loop represents the measured value as a current between 4 and 20 milliamps: 4 mA at the low end of the range, 20 mA at the high end. The choice of 4 mA as the "live zero" is the reason the standard survived.
The current loop is also naturally immune to voltage drops: as long as the loop supply can push the current, the measured current is the same at the receiver regardless of wire resistance. This is why 4–20 mA works reliably over long cable runs where voltage signals would sag.
Live zero: the killer feature
With a 0–20 mA (or 0–10 V) signal, a broken wire looks exactly like a legitimate zero reading. With 4–20 mA, a reading below about 3.6 mA is unambiguous: the loop is broken, the device has lost power, or the transmitter has detected a fault.
This "live zero" gives engineers a built-in integrity check on every measurement, which is why the standard is specified in safety-critical and high-availability loops.
Loop-powered devices
Many 4–20 mA transmitters are loop-powered: they draw their operating power from the same two wires that carry the signal. Two-wire loop-powered instruments need no separate power wiring, which simplifies installation and reduces cost. The loop supply (typically 24 V DC) drives the loop; the transmitter regulates the current.
Common loop components: the transmitter, the two-wire loop, a 24 V supply, and a receiver (PLC analog input, indicator, or recorder) with a precision burden resistor (often 250 Ω, producing 1–5 V across it).
Limits of the analog loop
The 4–20 mA loop carries one variable per pair of wires. It cannot carry diagnostics, multiple variables, or configuration data. Calibration must be done at the transmitter, and a loop is point-to-point — no multi-drop without HART or a digital protocol.
This is why HART superimposes a digital FSK signal on the 4–20 mA loop: the analog value keeps working for control, while digital communication carries configuration, diagnostics, and secondary variables.
When to move to digital
Fieldbus and Ethernet instruments add diagnostics and multi-variable data, at the cost of more complex engineering. The pragmatic position most plants take: keep 4–20 mA for critical control loops where a simple, universally understood, break-detectable signal is an advantage, and use digital protocols where you need diagnostics, configuration, or many variables from one device.