If wire resistance can't change the current in the loop, why does loop resistance matter at all for 4-20mA?

The current stays fixed by the transmitter, but the loop still needs enough supply voltage to push that current through the total resistance of the wire, the receiver's input impedance, and any other loop components (Ohm's law: V = I × R for the loop as a whole). If the available supply voltage is too low for the total loop resistance, the transmitter can't maintain the correct current at all and the loop "runs out of headroom" — but as long as there's enough headroom, the current value itself is unaffected by how much of that resistance is in the wire versus elsewhere.

Is 0-10V ever the right choice?

Yes — for short, local wiring runs where wire resistance and noise pickup are negligible (a sensor a few feet from its input card, or inside a single control panel), 0-10V is simpler to generate, doesn't require the transmitter to be "loop-powered," and is easy to read directly with a basic analog input. The tradeoffs that favor 4-20mA mainly become significant once the run gets long or passes through an electrically noisy environment.

What does "live-zero" actually mean?

It means the bottom of the signal range is deliberately set above absolute zero (4mA instead of 0mA) so that "zero" output current can be reserved exclusively for a fault condition. A "live" 0% reading is still a nonzero, present signal (4mA); true 0mA only happens when the loop itself is broken or dead.

Does a 4-20mA loop detect every kind of fault, or just open circuits?

The 4mA live zero specifically detects open-circuit conditions — a broken wire, a disconnected terminal, a failed transmitter that stops sourcing current entirely. It does not, by itself, detect other fault types such as a short circuit or a transmitter that is powered but reporting an incorrect value within the valid 4-20mA range; those typically require additional diagnostics (HART communication, redundant sensors, or process cross-checks).

Can HART communication ride on a 4-20mA loop?

Yes — HART (Highway Addressable Remote Transducer) protocol superimposes a low-level digital signal on top of the same 4-20mA analog current loop, letting the transmitter send additional diagnostic and configuration data over the same two wires without disturbing the primary analog value.

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Concept Explainer · SCADA

4-20mA vs. 0-10V Signals

Why industrial instrumentation prefers current over voltage the moment a wire run gets long.

Both 4-20mA and 0-10V exist to do the same basic job: let a field transmitter send an analog process value — pressure, level, flow, temperature — down a pair of wires to a receiving instrument. A 0-10V transmitter outputs a voltage proportional to the measurement; a 4-20mA transmitter outputs a current instead. That single difference — voltage vs. current — turns out to matter enormously once the wire run gets long, because voltage and current behave completely differently when they travel down a real conductor with real resistance.