What is a 4-20 mA signal and why is it used?

4-20 mA is an analog current signal standard where 4 mA represents 0% of measurement range and 20 mA represents 100%. Advantages: current signals are immune to voltage drops in long cable runs, 4 mA live zero allows detection of wire breaks (0 mA indicates fault), and linear relationship simplifies scaling. It powers 2-wire transmitters (loop-powered). Gradually being supplemented by digital protocols (HART, Foundation Fieldbus, Profibus) which can overlay digital communication on 4-20 mA signal or replace it entirely.