Why is 4-20 mA the standard signal range in process instrumentation?

4-20 mA advantages: current signal is immune to voltage drops over long cable runs (unlike voltage signals), 4 mA live zero allows detection of cable breaks (0 mA indicates fault), linear relationship between current and measured value, can power two-wire transmitters through same pair (loop power), and standardization enables interchangeability. The 16 mA span (4 to 20) provides good resolution while 4 mA offset allows fault detection. This standard has been dominant in process industries for decades due to its reliability and noise immunity.