Why is 4-20 mA used as a standard signal in industrial instrumentation?

The 4-20 mA current loop is standard because: current signals are immune to voltage drops over long cable runs (unlike voltage signals), 4 mA offset allows detection of zero signal vs wire break (0 mA indicates fault), the signal powers the transmitter (two-wire operation), and it has good noise immunity. The 16 mA span provides adequate resolution. 4 mA represents 0% of range and 20 mA represents 100%. This standard enables interchangeability of instruments from different manufacturers.