4-20 ma

4–20 ma is an analog electrical transmission standard for industrial instrumentation.

"ma" is a shorthand notation for milliamps, or 1/1000 of an amp. The signal is a current loop where 4 milliamps represents zero percent signal, and 20 milliamps represents the one hundred percent signal. The reason zero is at 4 ma and not 0 ma is that this "live zero" allows the receiving instrumentation to differentiate between a zero signal and a broken wire of a dead instrument. This standard was developed in the 1950s and is still widely used in industry today, even though many attempts have been made to replace it with digital forms of communication such as fieldbus and Profibus. Its benefits of being a widely followed standard, low cost, reliability, and immunity to electrical noise keep it in regular use.