It fascinates me that the accepted answers (listed here) to this question of “Why 4-20 mA?” reflects ignorance as to what the real primary reason was in the development and acceptance of a live zero electronic replacement for 3-15 psi (20-100 kPa) pneumatic process signals.
The core reason for the development of 4-20 mA was for 2 wire, loop powered instrumentation, where 3.5-3.6 mA is used for powering the instrument. Without a minimum 3.6 mA field instruments would not have any energy or power to run themselves! They’d be 3 or 4 wire devices with separate wiring for DC power.
You can’t have 2 wire, loop powered 0-20 mA or zero to anything mA because there is no energy to power the transmitter at zero mA. There is energy to power the transmitter with an ‘elevated zero’ at 4 mA.
3.8 mA has become a range for low failsafe; 3.9 mA is a range for underflow leaving 4.0 mA for a true engineering ‘elevated’ zero.
There are multiple advantages of 2 wire loop power 4-20 mA, as the accepted conventional wisdom lists:
- live zero is handy to indicate a broken circuit or a dead transmitter
- the loop can be made intrinsically safe
- it has the advantages of less susceptibility to noise that is inherent in current vs voltage signals
- a current loop adjusts for minor resistance changes in loop wiring (rusty terminals)
- it does create 1-5 volts when shunted through a 250 ohm resistor, but there neither was (historically) nor is anything particularly superior about 1-5Vdc. The 1:5 ratio of 4-20mA is the same as its predecessor, 3-15 psi (20-100kPa) pneumatics, which is nice, but again, there’s nothing inherently superior about one ratio vs another.
But those are advantages only, none are the real primary reason for an elevated live zero: 2 wire loop powering of the instrument.
Don’t fail your interview because the interviewer doesn’t know either, but if you’re a professional in the field, you should know the real reason for the 4-20mA standard.