Why the industry's standard current standards are from 4mA to
20mA,not from 0mA or any other range?

Answer Posted / fidelis

This is for the purpose of identifying a fault either in
the instrument or in the control loop. If there is a fault
in an instrument, the reading on that instrument will be
zero as when the transmitter is not reading any data due to
maybe a broken loop, so one may not be able to tell what
the problem is. But with the 4-20mA standard, it will be
simple to tell if the transmiter is faulty or not. This
makes trouble-shooting easy.

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