Answer Posted / carl_ellis
@Marmkijani
The main reason for 4-20mA is that 2-wire field instruments need to use 3.5mA to power their own electronics in order to make a measurement and to control the regulated output.
4-20mA is also, as you pointed out, a range in which the energy in the circuit is low enough to qualify for Intrinsic Safety approval (when used installed properly with an approved DC power supply and I/S barrier).
The "live zero" is nice and convenient as a diagnostic for an open circuit fault, but it was not the main reason for 4-20mA; powering field instruments was the main reason.
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