Standard signal in fields Current and Pneumatic

[amshah]

I think you are not from instrument field,

1) 3 requires for differentiate live zero and dead zero (instrument fail), 15 requires to maintain linearity with flapper nozzle design. Also you can find other signals as 3 to 30 psi, 3 to 50 psi in rare cases for specially designed instruments.

2) same way for 4 ma requires for live and dead zero differentiate, 20 ma and 4 ma combination requires to convert to voltage through 250 ohm std. rasister linearly to 1 to 5 v dc. Above 25 ma as per biomedical laws a human can get shock( internal to body cell) with 24 vDC. there are some instrument which have option of 0 to 20 ma span of measurement.