Answer / rejith prasad

In 4 to 20mA system, min. value is 4mA n max. value is 20mA and if there is a wire breakage, the value comes down to 0mA. Which means wire brekage can easily b detected. While in 0 to 20mA system, min. value is 0mA n wire breakage value will also be 0mA...... that is the reason y we use 4 to 20mA rather than 0 to 20mA.......

Answer / simon marshall

The 4-20mA loop can be used with active sensors. The
minimum current of 4 mA powers the sensor. (The sensor then
must act as a variable current sink through its supply
terminals varying the current it draws in proportion to the
quantity it is sensing such as the millivolts or so from a
thermocouple, for a 4-20mA temperature gauge.) I do not
know how the powering current of 4mA was arrived at. The
specifications allow a range of maximum voltages for the
loop to operate on and the sensor power can then be
satisfied in different designs by altering the minimum
voltgage drop on the device. The item powering the loop
then must also be designed to measure the current drawn in
the loop but read out on a display with the transfer
funtion zero to full scale deflection = 4mA to 20mA

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