Answer / tang

Because we want to know the "life zero".
Life zero is a condition where there's a signal when the process is at 0%. For an example if we use 0mA instead of 4mA, we might don't know either the process is really 0% or the measuring unit is faulty(that will also indicate 0mA).

Answer / venkadesh

because the transistor operating region range is 4-20 ma,these in between range is transmitter working in safe level

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