why 4-20mA is standard?

Answer 1

It is more stable and immune to noise than, say, a 0-5V output.

By having the low end be something other than 0mA or 0V, the common-mode noise rejection range of the receiving circuit is improved.

Answer 2

I don't know why 4-20mA is the standard for current loop analog data, but it's been that way for a long time. The is the best way to transmit analog signals from one place to another in an electrically noisey environment, like a factory. High impedance inputs and voltage signals are far to unreliable over a long wire run.

Answer 3

in addition to answers 2 and 3, there is also 2-10mA standard.

in same cases 0-10mA or 0-20mA can be used. for example in a lab application, where a open loop is not a problem but an increase sensitivity is required.

Answer 4

You don't want to go 0 to 20 mA, because at 0, you couldn't tell the difference between an open loop (fault) or a 0 mA signal. Also, for a closed loop, induced noise would show up as a false signal

4 mA is far enough above the noise floor to be a reliably detectable signal.

Anything above 20 mA and power losses start to become a factor and accuracy decreases.

.