Why 4 - 20mA? why not start with 1 or 2 or 3 - 20mA?

Question

i know it should not start with 0 to differentiate between zero signal and dead signal or break cable but then why start with 4?

Answer 1

That may have been one of those arbitrary decisions that is sometimes made when a standard is developed. In addition to providing a non-zero minimum to allow detecting signal loss, the minimum signal voltage provides for a margin between the minimum signal and any noise that might be present. Perhaps someone determined that 4 was large enough to help with noise and thought that the numbers worked out conveniently. A 16 volt difference between min and max, difference = 4 X min, range of 20% to 100%, perhaps seemed easier to work with than 17 volts, 5.67 X min and 15% to 100% or 18V, 9 X & 10% to 100%.

Answer 2

*click:

http://www.eurotherm.com/training/tutori...

http://www.kele.com/tech/Electric/TRefCL...

Answer 3

One of the advantages of 4-20ma analog signaling is that you can deliver power to the remote device for it to use (in fact you must deliver some, even it is not used)  4 ma provides enough to positively operate a device.  

http://en.wikipedia.org/wiki/Current_loo...

http://www.control.com/thread/1026240195