Current Standards

Dear All,

Why is (4-20)mA is kept as an standard and not any other value?
and similarly what is the standard for Voltage and why is it maintained at that particular value?

Like any standard the purpose is to allow others to interface with a control, sensor, etc. If I am not mistaken 4 ma thru a specific resistor is 1 volt and 20 ma is 5 volts - the resistor is 250 ohms. One standard is 1 to 5 volts but it seems that either 1 to 10 or 2 to 10 volts is more common.

Like other standards, they probably were convenient round numbers or were the standards adopted by a popular manufacturer some time ago and everyone else just followed.