Solution
There are various reasons for the 4-20mA current loop protocol to be considered as the defacto standard, especially with loop powered devices (e.g. 2-wire transmitters). Two of the important ones are as follows:
- Can be effectively used to detect open circuits in a DC system. If the line breaks in a 4-20mA system the current drops to zero raising an alarm, whereas in a 0-20mA system this cannot be done thus open circuit cannot be differentiated from a live circuit carrying minimum current.
- Most two wire devices are self powered and require a minimum of 4mA to operate. Also, in 4-20mA communication systems the sensor transmitter can be powered by the same two wires that carry the loop signal. This helps in lowering wiring cost.
Historically a 10-50mA standard was used, however, because of higher current losses while using I2C communication protocol it was replaced with 4-20mA protocol.