Why 2 wire system has a current of only 4-20 ma?

The current loop scale of 4-20 milliamps has not always been the standard for current instruments: for a while there was also a 10-50 milliamp standard, but that standard has since been obsoleted. One reason for the eventual supremacy of the 4-20 milliamp loop was safety: with lower circuit voltages and lower current levels than in 10-50 mA system designs, there was less chance for personal shock injury and/or the generation of sparks capable of igniting flammable atmospheres in certain industrial environments.

Hi,

For 2 wire transmitter the supply (actually the supply voltage for the transmitter) coming from the signal reader. The internal consumption of the transmitter is near about 4 mA. So the signal is 4-20 mA.

The healthiness of the line identification is added advantage of the configuration.

Regards

ddg