Why does the industry use 4-20mA signals instead of 0-20mA signals?

The most widely used standard analog electrical signal in the industry is to use 4-20mA DC current to transmit the analog. The reason for using the current signal is that it is not easy to be interfered, and the internal resistance of the current source is infinite, and the resistance of the wire in series in the loop does not affect the accuracy, and it can transmit hundreds of meters on the ordinary twisted pair wire. The upper limit is 20mA because of the explosion-proof requirement: the spark energy caused by the on-off of the 20mA current is not enough to ignite the gas. The reason why the lower limit is not set to 0mA is to be able to detect disconnection: it will not be lower than 4mA during normal operation. When the transmission line is broken due to a fault, the loop current drops to 0, and 2mA is often used as the disconnection alarm value.