4 to 20 mA is live zero or dead zero

The well established 4 - 20 mA current range is used to differentiate live zero and dead zero.

A live zero is indicated by 4 mA and a dead zero by 0 mA. If the control signal is itself zero, it is live zero (4mA). Under fault conditions, the electronic signal would fall to zero (dead zero).

There are also chances for the signals to go above 20 mA or get stucked at an intermediate value. The real reason for live zero is that it enables the transmitter electronics to be powered by the receiver with the help of only two conductors.

Thus, 4-20 mA standard supports two wire transmission. If the dead zero range 0 - 20 mA range is used, then an additional two conductors are needed to power the transmitter.