Purpose of using 4-20 MA as analog signal
4 is our live zero so that we will be able to know whether it is z power or not, or the Instrument is faulty|
To check active zero 4 to 20 ma is preferred and current cannot be dropped
Because if we use 0 to 20 ma and the measured signal is 0 ma, we won’t know that the signal is really 0ma or the wire is broken|
2 wire, loop powered field instruments use up to 3.5mA of the loop current to power their own operation. The cost savings of 2 wire loop power over 3 or 4 wire power is the driving economic reason for using 4-20mA loop power in field instruments.
A 0-20mA ‘loop’ would have no available current for the field device to use to operate. 4-20mA allows the field instrument to use 3.5mA of current for operation.
The 4mA live zero is convenient for troubleshooting, but it is an artifact that comes with powering the field instrument using the same two wires as the signal uses.