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The 4-20 mA signal is used instead of 0-20 mA oR 0-16 mA for reasons as below:
1. For a 2 wire transmitter the device itselg requires some power for operation which is equal to 3.98 mA.
2. The 4-20 mA current signal helps to differentiate between live zero (Process parameter at 0) when Transmitter reading is 4 mA & Dead Zero ( Transmitter Fault Or Circuit faul) when transmitter readiing is less than 4 mA
because you cannot determine if the line is cut if you start with zero. And 4ma is what we called a livezero, because at that ampere rating is where the current is stable.
instrument transmission 4-20 ma is correct...
starting 4ma is transimission without dead
starting 0ma is transimission dead condication so using 4-20 ma.
beacuse in case of cable cut thier is no signal will be sent to teh control system - as in case of open signal if we use 0-20 ma or 0-16 ma- so it will make wrong indication
A good answer is already added by shipla joshi.
because of the live zero, where the standard voltage for electronics is 1v, 5v
4ma*250ohm=1v
20ma*250ohm=5v
the standard measurement of instrument 4-20mA which is 4mA equal to 0% and 20mA equal to 100% is used of 3 wire sinal cable 2 wire connected terminal to +/- 24vdc and 1 wire connected to 4-20mA terminal which fadeback to the HMI
because in case of 4-20 mA Zero output indicating an open loop condition(error condition). but for 0-20 mA we cannot identify actual zero output from open loop (line cute) condition. also 20 mA is used as maximum value( URV ) because the human heart can withstand up-to 30 mA of current only. so,as a safety point of view 20 mA is chosen
To ensure loop monitoring in case of cable failure. Also there NAMUR range to monitor connected device.
