Why 4-20mA signal is used as a standard in process control?

The dominant 
International Standard

4-20mA is an International Standard which is dominantly used as process control signal in most of the industries.

Earlier pneumatic pressure signal 3-15psi was the standard in process control. The important reasons to use 3-15psi of compressed air are,

1. Signals below 3psi are difficult to detect and need expensive sensing system.
2. Need to differentiate the live zero (3 psi) and system failure (0 psi)

After 1950s electronic systems entered process control, current signal became the most preferred and efficient one. The 4-20mA range became the standard and still it is being used.

Why 4-20mA signal is used as a standard?

1. The lower and upper limit of the range (4-20 mA). 

• The minimum current output must be capable to drive semiconductor devices. The electronic devices need at least a 3mA of current for its functioning. So, the lower limit must be a value greater than 3mA.
• The upper limit of the current output must guarantee safety of the technicians who work when the system is live. Human heart withstands upto 30mA of current which means current above 30mA kills a person. So, the upper limit must be less than 30mA.
• There is a need for a standard which is both linear and follows the same 1:5 ratio of 3-15psi for easy conversions. 
• 4-20mA and 5-25mA are the ranges that satisfy these three criteria between 3-30mA band. the 4-20mA is most recommended as the calculations will be easier if the current values are in multiples of 2.

2. 4-20mA is immune to noise pickup that is Signal to Noise Ratio(SNR) is high.It is also free from interference and cross talk of other signals.

3. The 4-20mA current signal and a 250Ω loop resistance gives a voltage output of 1-5V which is the range of operation of digital devices.

4. The 4-20mA current signal ensures lossless transmission in process loops and therefore it can be used for long distance transmissions.

5. 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).

6. Here, 4mA is the live zero and it enables the transmitter electronics to be powered by the receiver with the help of only two conductors. Thus, 4-20mA 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.

These are the major reasons for the 4-20mA standard to rule the process control for decades after decades!