Reason for 4-20mA current as a standard signal for transmitting data to and from PLCs

[Heartnet]

Hi, What is the reason behind the 4-20mA current as a standard signal for transferring data? Well for starters, we choose current over voltage because there is less signal / data loss when the signal is being transferred over. How about 4-20mA? Well the purpose of having 4mA as a minimum is to be able to make sure that process actuators are "on" so that SCADA is able to detect any possible fault. Is that right? how about 20mA?

[DwalterE]

In the early days, industrial instrumentation used pneumatic signals. 3 - 15 psi was the standard, although 6 - 30 psi was also not uncommon. When electronics were introduced, a lot of the pneumatic techniques were kept. ie force balance transmitters and the elevated zero. The use of current instead of Voltage eliminates wire resistance as a factor in signal accuracy. It also permits the use of 2-wire transmitters, where the same pair of wires carries the signal, but also provides the operating power to the field device. This is also one reason for the elevated zero - 4 mA. Early on, 4 - 20 mA was not the only signal in use. Some manufacturers, Foxboro for one, used 10 - 50 mA. As far as the choice of 4 and 20 mA values, a non-engineering friend of mine suggests that these numbers were chosen by a couple of engineers in the bar late one evening. Until someone comes up with a better explanation, I think I'll stick with that one.

[BobB]

Two engineers in a bar? Bit fanciful. The 4ma allows detection of a failed connection (loss of signal) and 4-20ma is far more immune to electrical noise than, say, 1-5v as the signal does not rely on voltage but current and so a small voltage injection will not change the value of the signal. Much prefer 4-20ma for that reason alone.

[DanW]

No. The purpose of 4mA as a minimum is to allow the transmitter to draw its power from the loop itself to avoid the requirement for power wiring. This is known as 'two wire loop powered'. The demand for two wired loop powered instrumentation came from the process industries that dominated the Instrument Society of America (ISA) at the time: refineries, steel mills, power plants, and paper mills. Those industries were central control room panel board oriented and home run wiring to the control room (or marshalling panels) was the norm. Running a 3rd or 4th wire for power would have been a deal breaker for anyone contemplating 3 or 4 wire field instruments. In 2 wire loop powered applications, the transmitter consumes about 3.6mA for its sensing and operation. All field instruments offer a choice between fail-low or fail-high indication, so the region around 3.6 to 3.8mA is used for fail-low indication, as the graphic from NAMUR shows. The 20mA span followed the the ratio of the pneumatic predecessors, 3-15psi, where the span to elevated zero ratio is 5:1. The indication of open circuit at 0mA is a great feature of 4-20mA but it wasn't the driving factor, 2 wire loop power was.

[TimWilborne]

Very good explanation Dan. Do you mind if I borrow your image?

[DanW]

I don't mind, but it isn't mine. I found the graphic in a NAMUR slide or document some time ago, so they're the owners, and I can't give permission to use something that I don't have title to. But if you right click and select "save picture as" . . . .

[TimWilborne]

Ok thanks

[Arjan]

Also keep in mind that 4mA instead of 0mA allows a 2 wire sensor to remain powered up.