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Why has 4-20 mA gained so much acceptance

and survived so long?

 

2 Votes

Last month, I presented a webinar for our customers. At the end ofControl 101, when we opened the

session up for questions, a customer asked about making the choice between 4-20 mA and digital signal

outputs. It got me thinking about this list I put together a while back — one that I pull out whenever

customers ask me “Is 4-20 mA still valid?”

The answer: Yes, it is. And here are the 18 reasons I came up with… so far.

1. 4-20mA has an inherent ‘live zero’. Zero engineering units is 4.0mA, not 0.0mA. 0.0mA indicates an open

circuit, a failure or a fault mode like cut wires or a dead transmitter. 0.0mA is not a valid reading of zero

engineering units and a wake-up call that there’s something seriously wrong in the loop. (electrical current level

below about 3.6mA is absolutely necessary because it is used to drive the electronics of loop powered devices).

2. Any electrical circuit needs two conductors. In 4-20 mA’s ‘two-wire, loop powered’ form, a field transmitter

gets its power over the same two wires as the signal, without a third or fourth wire for power. This is a major

plus when large numbers of field instruments or long wire runs are involved.

3. 4-20 mA can be designed for Intrinsically Safe approval for use in hazardous areas, which allows

troubleshooting the circuit while it’s ‘live’ and powered.

4. 4-20 mA can carry HART® digital data superimposed on its primary DC signal, without sacrificing any of the

properties above and without interfering with non-HART enabled analog inputs.

5. The 4-to-20 span and offset (with live zero at 20% of the span from zero) has the same 1:5 ratio as the

widely used 3-15 psi pneumatic control signal.

6. Values slightly above and below the nominal 4-20mA can be and are used to signal a major fault. (See

theNAMUR NE43 standard.)

7. A 4-20 mA current signal is less susceptible to noise pickup than a voltage circuit, so 4-20 mA is relatively

free from interference and cross-talk from other signals.

8. 4-20 mA has little, if any, signal loss in its circuit; current regulation compensates for minor resistances in

terminal boxes, whereas voltage drop across resistances for voltage signals creates an error.

9. 4-20 mA works at relatively long distances (>1km at a nominal 24 VDC). Boosting the power supply voltage

will drive the signal even farther.

10. Two wire 4-20mA commercial instruments are isolated and floating so ground loops and their associated

errors and faults are relatively rare.

11. 4-20 mA is generally tolerant of copper cable length and type.

12. The response of 4-20mA is adequate for the vast majority of process sensing needs.

13. The original published standard for 4-20 mA, ISA SP50, was published in 1966, almost 50 years ago,

generating widespread acceptance and creating an installed base of millions of points.

http://www.lesman.com/train/webinars/Webinar-Process-Control-101.htmhttp://blog.lesman.com/2012/02/17/what-does-namur-ne-43-do-for-me/http://blog.lesman.com/2012/02/17/what-does-namur-ne-43-do-for-me/http://blog.lesman.com/2012/02/17/what-does-namur-ne-43-do-for-me/http://www.lesman.com/train/webinars/Webinar-Process-Control-101.htm

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14. Electricians find it easy to wire and troubleshoot 4-20 mA. Its working principles are easily understood DC

electronics.

15. When used with a nominal 24 VDC loop power supply, no one who touches the wire terminals gets shocked

or even feels a tingle.

16. A cheap, $10 digital multimeter meter can be used to troubleshoot a 4-20 mA loop circuit.

17. 4-20 mA can easily be measured in multiple ways:

o With a milliameter inserted into the circuit or across a diode in the circuit

o With a voltmeter as a voltage drop across a dropping resistor

o With a Fluke 77x clamp meter

18. 4-20 mA offers a wide choice of resistor values when converting to a IR voltage drop across the receiver

input. Lots of resistor values, from 1 ohm to 500 ohms, have been used (250 ohms is the most common).

In summary, 4-20mA is simple and rugged. It works well. As a contributor to a controls web forum once

stated, “beat that”.