What is 4-20 mA?
4-20 mA is the most widely used analogue signal in the process industry: a transmitter converts a measured value into a direct current between 4 and 20 milliamperes, where 4 mA represents the start and 20 mA the end of the measuring range. The PLC or DCS measures that current through an analogue input and converts it back into, for example, a temperature, pressure or level. The signal dates back to the 1950s and is standardised in IEC 60381-1.
๐ง How does a 4-20 mA current loop work?
A current loop consists of four elements in series:
- Power supply โ usually 24 V DC, often provided by the analogue input card
- Transmitter โ regulates the current through the loop according to the measured value
- Cabling โ two conductors, often shielded and twisted
- Receiver โ the PLC input measures the current across a resistor (for example, 250 ฮฉ gives 1โ5 V)
Because the current in a closed loop is the same everywhere, the length or resistance of the cable does not matter within limits. That makes 4-20 mA suitable for cable runs of hundreds of metres through an electrically noisy plant.
๐ง Why 4 mA and not 0 mA?
| Signal | Meaning |
|---|---|
| 0 mA | Cable break or failed transmitter: recognisable as a fault |
| 3.6โ3.8 mA | Below range (NAMUR NE 43): measured value too low or faulty |
| 4 mA | 0% of the measuring range (live zero) |
| 12 mA | 50% of the measuring range |
| 20 mA | 100% of the measuring range |
| โฅ 21 mA | Above range or fault (NAMUR NE 43) |
Thanks to the live zero, the system can distinguish a 0% reading from a broken cable. In addition, a two-wire transmitter can draw its own power from that first 4 mA, so no separate power cable is needed.
๐ What is the difference between 4-20 mA, HART and fieldbuses?
| 4-20 mA | HART | Fieldbus / Ethernet | |
|---|---|---|---|
| Signals per cable | One measured value | One measured value + digital extras | Many values and devices |
| Diagnostics | Only out-of-range | Status, configuration, diagnostics | Extensive |
| Complexity | Very low | Low | Higher |
| Security risk | Virtually none | Limited | Network attacks possible |
HART superimposes a digital signal on the analogue 4-20 mA loop, allowing existing cabling to carry additional information. More and more installations move to Foundation Fieldbus, PROFIBUS PA or Ethernet-APL, but 4-20 mA remains widespread thanks to its simplicity and robustness.
๐ญ Where is 4-20 mA used?
- Measurements โ pressure, temperature, flow and level via transmitters
- Actuation โ an analogue output drives a control valve or variable frequency drive with a 4-20 mA signal
- Safety systems โ many SIS sensors use 4-20 mA because of its predictable fault detection
- Hazardous areas โ combined with intrinsically safe barriers (ATEX)
๐งฎ How do you convert a 4-20 mA signal into a measured value?
The relationship between current and measured value is linear. The formula is:
Measured value = range-min + (I โ 4) / 16 ร (range-max โ range-min)
Example: a pressure transmitter has a range of 0 to 10 bar and outputs 13.6 mA. (13.6 โ 4) / 16 = 0.6 โ 0.6 ร 10 bar = 6 bar.
| Current | Pressure (range 0โ10 bar) | Temperature (range โ50 to 150 ยฐC) |
|---|---|---|
| 4 mA | 0 bar | โ50 ยฐC |
| 8 mA | 2.5 bar | 0 ยฐC |
| 12 mA | 5 bar | 50 ยฐC |
| 16 mA | 7.5 bar | 100 ยฐC |
| 20 mA | 10 bar | 150 ยฐC |
In the PLC the current is usually first converted into a raw integer value (with Siemens, for example, 0โ27648) and then scaled to the engineering unit. A common mistake is a range in the PLC that does not match the transmitter, causing all measured values to deviate systematically.
โ Frequently asked questions
What is the difference between a two-wire and a four-wire transmitter?
A two-wire transmitter draws its power from the 4-20 mA loop itself and therefore needs only two conductors for both power and signal. A four-wire transmitter has a separate power supply, for example 230 V AC or 24 V DC, plus two conductors for the signal. Four-wire transmitters are used when the measurement needs more energy than the loop can supply.
How long can a 4-20 mA cable be?
A 4-20 mA cable can be hundreds of metres to more than a kilometre long. The maximum length is determined by the supply voltage, the minimum voltage the transmitter needs and the total resistance of cable and receiver. As long as the loop retains enough voltage, cable length has no effect on the measured value.
Why is 4-20 mA used instead of 0-10 V?
4-20 mA is preferred over 0-10 V because a current signal is immune to voltage drop over long cables and is less affected by electromagnetic interference. In addition, 4-20 mA detects a cable break thanks to the live zero of 4 mA, whereas 0 V on a voltage signal cannot be distinguished from a genuine zero reading.
What is NAMUR NE 43?
NAMUR NE 43 is a recommendation from the German user association NAMUR that defines which current values outside the measuring range indicate a fault. Values below 3.6 mA or above 21 mA mean a fault in the transmitter or the loop. This allows a controller to distinguish between an extreme reading and a defect.
Can a 4-20 mA signal be hacked?
A purely analogue 4-20 mA signal cannot be attacked over a network, because it carries no digital data. An attacker would need physical access to the cabling. The PLC that reads the signal, or digital protocols such as HART on the same loop, can however be targets of an attack.
๐ In summary
4-20 mA is the standard analogue current signal that transmitters use to send measured values robustly and over long distances to a PLC or DCS, with 4 mA as a โlive zeroโ to detect cable breaks. It is simple, immune to interference and virtually impossible to attack over the network, which is why it remains the foundation of process measurement.