ホームニュース Why 4-20mA is the Universal Language of Industrial Automation

Why 4-20mA is the Universal Language of Industrial Automation

Technical Guide

Why 4-20mA is the “Universal Language” of Industrial Automation

Understanding the Engineering Principles Behind the World’s Most Widely Used Process Control Signal

4-20mA

Current Loop Standard

2-Wire

Power & Signal

24VDC

Standard Supply

ハート

Digital Overlay

Introduction: The Industry Standard

If you ask what is the most classic and widely used signal in industrial automation, the answer is undoubtedly the 4-20mA current signal. From oil refinery units in petrochemical plants to fermentation tanks in pharmaceutical facilities, from water level control in wastewater treatment to temperature regulation in power plants, the 4-20mA signal is everywhere.

It is known as the “universal language” of industrial automation—much like English in international communication. But why exactly 4-20mA? Why not 0-10V? Why not 0-20mA? The answer lies in profound technical principles and engineering wisdom developed over decades.

Part 1: What is the 4-20mA Signal?

1.1 Basic Concept

4-20mA (4-20 milliamperes) is an analog current signal standard where:

4mA: Corresponds to the measurement zero point (lower limit)
20mA: Corresponds to the full-scale measurement (upper limit)
Intermediate values: Linearly correspond to the measured parameter

Example: Pressure Transmitter with 0-500 kPa Range

Measured Pressure	Output Current
0 kPa	4 mA
250 kPa (50%)	12 mA
500 kPa (100%)	20 mA

Calculation Formula:
Output Current = 4 + (Measured Value / Range) × 16 mA

Reverse Calculation:
Measured Value = (Output Current – 4) / 16 × Range

1.2 Why Not Other Signals?

Voltage Signal vs Current Signal

Comparison Item	Voltage Signal (0-10V)	Current Signal (4-20mA)
Transmission Distance	<100 meters	Up to several kilometers
Anti-Interference	Weak (susceptible to EMI)	Strong (current source characteristics)
Cable Voltage Drop	Significant (direct signal loss)	None (constant current unaffected)
Broken Wire Detection	Difficult (0V could be zero or fault)	Easy (<4mA indicates fault)

Why 4mA Instead of 0mA?

This is the most clever design aspect of 4-20mA:

Broken Wire Detection: When the signal drops below 4mA (e.g., 0mA or 2mA), the receiving device can immediately detect a broken wire or fault condition
Two-Wire Power Supply: The 4mA minimum current can power two-wire transmitters (the “live zero” concept)
Fault vs True Zero Distinction: 0mA could be a true zero value or a fault; 4mA clearly indicates the zero point

Part 2: Four Core Advantages of 4-20mA

Advantage 1: Long-Distance Transmission Capability

Principle: Constant Current Source Characteristics

A 4-20mA signal source is a constant current source, meaning that regardless of load resistance variations (within allowable limits), the output current remains constant.

Ohm’s Law: U = I × R

When I (current) is constant: Resistance R changes → Voltage U changes proportionally, but current I remains unchanged.

Practical Application Example

Scenario: A pressure transmitter is installed in the field, 2000 meters from the DCS control room.

パラメータ:

Cable resistance: ~40Ω/km × 2km = 80Ω (round trip)
DCS input resistance: 250Ω
Total load: 80 + 250 = 330Ω

Calculations:

At 20mA voltage drop: U = 0.02A × 330Ω = 6.6V
At 4mA voltage drop: U = 0.004A × 330Ω = 1.32V
Required supply voltage: ≥6.6V + transmitter operating drop (~3V) = 9.6V

Conclusion: With a standard 24VDC supply, 2000-meter transmission is completely feasible! With 0-10V voltage signals, cable voltage division would severely degrade the signal, potentially reducing the actual voltage reaching the DCS to only 7-8V with enormous error.

Advantage 2: Strong Anti-Interference Capability

Industrial Electromagnetic Environment

Industrial sites contain numerous interference sources: large motors, variable frequency drives, high-voltage cables, power lines, welding machines, contactors, and wireless communication equipment.

Why Current Signals Resist Interference

The “immune mechanism” of current signals:

Constant Current Source Characteristics: Externally induced interference voltage cannot change the current magnitude
Low Loop Impedance: Current loop impedance is much lower than voltage signal circuits, making it difficult for interference voltage to couple
Differential Reception: Receiving devices only care about current magnitude, not common-mode interference

Analogy: Voltage signals are like “water flow”—easily affected by external influences. Current signals are like “fixed water volume”—regardless of how complex the path, the volume remains constant.

Advantage 3: Powerful Fault Diagnosis Capability

The Genius of “Live Zero” Design

Current Value	意味
<3.6 mA	Broken wire or instrument fault
3.6-4.0 mA	Below zero (negative overrange)
4.0 mA	Normal zero point
4.0-20.0 mA	Normal measurement range
>20.5 mA	Overrange upper limit or fault
>22.0 mA	Severe fault (e.g., sensor damage)

Smart Diagnostic Applications

Modern smart transmitters support the NAMUR NE43 standard, communicating fault types through current values:

Instrument Technician’s Diagnostic Logic:

Seeing 0mA → Immediately judge broken wire, check wiring
Seeing 3.5mA → Transmitter internal fault, replace
Seeing 21mA → Overrange, check range settings or process conditions

Advantage 4: Two-Wire System Support, Cost Savings

What is Two-Wire?

Two-wire transmitters use just two wires that simultaneously perform two functions:

Transmitting the 4-20mA signal
Powering the transmitter

No additional power wiring required!

Cost Comparison

Item	Four-Wire System	Two-Wire System
Cable Cores	4 cores (2 power + 2 signal)	2 cores
Cable Cost	High	Reduced 50%
Terminals	Many	Few
Installation Difficulty	より高い	Simplified
Maintenance Difficulty	より高い	Simplified

Factory Project Example: Assuming 1000 instruments
Two-wire savings: 1000 sets of cable terminals, wiring labor
Total cost savings: Can reach hundreds of thousands of dollars

Part 3: 4-20mA System Components

3.1 Typical 4-20mA Loop

Power Supply (24VDC)

|

├─ [Resistor 250Ω] ──┬─ Ammeter (display mA)

│                    |

└────────────────────┴─ [Two-Wire Transmitter]

|

Field Measurement

3.2 Key Components

Transmitter (Signal Source)

Function: Converts physical quantities to 4-20mA current
種類だ： Pressure, temperature, flow, level transmitters, etc.
Output Characteristics: Constant current source output

Load Resistor

DCS/PLC input resistance: typically 250Ω

Function: Converts current to voltage for A/D conversion
Calculation: 20mA × 250Ω = 5V (standard input voltage)

電源

Standard: 24VDC ±10%
種類だ： Switching power supply, linear power supply
Requirements: Stable output, low ripple, short-circuit protection

Shielded Cable

Specification: Twisted pair shielded cable (e.g., RVVP 2×1.5)
Shield Layer: Single-end grounded (prevents ground loops)

3.3 Loop Calibration Methods

Using a Calibrator

Disconnect transmitter from DCS connection
Connect calibrator in series with the loop
Have calibrator output 4mA, 12mA, 20mA respectively
Check if DCS/PLC readings correspond correctly
Record errors, adjust or replace as needed

Field Calibration Tools

Signal Generator: Simulates 4-20mA signals
Multimeter: Measures actual current
Standard Meter: Compares accuracy

Part 4: Common 4-20mA Fault Diagnosis

4.1 Typical Fault Symptoms and Causes

Fault Symptom	考えられる原因	解決
Reading 0mA	Signal line open circuit	Check wiring, replace cable
Reading <4mA	Insufficient power, transmitter fault	Check supply voltage, replace transmitter
Reading fixed at value	Transmitter fault, signal line short	Check for short, replace transmitter
Reading fluctuates greatly	EMI, poor grounding	Check shielding, improve grounding
Reading doesn’t match actual	Range setting error, zero drift	Recalibrate, adjust range

4.2 Fault Troubleshooting Steps

Case Study: DCS Shows Flow Value Stuck at 4mA (Zero)

トラブルシューティング手順：

Check local indicator
- Local indicator has reading → Problem in signal transmission or DCS
- Local indicator also shows zero → Problem with field instrument
Assuming local indicator is normal, measure mA signal
- Use multimeter to measure mA in series → Actual signal is 12mA → Problem in DCS card
- Actual signal is only 4mA → Problem with transmitter or wiring
Further inspection
- Check terminal connections → Found loose connection
- After tightening, returns to normal → Fault cleared

4.3 Preventive Maintenance

Regular Inspection Content

Signal value stability check
Terminal connection tightness
Power supply voltage measurement
Environmental conditions (temperature, humidity)

Preventive Measures

Use high-quality cables and connectors
Ensure proper shielding and grounding
Regular calibration (1-2 times per year)
Maintain maintenance records

Part 5: Modern Developments in 4-20mA

5.1 Smart Transmitters

HART Protocol

HART (Highway Addressable Remote Transducer):

Digital signal superimposed on 4-20mA analog signal
Frequency: 1200Hz FSK modulation
Advantage: Compatible with traditional 4-20mA while supporting digital communication

Enhanced Functions

Remote diagnostics
Online calibration
Multi-variable transmission
Asset management

Digital Communication Advantages

Traditional 4-20mA	HART/Digital
Transmits measurement value only	Can transmit multiple parameters
Requires field calibration	Remote configuration
Limited diagnostic information	Rich self-diagnostics
Manual record-keeping	Automated management

5.2 Next-Generation Instrument Protocols

FOUNDATION Fieldbus:

Fully digital, bidirectional communication
Control functions distributed to field devices
Higher accuracy and reliability

Profibus PA/DP:

Open fieldbus standard
Widely used in process industries

WirelessHART:

Wireless transmission
Suitable for locations difficult to wire

But! 4-20mA remains the mainstream:
✓ Excellent compatibility
✓ Mature technology
✓ Low cost
✓ Simple maintenance

Part 6: Real-World Case Studies

Case 1: Long-Distance Pressure Measurement

Background: A new unit at a refinery with pressure transmitters 1500 meters from the DCS control room, using 4-20mA signal transmission.

Design Calculations:

Cable resistance: 1.5km × 40Ω/km × 2 = 120Ω
DCS input resistance: 250Ω
Total load resistance: 370Ω

Voltage drop at 20mA: 0.02A × 370Ω = 7.4V
Required supply: 7.4V + 3V (transmitter) = 10.4V
Selected 24VDC supply: Sufficient margin ✓

Implementation:

Selected RVVP 2×1.5 shielded cable
Shield grounded at DCS side only
Field terminals properly waterproofed
Commissioned successfully on first attempt

Case 2: Solving Interference Problems

Background: A flow meter in a pump room showed severe reading fluctuations, making it unusable for control.

Fault Diagnosis:

Symptom: Flow readings fluctuating rapidly within ±10% range

Investigation:

Measured mA signal with multimeter → Confirmed fluctuation
Disconnected transmitter, connected standard signal generator → DCS reading stable
→ Conclusion: Problem with transmitter or field conditions
Inspected field: Large VFD motor adjacent to transmitter
→ Conclusion: Electromagnetic interference

解決：

Rerouted signal cable through metal conduit (magnetic shielding)
Grounded shield at both ends (eliminates interference)
Added filter to transmitter power supply
Increased spacing between signal and power cables (>30cm)

Result: Readings stabilized within ±0.5% range

まとめ

Why is 4-20mA the “Universal Language” of Industrial Automation?

アドバンテージ	説明
Long-Distance Transmission	Constant current characteristics overcome cable voltage drop
Strong Anti-Interference	Stable and reliable in complex electromagnetic environments
Fault Detection	4mA “live zero” design, broken wire immediately detectable
Two-Wire System	Simplified wiring, reduced costs
Standardization	Global standard, equipment compatibility
Mature Technology	Decades of application, high reliability

Significance for Instrument Technicians

Understand the principles: Knowing why we use it enables us to use it well
Master fault diagnosis: 4-20mA faults are the most common in daily work
Perform proper maintenance: Regular calibration, wiring checks, ensuring reliability
Embrace new technology: HART and smart transmitters are the evolution of 4-20mA

Looking Ahead

Although various digital bus technologies have emerged, 4-20mA will continue to hold an important position in industrial automation. Just as analog and digital signals will coexist long-term, this “universal language” will continue serving industrial production.

The 4-20mA signal may seem simple, but it embodies profound engineering wisdom. Mastering it is fundamental knowledge for every instrument technician.