4–20 mA Is Not Just a Signal — It Is a Safety Decision

Industrial automation continues to evolve rapidly, incorporating digital networks, smart instrumentation, and industrial Ethernet protocols. Despite these advancements, analog current loops—specifically 4–20 mA—remain the most widely deployed communication method for transmitting process variables such as pressure, temperature, level, and flow from field instruments to control systems like PLCs, DCS, and SCADA.

Textbooks and engineering training traditionally highlight two main reasons for its widespread use:

1. Strong noise immunity in electrically harsh environments
2. Stable signal transmission over long cable distances

While these points are correct, they only address the operational advantages. The deeper, less frequently discussed reason is that the 20 mA upper limit was intentionally chosen to remain below the dangerous electrical current threshold that can cause fatal physiological effects in humans, as defined by the IEC 60479-1 standard. In other words, 4–20 mA is not only an instrumentation decision—it is a safety decision grounded in human biology.

The endurance of the 4–20 mA standard therefore is not due to legacy inertia, but because it satisfies a rare intersection of electrical safety, engineering reliability, diagnostics, and deterministic behavior in critical industrial systems.

The Origin of 20 mA — A Human Safety Boundary

The IEC 60479-1 international standard defines how electrical current passing through the human body affects physiological functions. The standard plots current magnitude against exposure duration, dividing human response into four zones:

IEC 60479-1 Zone	Physiological Effect
AC-1	No perceptible effect
AC-2	Painful shock, reversible muscle contraction
AC-3	Involuntary muscle lock, breathing difficulty
AC-4	Ventricular fibrillation, cardiac arrest, severe burns, death

When 20 mA is plotted on the curve, it sits at the upper boundary of AC-2 and the lower boundary of AC-3, meaning:

• Pain and reflexive contraction may occur
• Breathing may be affected in prolonged exposure
• It remains below AC-4, the zone associated with ventricular fibrillation, the primary cause of death from electrical shock

Common biological thresholds include:

• 1 mA → Tingling sensation
• 5 mA → Painful shock, still controllable
• 10 mA → Loss of voluntary muscle control (“cannot let go”)
• 20 mA → Possible respiratory interference
• 30 mA+ → Risk of ventricular fibrillation (fatal arrhythmia)

Thus, choosing 20 mA as the upper limit ensures functionality while remaining below the fatal threshold, embedding safety directly into the physical layer—no software or protection logic required.

Why 4 mA? — The Value of Live Zero Diagnostics

The lower boundary of 4 mA is equally intentional. Instead of using 0 mA as the baseline, the range begins at 4 mA for one primary reason:

0 mA can mean two different things — a valid measurement or a broken wire.
4 mA eliminates ambiguity.

This concept, called live-zero, enables immediate fault detection:

Loop Current	Meaning
0 mA	Broken wire, power loss, open loop fault
0–3.8 mA	Fault condition (NAMUR NE43 standard)
4–20 mA	Valid measurement range
> 20.5 mA	Sensor or loop saturation / fault

This built-in diagnostic capability is one reason 4–20 mA remains essential in safety-critical industries such as power generation, oil & gas, chemical processing, mining, and pharmaceuticals—where silent failures are unacceptable.

Electrical Safety Proven Through Ohm’s Law

To understand why 20 mA is inherently safer than other signal standards, consider basic circuit behavior using Ohm’s Law:

I = V/R

Typical human body resistance varies:

• Dry skin: 5,000 – 7,000 Ω
• Wet skin: 500 – 1,000 Ω

Example 1 — Accidental contact at 50 V with 5,600 Ω body resistance:

I = 50/5600 which is approximately 8.9 mA

This is already near the “loss of grip” threshold (~10 mA).

Example 2 — To reach 20 mA through 5,000 Ω:

V = 0.02 x 5000 = 100 V

However, 4–20 mA loops typically operate at 12–24 VDC, far below the voltage required to push dangerous current through the body. Therefore:

✅ The system is self-limiting by electrical design
✅ It cannot deliver life-threatening current even in fault scenarios
✅ No additional active protection is required to maintain electrical safety

Engineering Benefits That Cemented Its Adoption

Beyond human safety, 4–20 mA survives because it excels in conditions where digital communication often struggles:

✔ Immune to electromagnetic interference (EMI) and ground noise
✔ Maintains accuracy over hundreds of meters of cable
✔ Unaffected by voltage drop, wire resistance, or termination impedance
✔ Simple to troubleshoot using only a multimeter
✔ Predictable failure behavior that is measurable and linear
✔ Works reliably in high-humidity, high-temperature, and high-vibration environments

Unlike voltage signals (0–10 V), where resistance and interference distort the signal, current remains constant through the loop regardless of cable length or conductor degradation.

Compatibility with Industrial Safety Standards

4–20 mA is not just common in automation—it is embedded in safety engineering frameworks:

Standard	Purpose
IEC 61508	Functional safety of electrical/electronic systems
IEC 61511 / ISA 84	Safety instrumentation for process industries
IEC 60079 (Ex i)	Intrinsic safety in explosive environments
NAMUR NE 43	Fault signaling for 4–20 mA systems

Additionally, Safety Integrity Level (SIL) loops often rely on 4–20 mA because the signal can indicate both process value and fault condition without ambiguity.

Modernization Through HART, Not Replacement

Many predicted the replacement of 4–20 mA by fully digital fieldbus systems, but in practice, the opposite occurred—digital functions were layered on top of 4–20 mA, not instead of it.

HART (Highway Addressable Remote Transducer) enables:

• Digital diagnostics over the same 2-wire loop
• Remote configuration and commissioning
• Device health monitoring
• Asset management and predictive maintenance

This hybrid model preserves analog reliability while gaining digital intelligence, extending the life of 4–20 mA well into Industry 4.0.

Intrinsic Safety and Hazardous Area Compliance

Since 4–20 mA loops never exceed 20 mA, they align naturally with Intrinsically Safe (Ex i) design principles used in hazardous environments such as:

• Oil and gas refineries
• Chemical plants
• Grain silos
• LNG processing facilities
• Mining operations

Energy-limiting barriers can certify these loops as safe for explosive atmospheres without complex engineering, because the electrical energy is already bounded by design.

The Core Philosophy: Deterministic, Observable, Safe

Unlike digital communication, which can fail silently due to packet loss or checksum errors, analog current loops fail observably. Changes in current reflect real system state:

• Broken wire → 0 mA
• Sensor failure → < 3.8 mA
• Instrument saturation → > 20.5 mA
• Normal operation → 4–20 mA scaled signal

This determinism and transparency are the foundation of safety-critical engineering, where knowing that something is wrong is often more important than the measured value itself.

Conclusion

The 4–20 mA standard remains dominant not because industry resists change, but because it uniquely satisfies fundamental requirements that newer technologies still struggle to match simultaneously:

✅ Safe for human interaction by design
✅ Deterministic and diagnosable failure behavior
✅ Resilient to noise, distance, and harsh environments
✅ Compatible with modern digital upgrades via HART
✅ Ideal for SIL, ISA 84, IEC 61508, and Ex safety systems

It is not merely a method of transmitting data—it is a safety-anchored engineering philosophy that balances reliability, observability, and human protection.

Also read:

• 4-20 mA Transmitters (2-wire, 3-wire and 4-wire Types)
• Features of Analog Signal Transmission
• Basic Facts about 4-20 mA current loop in Process Instrumentation
• Standard Process Signals for Industrial Instrumentation
• NAMUR NE 107 Standard for Modern Instrumentation Diagnostics