A Practical Engineering Guide for Process Automation and Instrumentation Systems
In industrial automation—especially in plants involving hazardous areas, field instrumentation, and PLC/DCS systems—two devices frequently appear in the same cabinet: intrinsically safe barriers (safety barriers) and signal isolators.
Although their appearance and wiring layout may look similar, their engineering purposes, operating principles, and compliance requirements are fundamentally different.
Misunderstanding or interchanging these devices may lead to serious safety incidents, unstable signals, and extended troubleshooting time.
This article provides a clear and structured explanation of:
What a safety barrier is
What a signal isolator is
The differences between them
How to select the correct device for each application
Key installation and engineering considerations
1. What Is a Safety Barrier?
Safety barriers are protective devices used when field instruments are installed in hazardous or potentially explosive atmospheres containing flammable gases, vapors, or dust.
Their purpose is to limit the electrical energy entering the hazardous area so that even in the worst-case scenario, it cannot ignite the surrounding medium.
Core Principle of Intrinsic Safety (Ex i)
Under normal and fault conditions, the circuit must not release enough energy to cause ignition.
A safety barrier ensures that any voltage, current, or spark entering the hazardous area is always lower than the ignition threshold.
Key Functions of a Safety Barrier
1. Energy Limitation (Voltage, Current, and Power)
Safety barriers employ components such as:
Current-limiting resistors
Zener diodes
Fuses
Isolation transformers (for isolated barriers)
These ensure that even if the control system experiences:
Short-circuit
Surge
Power supply failure
Lightning impact
…the hazardous-area loop will never receive more energy than permitted.
2. Compliance with Intrinsic Safety Standards
Safety barriers must comply with strict international standards:
IEC 60079-11 (intrinsic safety)
ATEX (EU)
IECEx
GB3836 (China)
These standards require the device to remain safe even under single-fault and double-fault conditions.
3. Types of Safety Barriers
There are two major types:
| Type | Description | Pros | Cons |
|---|---|---|---|
| Zener Barrier | Uses Zener diodes + resistors; requires a solid ground | Low cost | Requires high-quality grounding; weaker isolation |
| Isolated Barrier | Uses galvanic isolation (transformer/optocoupler) | No grounding required; high noise immunity | Higher cost |
Today, isolated safety barriers are the industry mainstream due to better EMC performance and simpler installation.
4. When Is a Safety Barrier Required?
Whenever a field device is marked:
Ex ia
Ex ib
…it must be connected through a certified safety barrier.
This is a mandatory requirement in intrinsic safety loop design.
Safety barriers are not signal conditioners.
They are energy-limiting protection devices designed to prevent explosions.
2. What Is a Signal Isolator?
Unlike safety barriers, signal isolators are not used for hazardous-area protection.
Their primary function is to maintain signal quality, stability, and reliability in industrial automation systems.
Key Functions of Signal Isolators
1. Breaking Ground Loops and Eliminating Noise
Signal isolators prevent issues related to:
Ground potential difference
Electromagnetic interference (EMI)
Noise from VFDs, pumps, motors, and switching devices
Signal drifting and unexpected spikes
Isolation technologies (transformer, optocoupler, or capacitive isolation) electrically separate circuits and block interference at the source.
2. Signal Conversion and Matching
Common conversions include:
4–20 mA ↔ 1–5 V
Thermocouple ↔ standard analog output
RTD → analog signal
Isolated power supply for two-wire transmitters
This ensures compatibility between instruments and control systems.
3. Protecting PLC/DCS Input Modules
Isolators act as protective buffers, preventing:
Overvoltage
Short circuits
High-energy transients
that may damage expensive control equipment.
4. Signal Distribution (1 Input → 2 or More Outputs)
One transmitter signal may need to feed:
PLC
DCS
Recorder
Controller
A signal isolator can distribute outputs without loading or interfering with each other.
A signal isolator solves “signal quality” issues—not “explosion protection” issues.
3. Why They Cannot Be Interchanged
Safety Barrier = Energy Limitation
Used in hazardous areas
Mandatory for Ex ia / Ex ib devices
Prevents ignition risk
Signal Isolator = Signal Stability & Isolation
Used in safe (non-hazardous) areas
Prevents noise, ground loops, and signal errors
Enhances measurement reliability
One protects lives; the other protects signals.
They belong to completely different categories:
| Category | Safety Barrier | Signal Isolator |
|---|---|---|
| Purpose | Prevent explosions | Improve signal quality |
| Standard | IECEx/ATEX/IEC 60079 | EMC & industrial signal standards |
| Installation area | Hazardous area boundary | Safe area (control room) |
| Main function | Energy limiting | Signal conditioning & isolation |
If both safety and signal stability are needed (e.g., hazardous-area loops with long cable runs), then isolated safety barriers should be used.
4. Engineering Guidelines for Safety Barrier Installation
To ensure correct and safe application:
✔ Must match the certified intrinsic safety parameters
Check the instrument’s:
Ui, Ii, Pi (input energy limits)
Ci, Li (internal capacitance and inductance)
These must be compatible with the barrier’s rated parameters.
✔ Zener barriers must be grounded with low-resistance earth
If grounding is not reliable, isolation barriers should be selected.
✔ Wiring must strictly follow the manufacturer’s terminals
Any wiring mistake could invalidate the intrinsic safety protection.
✔ Ensure the entire loop meets Ex requirements
A complete intrinsic safety loop includes:
Field device
Safety barrier
Cabling
Power supply
All of them must meet the same explosion-proof rating.
5. Engineering Guidelines for Signal Isolator Installation
✔ Verify isolation strength and EMC performance
Especially in areas with:
Variable-frequency drives (VFDs)
High-power motors
High-noise environments
✔ Consider accuracy, temperature drift, response time
These determine the stability of your control loop.
✔ Select correct wiring method (2-wire, 3-wire, 4-wire)
Improper wiring can cause measurement errors.
✔ Check load capability for signal splitting
Avoid overloading the source signal.
✔ Ensure proper cold-junction compensation for thermocouples
Critical for temperature measurement accuracy.
6. How Safety Barriers and Isolators Work Together
A typical automation signal chain:
Safety barrier = Protection
Signal isolator = Stability
Both together create a safe, stable, and compliant automation system.
Conclusion
Safety barriers and signal isolators may look similar, but they serve completely different engineering purposes:
Safety barriers ensure explosion protection by limiting electrical energy.
Signal isolators ensure signal integrity, EMC immunity, and system reliability.
Correct selection and proper installation of both devices are essential for building a safe, stable, and compliant industrial control system.