Intrinsic Safety Explained: Ex i Barriers and IS Circuits Made Simple

Intrinsic safety is one of the most common protection methods used for instruments in hazardous areas.

It is also one of those topics that seems simple until you actually need to select a barrier, check entity parameters, confirm cable capacitance and inductance, or explain why an output will not drive a solenoid.

The basic idea is straightforward: limit the energy going into the hazardous area so that, even under fault conditions, there is not enough energy available to ignite the gas or vapour.

Quick answer: Intrinsic safety, often shown as Ex i, is a protection method that limits electrical and thermal energy in a circuit. The aim is to make sure the circuit cannot create a spark or hot surface capable of igniting the hazardous atmosphere, even under certain fault conditions.

Disclaimer: This article is a plain-English explanation of intrinsic safety principles. It is not a design approval, inspection certificate or replacement for the relevant standards, equipment certificates, hazardous area classification documents or competent engineering review. Always check the actual certificates and project requirements before selecting or installing equipment.

What Is Intrinsic Safety?

Intrinsic safety is a way of protecting electrical circuits used in hazardous areas by limiting the available energy.

In simple terms, the circuit is designed so that it should not be capable of producing enough electrical or thermal energy to ignite the surrounding explosive atmosphere.

This is why intrinsic safety is commonly used for instrumentation circuits such as transmitters, switches, sensors and other low-power field devices.

Intrinsic safety is not just “low voltage”. It is a certified system where the barrier or isolator, field device, cable and installation all need to be assessed together.

Why Do We Need an IS Barrier or Galvanic Isolator?

An IS barrier or galvanic isolator is used to limit the energy that can enter the hazardous area.

If a fault occurs, the barrier or isolator helps prevent excessive voltage, current or power from reaching the field device and hazardous area wiring.

One simple way to think about it is like a coffee machine.

Imagine a coffee machine filling a cup.

The cup can only safely hold a certain amount of water. If the machine pours too much in, the cup overflows.

The coffee machine controls the flow so the cup does not overflow.

An IS barrier or galvanic isolator does something similar with electrical energy. It limits what can be sent into the hazardous area so the circuit cannot “overflow” with enough energy to ignite the gas.

Common Intrinsically Safe Equipment

In my experience, intrinsic safety is commonly used with instruments and low-power field devices.

Typical examples include:

Flowmeters
Level transmitters
Pressure transmitters
Temperature transmitters
Level switches
Proximity sensors
Solenoids
Reed switches
Simple apparatus

Intrinsic safety is especially useful for instrumentation because the power levels are usually low enough to make energy limitation practical.

Real-world note: I mostly see intrinsic safety used around instrumentation: flowmeters, transmitters, level switches, proximity sensors and solenoids. It is very common in process and hazardous area work.

Zener Barrier vs Galvanic Isolator

People often use the word “barrier” generally, but there are different types of associated apparatus used in intrinsic safety.

Two common examples are:

Zener barriers
Galvanic isolators

Zener barriers rely heavily on correct earthing arrangements. Galvanic isolators provide isolation between the safe-area side and the hazardous-area side and are often simpler from an installation point of view.

In my own work, I typically see galvanic isolators rather than traditional Zener barriers.

Practical note: In this article, I use “barrier” and “isolator” together because both are commonly used terms. The exact equipment type should always be checked against the certificate and project design.

What Does Ex ia, Ex ib and Ex ic Mean?

Intrinsic safety markings are usually shown as Ex i, with different levels such as Ex ia, Ex ib and Ex ic.

Marking	General Meaning	Typical Use
Ex ia	Highest intrinsic safety level	Often used where circuits may enter Zone 0, depending on full marking and certification
Ex ib	Intermediate intrinsic safety level	Often associated with Zone 1 and Zone 2 applications, depending on marking
Ex ic	Lower intrinsic safety level	Often associated with Zone 2 applications, depending on marking

In practice, I usually see Ex ia, especially with barriers and isolators.

One thing to watch is dual marking. Associated apparatus may have one marking for where the device itself can be installed, and another marking in square brackets showing what it can transmit into.

For example, a galvanic isolator may be installed in a Zone 2 panel but have an output circuit suitable for an Ex ia loop into Zone 0 or Zone 1, depending on the full certificate and marking.

The square brackets matter because they help show that the associated apparatus is not necessarily installed in the same area as the field device.

The Whole IS Loop Matters

One of the biggest mistakes with intrinsic safety is only looking at one part of the loop.

A field instrument being Ex ia does not automatically make the full circuit suitable.

The full loop normally includes:

The safe-area equipment
The barrier or galvanic isolator
The hazardous-area field device
The interconnecting cable
Any simple apparatus
Earthing and screening arrangements where applicable
The installation conditions

Important: Intrinsic safety is a loop assessment. Do not check the field device on its own and assume the whole circuit is acceptable.

Entity Parameters Explained

Entity parameters are the values used to prove that the barrier or isolator is compatible with the field device and cable.

The field device has limits that should not be exceeded. These values are normally tested and stated by the manufacturer on the certificate or datasheet.

The barrier or isolator has output values. The IS calculation checks that the barrier cannot supply more voltage, current or power than the field device can safely accept.

The field device has limits. The barrier has outputs. The cable adds capacitance and inductance. The IS calculation checks that the combination is still safe.

Basic IS Calculation Checks

A basic intrinsic safety entity parameter check will usually compare the output parameters of the barrier or isolator against the input parameters of the field device.

Typical checks include:

Uo ≤ Ui
Io ≤ Ii
Po ≤ Pi
Co ≥ Ci + Ccable
Lo ≥ Li + Lcable

Parameter	Simple Meaning
Uo	Maximum output voltage from the barrier or isolator
Ui	Maximum input voltage the field device can safely accept
Io	Maximum output current from the barrier or isolator
Ii	Maximum input current the field device can safely accept
Po	Maximum output power from the barrier or isolator
Pi	Maximum input power the field device can safely accept
Co	Maximum external capacitance allowed by the barrier or isolator
Ci	Internal capacitance of the field device
Lo	Maximum external inductance allowed by the barrier or isolator
Li	Internal inductance of the field device

The exact calculation can depend on the protection concept, gas group, certificate values, cable details and project requirements.

Design note: The simple checks above explain the principle. Real IS calculations should be completed using the actual equipment certificates, cable data, applicable standards and project requirements.

Why Cable Capacitance and Inductance Matter

The cable is part of the IS loop.

That means the cable capacitance and inductance need to be included in the calculation.

In many cases, short cable lengths will pass without much issue. However, long cable runs can become a problem, especially if the calculation is close to the limit.

One mistake I have seen is when the cable length changes but the IS calculation is not updated.

Important: If the cable route changes, the cable length changes, or the cable type changes, the IS calculation may need to be reviewed.

The 1% Rule

In some IS assessments, the relationship between inductance and capacitance needs extra care, particularly when both are present at meaningful levels.

You may hear people refer to the “1% rule” when checking whether capacitance and inductance values can be treated in a simpler way.

The key point for a beginner is not to blindly assume the cable values are irrelevant.

Practical note: Cable capacitance and inductance often seem like small numbers, but they still need to be checked correctly. Long cable lengths and wrong units can easily cause mistakes.

Common Mistake: Wrong Units and Decimal Places

IS calculations are usually straightforward in principle, but they are easy to get wrong when entering data manually.

One of the simplest mistakes is using the wrong unit or decimal place.

For example:

nano instead of micro
micro instead of milli
watts instead of milliwatts
0.993 W entered as 993 W
certificate values rounded incorrectly

Real-world warning: It is easy to see a calculation pass and assume it is fine. But if the data has been entered wrongly, the pass result is meaningless.

Common Mistake: Poor IS and Non-IS Segregation

One of the biggest issues I have seen in live panels is poor segregation between intrinsically safe and non-intrinsically safe circuits.

This can still happen on commissioned systems.

Common issues include:

IS and non-IS trunking too close together
IS and non-IS glands too close together
Internal wiring not segregated properly
IS cables not clearly identified
Cable identification missing
Spare cores not terminated correctly
Wiring not matching the drawing

Segregation is not just about making a panel look tidy. It is part of maintaining the safety of the installation.

Real-world note: I have seen IS circuits wired to the wrong channel, spare cores not properly managed, and drawings not matching what is installed. These are exactly the sorts of things that cause problems during inspection and commissioning.

Common Mistake: Assuming ATEX Means Intrinsically Safe

Another common misunderstanding is assuming that any ATEX rated device is automatically suitable for an intrinsically safe circuit.

That is not true.

An Ex d device may be ATEX certified, but that does not mean it is intrinsically safe.

The protection concept matters.

Important: ATEX rated does not automatically mean Ex i. Always check the full marking and certificate.

Common Mistake: Misunderstanding Simple Apparatus

Simple apparatus can be misunderstood.

People sometimes assume that most basic devices can be treated as simple apparatus because they look simple.

Examples might include:

Mechanical switches
Reed switches
Thermocouples
RTDs
Simple contacts

But it is still worth checking properly. Some devices may include components or stored energy that mean they should not simply be assumed to be simple apparatus.

Practical advice: If you are unsure whether something qualifies as simple apparatus, confirm it with the manufacturer, certificate information or competent design authority rather than guessing.

Common Mistake: Outdated or Difficult Certificates

ATEX and IECEx certificates can sometimes be difficult to find, awkward to read, or out of date.

It is not always obvious whether the certificate you have is the latest version.

Pay attention to:

Certificate supplements
Specific conditions of use
X markings on certificates
Tables where values change by gas group
Tables where values change by ambient temperature
Missing or incomplete entity parameters

Some certificates do not present the values in the neat way you might expect. Sometimes the value you need depends on another condition in the table.

Important: Always check certificate supplements and special conditions. The first page of the certificate may not contain everything you need.

Common Mistake: The Output Cannot Drive the Device

There is another practical issue that is not always treated as a hazardous area problem, but it still matters.

Sometimes the barrier or isolator is safe from an entity parameter point of view, but it cannot actually drive the field device properly.

Solenoids are a common example. The available current may be too low.

This creates a different kind of problem: the circuit may be safe, but it does not work.

Passing the IS calculation is not the only check. The circuit still needs to function. If the isolator cannot provide enough usable output to operate the solenoid, the design needs reviewed.

Intrinsic Safety Inspection Issues

During inspections, the problems are often practical installation issues rather than complex calculations.

Common inspection findings include:

IS cables not identified correctly
Blue identification missing or inconsistent
IS and non-IS cables not segregated
Glands too close together
Spare cores not terminated correctly
Terminations not matching drawings
Incorrect barrier channel used
Labels missing or unclear
Loop drawings not matching site wiring

These issues can delay commissioning and create unnecessary rework, especially when they are only found late in the project.

When Should IS Be Checked?

Intrinsic safety should be checked during the design stage.

Waiting until the equipment is installed or ready for commissioning is asking for trouble.

Early checks help avoid:

Wrong barriers or isolators being ordered
Field devices that are not compatible
Solenoids that cannot be driven
Cable lengths that fail the calculation
Inspection failures
Commissioning delays
Late redesign and replacement costs

The earlier you check the IS loop, the cheaper and easier it is to fix any problems.

Quick IS Design Checklist

Before approving an intrinsically safe loop, I would check:

Is the field device suitable for the hazardous area?
Is the barrier or galvanic isolator suitable?
Are the entity parameters compatible?
Have Uo, Io and Po been checked against Ui, Ii and Pi?
Have Co and Lo been checked against device and cable values?
Has the cable length been confirmed?
Has the cable type been confirmed?
Have gas group and temperature class been checked?
Are IS and non-IS circuits segregated?
Are the certificates current?
Have certificate supplements been checked?
Are there any X certificate conditions?
Does the circuit actually function, not just pass the calculation?
Do the drawings match the installed wiring?

Frequently Asked Questions About Intrinsic Safety

What does intrinsic safety mean?

Intrinsic safety is a protection method that limits electrical and thermal energy in a circuit so that it should not be capable of igniting a hazardous atmosphere, even under certain fault conditions.

What does an IS barrier do?

An IS barrier or galvanic isolator limits the voltage, current and power that can enter the hazardous area. This helps keep the circuit energy below ignition-capable levels.

Is low voltage the same as intrinsically safe?

No. Low voltage does not automatically mean intrinsically safe. Intrinsic safety requires certified equipment, compatible entity parameters, suitable cable checks and correct installation.

What is the difference between Ex ia and Ex ib?

Ex ia provides a higher level of intrinsic safety than Ex ib. The permitted use depends on the full equipment marking, hazardous area classification and certificate conditions.

Can an ATEX device be used in an IS loop?

Only if it is suitable for intrinsic safety and the full loop assessment passes. An ATEX device with another protection concept, such as Ex d, is not automatically suitable for an IS loop.

Why does cable length matter in intrinsic safety?

Cable length matters because the cable adds capacitance and inductance to the loop. Long cable runs can affect whether the IS calculation passes.

What is simple apparatus?

Simple apparatus is basic equipment that may be allowed in an IS circuit under certain conditions. However, it should not be assumed without checking the relevant requirements and manufacturer information.

Final Thoughts

Intrinsic safety is simple in principle but easy to get wrong in practice.

The idea is to limit the energy going into the hazardous area so that the circuit cannot ignite the gas or vapour, even under certain fault conditions.

The mistakes usually happen when people look at one part of the loop and forget the rest.

The field device matters.
The barrier or isolator matters.
The cable matters.
The certificate matters.
The installation matters.

My simple rule is this: do not assume a circuit is intrinsically safe just because the voltage is low or one device has an ATEX marking. The whole loop needs to be checked.

You may also find these useful: