Eejit's Guide

Intrinsic safety calculations look fairly simple when they are reduced to five equations.

Check the voltage. Check the current. Check the power. Check capacitance. Check inductance.

Done.

Unfortunately, the maths is usually the easy bit.

The more important part is making sure you have the correct equipment, the correct certificate, the correct certificate issue, the correct table, the correct units and the correct cable information before you start entering numbers into a spreadsheet.

In this article, I’m going to work through a practical intrinsic safety example using a Siemens SITRANS LVL200 level switch and a Pepperl+Fuchs galvanic isolator.

What Are We Trying to Prove?

The basic purpose of the intrinsic safety calculation is to prove that the energy available from the associated apparatus cannot exceed the limits of the field device and that the external capacitance and inductance remain within the permitted values.

For a basic entity-parameter assessment, we are normally checking:

If you’re new to intrinsic safety, you can read my broader guide first:

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

Before We Start: Does This Equipment Actually Need to Be ATEX?

I’ve seen projects where hazardous-area requirements are assumed from the start, only for the actual classification to be unclear or completed much later.

I’ve also experienced the opposite.

A project starts with the understanding that an area is not hazardous. Design progresses, equipment gets selected, procurement starts and then later someone produces the hazardous-area assessment and announces that parts of the installation are classified.

That can become expensive very quickly.

You’re potentially looking at:

• Redesign
• Re-procurement
• Replacement equipment
• Reinstallation
• Additional inspection
• Recommissioning
• Extra engineering hours
• Teams returning to site after they thought the job was finished

Any money that appeared to be saved by delaying the hazardous-area work can disappear very quickly while everyone tries to finish the project.

Get the HAC Documentation Done

If hazardous areas are possible, get the hazardous-area classification completed early and make sure the HAC drawings are kept current.

Know where the Zones actually are.

Then specify the equipment to suit the classified area.

Don’t simply fill an entire project with expensive Ex equipment because it feels safer.

More certification is not automatically better engineering.

Old Standards Aren’t Automatically Wrong

Another frustration of mine is being referred to an old company specification as though the fact that it exists means every requirement in it should remain forever.

Sometimes the document was written decades ago.

That doesn’t automatically make it wrong.

But it should probably make someone ask:

When was this last technically reviewed?

The equipment may have changed. Technology may have changed. The site may have changed. The applicable standards may have changed. The people who originally wrote the document may no longer work there.

Yet requirements can survive because:

“That’s how we’ve always done it.”

The same applies to specifying Ex d panels for everything.

If Ex d is genuinely required, use it.

If another protection concept or modern system architecture satisfies the actual requirement more efficiently, it is worth considering.

The Equipment in This Example

Siemens SITRANS LVL200

The field device in this example is:

Siemens SITRANS LVL200 – 7ML5747-4CC47-3AA0-Z

We are using it as a high-level switch on an ethanol tank within a Zone 1 hazardous area.

The certificate used for this example is:

PTB 17 ATEX 2006 X – Issue 0

The certification covers different installation arrangements and EPLs, including markings within the LVL200 family such as:

II 1G / 1/2G / 2G Ex ia IIC T6…T1 Ga / Ga/Gb / Gb

The exact marking applicable depends on how the equipment is installed and the conditions stated in the certificate.

Pepperl+Fuchs KFD2-SR2-Ex2.W

The associated apparatus in the safe-area panel is:

Pepperl+Fuchs KFD2-SR2-Ex2.W

For this UK example, the certificate being referenced is:

CML 21UKEX2786 – Issue 0

The intrinsically safe circuit is marked:

II (1) G [Ex ia Ga] IIC

The square brackets are important.

They indicate that the intrinsically safe circuit from the associated apparatus can be suitable for equipment in the stated hazardous-area level, while the barrier itself is not necessarily installed in that hazardous area.

In our example, the barrier is kept in the non-hazardous area.

Step 1: Get the Certificate Data

Before doing any calculation, I like to put the important certificate values into a simple table.

*For this worked example, I am using the IIC values from the certificate table applicable when external capacitance and inductance occur together.

Read the Correct Table

This is an important point.

The Pepperl+Fuchs certificate also contains larger values for external reactances when they occur individually.

For IIC, you may see values such as:

• Co = 2.41 µF
• Lo = 210 mH

It would be very easy to find those values, put them into an IS spreadsheet and move on.

But our cable introduces both capacitance and inductance.

For this example, the relevant certificate table gives:

• Co = 620 nF
• Lo = 3 mH

Step 2: Convert the Units First

Before comparing anything, I like to convert the values into consistent units.

This sounds basic, but it removes one of the easiest ways to make a serious mistake.

Nano, micro and milli are very easy to mix up when you are copying values between certificates, datasheets and calculation sheets.

A decimal point can make a massive difference.

Is the value 993 W or 0.993 W?

A spreadsheet won’t know that you typed it incorrectly.

Step 3: Check Voltage – Uo ≤ Ui

The barrier maximum output voltage must not exceed the maximum input voltage permitted by the field device.

The barrier cannot supply more voltage than the LVL200 is certified to accept.

Step 4: Check Current – Io ≤ Ii

Next, compare the maximum barrier output current with the permitted field-device input current.

Step 5: Check Power – Po ≤ Pi

Now check the maximum power.

Again, the available barrier output is below the permitted instrument value.

Step 6: Add the Cable

The cable is part of the intrinsically safe loop.

For this worked example, I am going to assume:

• Cable length = 50 m
• Cable capacitance = 200 pF/m
• Cable inductance = 1 µH/m

Step 7: Calculate Cable Capacitance

Cable capacitance is:

Convert that into more useful units:

The LVL200’s internal capacitance for the version without fixed cable is stated as negligibly low.

So for this example:

Step 8: Check Capacitance – Co ≥ Ci + Ccable

Step 9: Calculate Cable Inductance

Using our assumed cable inductance:

Convert to mH:

The LVL200 Li is stated as negligibly low for the version without fixed cable.

So:

Step 10: Check Inductance – Lo ≥ Li + Lcable

Final IS Calculation

The LVL200 Certificate Has an X – Don’t Ignore It

The Siemens certificate number ends in:

PTB 17 ATEX 2006 X

That X matters.

It tells us that there are specific conditions of use which need to be checked.

Section 17 of the certificate contains conditions relating to matters such as:

• Possible electrostatic charging of certain surfaces
• Installation considerations involving aluminium for particular equipment categories
• Equipotential bonding requirements for certain Category 1 and Category 1/2 arrangements
• Specific process temperature and pressure conditions for certain installations

Temperature Class Isn’t Just a Label Either

The LVL200 certificate also shows how the permissible sensor and electronics temperatures relate to the temperature class.

For Category 2 equipment, for example, the permitted conditions become more restrictive as you move towards T6.

*The certificate includes additional temperature-adapter conditions for some of these ranges. Always read the notes with the table.

This is another reason I don’t like people looking at an instrument, seeing “T6…T1” and assuming that is all they need to know.

The operating conditions matter.

If you want a broader explanation of this, see:

ATEX Temperature Classes Explained: T1 to T6 Made Simple
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Certificates Shouldn’t Be a Treasure Hunt

This one is aimed at manufacturers.

If you sell equipment for hazardous areas, please make the certificates easy to find.

And preferably make it obvious:

• What the latest certificate issue is
• Whether supplements exist
• Which exact product variants are covered
• Where the special conditions are
• Whether older certificates have been superseded

Engineers shouldn’t need to spend half an afternoon hunting through websites and old PDFs just to establish whether an instrument is suitable.

Certificate revision control matters because an IS calculation is only as good as the information used to build it.

A PASS Does Not Prove the Circuit Will Work

There is another important distinction.

An intrinsic safety calculation proves an aspect of the safety compatibility of the loop.

It does not automatically prove that the circuit will function correctly.

I’ve seen this particularly with intrinsically safe solenoids.

The entity parameters can pass, but there simply isn’t enough usable current available through the interface and cable to switch the device properly.

I’m deliberately not going deeply into functional checks in this article because that deserves a separate worked example.

Traditional Barriers vs Newer Architectures

Barriers and galvanic isolators are still extremely common.

I’ve worked on panels where there might be 40 individual barriers sitting in a marshalling section.

There is nothing automatically wrong with that approach. It is well understood and can work very well.

But it can also become time-consuming to design, wire, document and modify.

Adding more channels later isn’t always particularly elegant either.

There are now other approaches available, including integrated intrinsically safe and remote I/O architectures, depending on the application and certification requirements.

Common Mistakes This Example Highlights

• Using the wrong certificate revision
• Missing a certificate supplement
• Ignoring the X suffix
• Using values from the wrong Co/Lo table
• Mixing nano, micro and milli units
• Rounding values unnecessarily
• Forgetting to update cable length
• Using assumed cable values instead of manufacturer data
• Assuming a spreadsheet PASS means the engineering is correct
• Assuming ATEX equipment automatically means it is suitable for an IS loop
• Checking safety compatibility but forgetting functional compatibility

What This Calculation Does Not Prove

Even though the entity parameter calculation passes, this article has not completed the entire hazardous-area design.

You would still need to consider items such as:

• Actual HAC documentation
• Zone and EPL suitability
• Gas group
• Temperature class
• Actual ambient and process temperatures
• All special conditions of use
• Actual cable manufacturer values
• Functional compatibility
• IS/non-IS segregation
• Earthing and bonding requirements
• Cable identification
• Termination of spare cores
• Installation inspection
• Drawing accuracy
• Applicable project and regulatory requirements

My Rule When Checking an IS Calculation

The equations aren’t particularly difficult.

The dangerous part is assuming the input information must be correct because the spreadsheet says PASS.

Check the certificate.

Check the issue.

Check the table.

Check the units.

Check the cable.

Then check them again before approving the loop.

Frequently Asked Questions

Final Thoughts

On paper, this intrinsic safety calculation was easy.

The voltage passed.

The current passed.

The power passed.

The cable capacitance passed.

The cable inductance passed.

But the useful part of the exercise wasn’t typing five equations into a spreadsheet.

It was making sure we used the correct certificate values in the first place.

We had to identify the correct equipment, check the certificate issue, notice the X suffix, use the correct reactance table, convert the units and include the cable.

That is where most of the engineering is.

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  Intrinsic Safety Explained: Ex i, Barriers and IS Circuits Made Simple
  
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  ATEX Temperature Classes Explained: T1 to T6 Made Simple
  
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